The ecology of <italic>Psorophora confinnis</italic> (the dark rice-field mosquito) in Coachella Valley, California

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Content TEE ECOLOGY OP PSOROPHORA CONFIENIS (THE DARK RICE-PIEID MOSQUITO) IE COACHELLA VALLEY, CALIFORNIA by Abdulla Azawl A Thesis Presented to the FACULTY OP THE GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree MASTER OP SCIENCE (Zoology) January 1956 UMl Number: EP67221 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. Dissertation Publishëng UMl EP67221 Published by ProQuest LLC (2014). Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106- 1346 UNIVERSITY OF SOUTHERN CALIFORNIA G R A D U A T E S C H O O L U N IV E R S IT Y P A R K LO S A N G E L E S 7 This thesis, written by Abdulla Azawi under the guidance of h.%.^..Faculty Committee, and approved by a ll its members, has been pre sented to and accepted by the Faculty of the Graduate School, in p artial fu lfillm e n t of the requirements fo r the degree of r . Bean Date..Im LXLBi:ir.,2A,^ 1.95.&............... Faculty Committee Chairman 2'- f ACKNOWLEDGMENT My special thanks are due to Dr. Robert M. Chew under whose guidance the present problem was developed. The help and advice of Dr. Ernest R. Tinkham were inval uable. Dr. Tinkham originally suggested the problem, and in the summer of 19$4, as director of the Coachella Valley Mosquito Abatement District, made available laboratory space and C.V.M.A.D. research data. Thanks are also due to Dr. John S. Garth whose instruction and encouragement were of particular importance in the completion of this work; to Dr. Walter E. Martin whose kindnesses throughout the course of this study were deeply appreciated; to Mr. Cavenoph, Manager of the C.V.M.A.D. in the summer of 1955? for the use of equipment and laboratory space; and to the trustees and members of his staff, and the employees of the District for their help and cooperation. TABLE OF CONTENTS CHAPTER PAGE I. THE PROBLEM................................... 1 Introduction............................... 1 Development of the problem ................ 5 Definitions of terms used ................... 6 Soil moisture terms....................... 6 Wet s o i l ............................... 6 Damp soil............................... 6 Moist s o i l ............................. 6 Dry s o i l ............................... 6 Density terms ............................. 6 Adult density........................... 6 Set of sweepings....................... 7 Larval-pupal density ................... 7 Gross density........................... 7 Habitat terms ............................. 7 D i k e ................ 7 L a n e ................................... 7 V CHAPTER PAGE II. DESCRIPTION OF THE AREA...................... 8 General location ........................... 8 S o i l .......................... 8 Sources of water ........................... 9 Climate ..................................... 9 Vegetation................................. 10 Description of the study area.............. 12 III. MATERIALS AND METHODS........................ 17 Adults and larvae of P. confinnis.......... 17 Measurement of adult population ............ l8 Weather conditions ........................ 20 Measurement of adult density .............. 21 Study of adult habits...................... 21 Measurement of larval and pupal densities . . 24 Resistance to drying...................... 25 Larval and pupal behavior .................. 25 IV. RESULTS AND DISCUSSION...................... 27 Adult population.......................... 27 Seasonal occurrence ...................... 27 Relationship between environmental temperature and si^e of the adult population............................. 28 Effect of habitat on adult density .... 35 Vi CHAPTER PAGE Adult density and presence of livestock . . 36 Adult flight range........................ 43 Adult habits............................... 44 Daytime activity ........................ 44 Night activity........................... 46 Feeding habits ........................... 48 Egg-laying habits ........................ 51 Adult longevity............................. 53 The aquatic population .................... 55 One generation per irrigation............ 55 Types of larval habitats................ 56 Microhabitats ............................. 66 Relationship between larval size and density................................. 68 Larval behavior ........................... 70 Effect of temperature.................... 75 Phototropism ............................. 7 Water p H ................................. 76 Pupation and adult emergence ............ 76 Predominance of Psorophora confinnis in temporary waters........................ 79 Resistance of larvae and pupae to desiccation............................... 79 vil CHAPTER PAGE V. OBSERVATIONS ON THE EFFECTIVENESS OF TOXAPHENE AGAINST P. CONFINNIS ............. 83 VI. SUMMARY AND CONCLUSIONS....................... 86 REFERENCES CITED ................................... 90 LIST OF TABLES TABLE PAGE I. Average Monthly Temperature and Precipitation for United States Date Gardens, Indio, California; Based on United States Weather Bureau Records, 1879 through 1 9 4 8 ......................... 11 II. Data on Light Trap Catches for the Years 1952-1955........................ • 29 III. The Relation of Soil Moisture to the Density of Adult Psorophora confinnis in the study a r e a ......................... 37 IV. The Relation of Soil Moisture to the Number and Sex of Adult Psorophora confinnis in the Study A r e a ............... 38 V. The Relation of Soil Cover to Density of Adult Psorophora confinnis in the study Area . ................................... 39 VI. Night Activity of Adult Psorophora confinnis as Indicated by Light Trap Collections During July 20 to August 3, 1955 ........... 47 IX TABLE PAGE VII. Night Activity of Adult Psorophora confinnis as Indicated by Light Trap Catches August 29 to September 12, 1955............. 49 VIII. Results of Experiments Testing Preference of Egg-laying Site................... 54 IX. Larval-Pupal Densities in Relation to Water Area and Time After Irrigation....... 58 X. A Comparison of Larval Densities in: (1) Grassy Versus Non-grassy Parts of the Same Patch of Water, and (2) Water With and Without Pruned Leaves of Date Palm ... 67 XI. Sizes of Fourth-instar Larvae From Water Patches of Differing Larval Densities ... 69 XII. Activity of Third and Fourth-instar Larvae of Psorophora confinnis .................. 71 XIII. Data on Attraction of P. confinnis Larvae and Pupae to Light................... 77 LIST OF FIGURES FIGURE PAGE 1. Map of Study A r e a ............................ 13 2. Flooded Date Ranch L a n e ...................... 15 3. Ranch House Experimental Quarters ............ 15 4. Light Trap in Study Area...................... 19 5. Cage Used for Egg-laying Preference Experiments................................. 23 6. Aquarium Used for Rearing and Marking Adults . . 23 7. Apparatus Used in Experiment on Larval Attraction to Light......................... 26 8. The Mean Daily Water Temperature and the Daily Catches of the One Light Trap at the Study Area......................................... 30 9. Records of Water Temperatures for 24 Hours During the Period of High P. Confinnis Population at the Study Area................ 31 10. The Mean Weekly Air Temperature and Weekly Adult Catches of Eight Light Traps ...... 32 11. The Mean Weekly Maximum Air Temperature and Weekly Adult Catches of Eight Light Traps . . 33 xi FIGURE PAGE 12. Map of Coachella Valley.................... . 40 13. Maie and Female Psorophora confinnis Adults at Rest on Grass Stems ............ 45 14. Engorged Female............................... 52 15. Extreme Pupal Density in Small Pool of Water . . 57 1 6. through 24. Photographs of Different Habitats of Larval P. confinnis ............ 60-65 26. Drawings of Larval Activity................... 73 CHAPTER I THE PROBLEM I. INTRODUCTION Psorophora confinnis. the dark rice-field mosquito, also called the Florida Glades mosquito and the three-day mosquito, is a member of the subfamily Culicinae of the family Culicidae, order Diptera, Matheson (1944) sum marizes the complicated nomenclatural history of Psorophora confinnis. Lynch Arribalzaga first described the species in 18911 and the present generic status was established by Aitken in 1940. P. confinnis is found only in the western hemisphere, and within the United States is restricted to the southern and central tiers of states. According to Carpenter and LaCasse (1955) this species is unreported for the northern tier: Washington, Oregon, Idaho, Montana, Wyoming, North Dakota, Minnesota, Wisconsin, Michigan, Vermont, New Hampshire, and Maine. P. confinnis was first reported in 2 California by Aitken (1940), and was the first species of this genus recorded from the Pacific Coast. Most of the studies of the ecology of P. confinnis have been made in the rice growing areas of Arkansas where this mosquito comprises 80 to 90 per cent of the mosquito fauna (Schwardt, 1939)» Horsfall (1942) reports that adult Pj^ confinnis are present in Arkansan rice fields between mid-May and mid- October, with the largest populations occurring in the period of mid-June to mid-September. Two prominent popula tion peaks were noted, the first in the middle of June and the second in late July and early August. These peaks coincide with the periods of greatest irrigation of the rice fields, and it appears that abundance of P. confinnis is largely determined by the time and amount of irrigation. The first adult mosquitoes emerge after the initial flood ing of the fields in June and disappear before the fields are drained for cultivation. The second population peak develops following reflooding of the cultivated fields. Since the fields are then commonly kept continuously flooded, few egg-laying sites are available, few adults are thus added to the general population, and as the older adults die, the population declines. 3 Egg-laying habits vary. Carpenter and LaCasse (1955)j and Horsfall (1942) report that females lay their eggs on moist soil. However, Matheson (1944) claims that females deposit on dry ground. Horsfall (1942) found that areas planted with rice provide good egg-laying sites, whereas areas bare of vege tation were of little importance. He found the larval period to require four to ten days, and the pupal period 24 hours. The flight range of the female is 8.8 miles (Horsfall, 1942). Feeding females cling to humans and livestock for considerable periods of time, and this may be an important means of dissemination. Numbers of mosquitoes have also been observed to remain within an automobile during an entire half-day of travel (Schwardt, 1939). Whitehead (195D made precipitin tests to determine the sources of blood collected from Psorophora confinnis females. He found cattle to be the chief source of blood. Horses, mules, and hogs are fed upon less frequently, and birds rarely. Only a small percentage of mosquitoes were found to have fed upon humans, possibly because the people living in P. confinnis areas are asleep and protected by screens during the hours when P. confinnis seeks its food. According to Horsfall (1942) the longevity of the If female depends on the availability of blood for food. One blood meal is insufficient to sustain the female long- enough to develop the first batch of eggs. It was found that mosquitoes that were experimentally fed only once died in five days or less. No evidence has been found that P. confinnis ac tually transmits any diseases. Laboratory maintained individuals have contained viable virus of western equine encephalitis for seven days, but failed to transmit it to experimental animals during this period (Haxamon and Reeves, 1943) . The mosquito is important economically as a pest. The females are severe biters and will attack any time of day or night, although they are most active at night (Schwardt, 1939) . In the Florida Everglades and in the rice fields of Arkansas, where P. confinnis probably reaches its greatest abundance, livestock are occasionally killed as a result of mosquito bites. Bishop (1933) re ported that during an especially severe mosquito season at least 173 heads of livestock and poultry died in the vicin ity of Miami and the Everglades from mosquito bites. Milk production was noticeably reduced. Death was attributed to loss of blood and mosquito "toxins." 5 II, DEVELOPMENT OF THE PROBLEM In the spring of 1954 the writer participated in a field trip to the Coachella Valley Mosquito Abatement District to observe the work of Dr. Ernest R. Tinkham. The writer was attracted to the area because of its similarity to his native country, Iraq. He obtained employment by the C.V.M.A.D. during the summer of 1954. This employment allowed time for work on the side toward a Master*s thesis; Dr. Tinkham suggested the biology of Psorophora confinnis as a suitable subject, and the scope of the work was outlined by Dr. Chew, with suggestions from Dr. Tinkham. In the summer of 1955 the writer spent his full time in the field studying P. confinnis. This problem was worthy of study for several reasons: 1. P. confinnis is the main pest species in the Coachella Valley in the summer and early fall, and little was known of its biology. 2. There had been no work on this species in the valley since Aitken's report in 1940 of its presence. 3. The heavily-irrigated date gardens planted in the valley within the past thirty years present 6 a habitat highly suitable for P. confinnis, but rather different from that of the rice fields in Arkansas. III. DEFINITIONS OF TERMS USED Since some of the terms used in this paper have meanings peculiar to mosquito biologists, it has been felt best to define them here. Soil Moisture Terms Wet soil is soil visibly covered with water. Damp soil is soil not visibly covered with water but too moisture-laden to permit one to walk upon it easily. Moist soil is soil which obviously contains water within its pores but which permits easy walking. Dry soil is soil containing no obvious water. These soil terms are those currently being used by personnel of the Coachella Valley Mosquito Abatement District (C.V.M.A.D,). Density Terms Adult density is expressed as the number of adults living over one square meter of area, as measured by I 4 .8 sweeps with an insect net. 7 A set of sweepings is 48 sweeps taken in a standard fashion with an insect net. Larval"PUPal density is the average number of individuals collected per dipping with a standard aquatic dipper, calculated on the basis of six dippings in a particular limited area. Gross density is a density value based on numerous dippings or sweepings in different parts of a relatively large area. Habitat Terms A dike is an embankment along which a row of date trees is planted. A lane is the long strip of ground between two adjacent dikes that is filled with water when the date groves are being irrigated. CHAPTER II DESCRIPTION OF THE AREA I. GENERAL LOCATION The Coachella Valley is located in the south-central part of Riverside County in southern California. The city of Indio, near the center of the valley, is 130 miles south-east of Los Angeles and about 90 miles north of the Mexican border. The valley is bounded on the north and east by the San Bernardino and Chocolate mountain ranges respectively, on the south by the Salton Sea, and on the south and west by the Santa Rosa and San Jacinto mountains. The narrow, fan-like valley extends from San Gorgonio Pass 60 miles south-east, to the Salton Sea, and includes an area of 344 square miles (220,160 acres). II. SOIL McFarlane and Winright (1951) report that the soils of Coachella Valley are largely alluvial deposits from 9 granite rock sources, laid down mainly by the Whitewater River. Most of the soils are sandy, ranging from very coarse sand to loamy soils. Adjacent to the Salton Sea the soils are fine-textured, having been deposited from waters that were still or very slow moving. Wind action has created a topography of low dunes. The soils in this region contain a high concentration of alkali salts. III. SOURCES OF WATER Whitewater River, arising on Mt. San Gorgonio, traverses the main axis of the valley and empties into the Salton Sea. There are about twenty other watersheds that also enter the valley from surrounding mountains, but their mean annual discharge is not large, since the greater part of the drainage area is desert land. Only during infre quent flood stages does surface water reach the Salton Sea. Almost all irrigation water in the Coachella Valley is obtained from wells or from the irrigation canal that brings water from the Colorado River. Drainage of this water maintains the Salton Sea. IV. CLIMATE Climatically, the Coachella Valley is a hot desert. Most of the rainfall comes in the months of December, 10 January, and February. Weather records of the United States Date Gardens in Indio, continuous since 1879, in dicate an average annual rainfall of 3.34 inches. Monthly distribution of precipitation is indicated in Table I. An all-time maximum air temperature of 125°F has been recorded at the Date Gardens. The temperature may get as high as lOQOF any time between March and October, and winter maxima often rise as high as 75°F. The winters are short and mild, with a high percentage of sunny days. Killing frosts seldom occur before the middle of November or after the middle of March. Usually, killing frosts occur only between December 1 and February 15 so that a frost-free season of approximately three hundred days is normally expected. Monthly average temperatures are given in Table I. V. VEGETATION Most of the 220,000 acres of the Coachella Valley support only a sparse desert vegetation; only about 29,500 acres are cultivated. According to McFarlane and Winright (1951) the major crops with their acreages are: grapes (6,600 acres), corn (5,700), dates (4,800), citrus trees ( 1,900), alfalfa (1,000), cotton, beans and other vege tables (9,500). Psorophora confinnis can breed and survive 11 TABLE I AVERAGE MONTHLY TEMPERATURE AND PRECIPITATION FOR UNITED STATES DATE GARDENS, INDIO, CALIFORNIA; BASED ON UNITED STATES WEATHER BUREAU RECORDS, 1879 THROUGH 1948 Month Average Temperature Average Percipitation in inches January 53.8°F .68 February 58.8 .50 March 64.9 .31 April 72.0 .11 Mày 79.2 .04 June 87.3 .01 July 93.1 .07 August 91.7 .29 September 85.6 .32 October 75 .0 .20 November 63.9 .21 December 55.5 .60 Yearly average 73.3°F 3.34 12 only within irrigated areas where water stands in the fields for three to seven days or more. VI. DESCRIPTION OF THE STUDY AREA The present study was carried out largely on four adjacent date ranches located about eight miles south of Indio in the small village of Thermal. The ranches border both sides of Monroe Street from Avenue 56 on the north to Avenue 58 on the south (see map, Figure 1), and cover an area of slightly more than 500 acres. Agricultural practices on these ranches are such that they are very suitable for breeding of P. confinnis. Most of the date trees on the ranches have grown to a height of more than 8 meters. The trees are planted in rows along irrigation dikes, the grass-covered lanes be tween dikes being about 50 meters long and 7 meters wide. The lanes are usually flooded twice a month through standpipes coming to the surface at the end of each lane (see Figure 2, page 15). Following inundation, standing water usually remains from three to six days. Records by the writer for 17 periods of irrigation, involving 109 lanes, show an average persistence of water for 5.1 days. Some water remained in six lanes for eight days, and in two lanes for seven days. Disking of the soil following 13 FIGURE 1 MAP OF STUDY AREA Date ranches. The house that was used as headquarters. Light traps. The trap beside the house was used only briefly for adult flight- range and night activity studies. The other trap was used continuously for the measurement of adult population fluctua tion. Release point for the adult flight-range study. y4ve- sè dye. A- FIGURE 1 MAP OF STUDY AREA 15 Figure 2. Typical flooded lane on date ranch. Lane offers ideal breeding place for P. confinnis. Figure 3* Ranch house used as experimental quarters. Weather equipment station under trees. 16 irrigation is practiced on two ranches, and this reduces the period of standing water to two or three days. Usually, groups of lanes are irrigated in rotation. Thus, during the irrigation period, some lanes are com pletely covered with water, others have damp or moist soil, and still others are dry and ready to be irrigated. Three of the four ranches kept one or more kinds of livestock. Only a few houses occur in the study area, at the edges of the ranches. The author rented a house located 50 meters within one of the ranches (see map, Figure 1, page 13; and Figure 3, page 15), and some of the experi mental work was done within this house. Weather recording instruments were installed under a tree outside the house and weather conditions were checked over a three-month period, mid-June to mid-September 1955- CHAPTER III MATERIALS AND METHODS I. ADULTS AND LARVAE OF P. CONFINNIS Adult Psorophora confinnis are easily identified in the field on the basis of features visible to the unaided eye or with hand lens. Both sexes have the same coloration and markings. The adult is a general yellowish-brown. The thorax has a hump. The abdomen is pointed and has pairs of triangular spots of white scales apically on tergites IV through VII. The proboscis is dark except for a yellowish- white median band. Each femur has a ring of white scales near the lower end; the tibiae are dark with numerous small white scaly spots on the lateral surface; and the tarsi are marked with broad white basal rings. The larvae of P. confinnis are distinguished from other species present in the study area mainly by their flattened siphon. The siphon has 3 to 5 widely separated teeth, and the gills are two to three times as long as the 18 saddle. II. MEASUREMENT OF ADULT POPULATION An insect light trap was hung on the wall of one of the buildings in the study area (see map, Figure 1, page 13). Figure h shows the trap in operating position. This trap was emptied each morning during mid-June to mid- September 1955, and the captured P. confinnis were sexed and counted. The nightly catch was used as an index of changes in the size of the adult P. confinnis population in the study area. Seven other light traps were placed in scattered locations in the valley outside the study area (see Figure 12, page 1 | . 0 ) , and these traps were emptied twice weekly (at three-and four-day intervals) by workers of the C.V.M. A.D. An additional trap was used in the study area from August 29 to September 15, 1955, for the study of adult flight range and night activity only. The type of trap used is known as the New Jersey light trap. The cylindrical body of the trap is about 23 cm. in diameter and 30 cm. in length. An electric fan inside the top of the cylinder blows a current of air down ward through a fine wire-mesh cone into a killing jar. A reflecting cone housing a 25-watt bulb is fastened over the 19 m • i f t FIGÜEE I f LIGHT TRAP IN STUDY AREA 20 top of the cylinder. The light bult and fan are operated by a time clock attached to the outer surface of the cylinder and set to turn on at 6:00 P.M. and off at 6:00 A.M. Insects, attracted to the light, enter the cylinder and are pushed downward by the air current into a cyanide killing jar. III. WEATHER CONDITIONS Weather conditions thought to be important to P. confinnis were recorded daily. Maximum and minimum air temperatures were recorded from a max-min. thermometer each morning at 7:00 A.M. Pacific Standard Time*-«’ Water tem peratures were measured at 7:00 A.M. and at if:00 P.M. There was no permanent body of water in the study area, so water temperature was recorded from temporary areas of standing water, wherever they were available. Measure ments were made on a particular pahch, through a series of days as long as the patch existed. A distance-bulb re cording thermograph was used to make continuous records of water temperatures in certain flooded lanes. Relative himidity was measured with an ”egg-beater” psychrometer dally at 7:00 A.M. and l^zOO P.M. Occurrences of rain and wind were noted. •x-All times in paper will be Pacific Standard. 21 IV. MEASUREMENT OF ADULT DENSITY Adult density was measured in terms of the number of individuals captured by ^8 uniform sweeps of a standard sized insect net. The net used had a circular frame 30 cm. wide and a handle 80 cm. long. The ^8 sweeps were taken uniformly right to left and left to right as the collector walked slowly forward. Each sweep followed an arc of about 50 cm. in length, and each set of sweeps was taken over a linear distance of about 10 meters. Sets of 48 sweeps were made at different heights (uniform height for any one set) over different types of soil (wet to dry) and over dif ferent soil covers. The height of sweeping was expressed as the distance between the lowest part of the frame of the net and the ground. A set of 48 sweeps taken as described is considered to capture a number of insects equal to that living over one square meter of surface in the area of sweeping. See Shackleford (1929) for the validity of this relationship. V. STUDY OF ADULT HABITS The amount of adult activity at different times of the day was measured in two ways: (1) by sweeping an insect net through the air from side to side while walking forward, or by swinging the net circularly about the body 22 while standing still, the height of sweeping varying from 40 to 160 cm., and (2) by making light trap collections at frequent regular intervals during the night. The biting behavior of adults was studied in the laboratory. Adults were kept in small shell vials for this study. Adult longevity was measured by confining adults in fine wire-mesh cages 9 cm. wide by 17 cm. long. Egg-laying habits were studied by keeping females collected from the ranches in an “egg-laying” box 46 x 34 x 47 cm. having top and one side of wire-mesh. A black cloth sleeve was attached to an opening in the front of the box to permit loading of the cage without escape of the mos quitoes (see Figure 5). Petri dishes 10 cm. in diameter were prepared in several ways and placed in the box to simulate different natural conditions. Damp soil waa simu lated by mud in a dish over a piece of filter paper over a layer of moistened cotton. To simulate dry soil, the cotton was not moistened and the mud was allowed to dry. Open water was simulated merely by a petri dish filled with water. A grassy surface was simulated by sticking grass stems into the “moist soil” in a petri dish. Four to five days after the females were placed in the box, the petri dishes were examined for eggs and the eggs were counted. 23 Figure 5« Cage with petri dishes for P. confinnis egg-laying preference experiments. Figure 6. Covered aquarium for raising and marking adults for flight-range experiments. 24 To study flight-range, adults were raised inside an aquarium covered with a black cloth (see Figure 6, page 23). As adults hatched and clung to the sides of the aquarium they were dusted with rhodamine-B powder blown into the aquarium, and then turned loose. To detect these marked adults following their capture in light traps, light trap collections were placed in a pan of water to which a few drops of the detergent tritin had been added. Dye re maining on the dusted adults is easily detected as a red stain in the water. (This marking and detecting technique is used successfully in gnat studies by the C.V.M.A.D.) VI. MEASUREMENT OF LARVAL AND PUPAL DENSITIES Larval and pupal densities were measured in terms of the average number of individuals captured per dipping of an aquatic dipper, on a basis of six dippings in a particu lar area. The dipper used was a standard entomological dipper 11.5 cm. wide and 7 cm. deep. Each dipping was made quickly over a distance of about 40 cm. with the bowl of the dipper about two-thirds submerged. Successive dippings were made about one meter apart. The larval-pupal densities of a particular lane were measured each day as long as water remained in the lane. 25 Density was measured at a number of different locations previously plotted on a lane map at the start of the study. VII. RESISTANCE TO DRYING The resistance of larvae and pupae to desiccation was measured by placing larvae of various instars and pupae in covered petri dishes having a moist filter paper insert. After the filter paper was moistened, any water not held adsorbed by the paper was removed with an eye dropper. Resistance was measured in terms of successful development of larvae to pupae and pupae to adults. VIII. LARVAL AND PUPAL BEHAVIOR Various aspects of behavior were observed in clear patches of irrigation water. Reaction to light at night was studied by hanging a flashlight over the water by means of a homemade frame (see Figure ?)• The height of the light was adjusted by bending the legs. Dippings could be made directly under the light since the construction of the holder left this area free of obstructions. Larval and pupal movements and the hatching of adults were observed in the laboratory. 26 FIGURE 7 APPARATUS USED IN EXPERIl-ffiNT ON LARVAL ATTRACTION TO LIGHT CHAPTER IV RESULTS AND DISCUSSION I. ADULT POPULATION Seasonal Occurrence The seasonal occurrence of adult P. confinnis was determined from the 1952-1955 light trap records of the C.V.M.A.D. The writer contributed to these records in the summers of 1954 and 1955* In 1952 the first adults were caught in traps on July 1, the population peak occurred in the second week of September, and the last adults were caught on October 21. In 1953 the first adults were caught on June 15, the population peak occurred in the fourth week of September, and the last adults were caught on October 23. In 1954 the first adults were captured on April 16, the population reached a peak the third week in August, and was last detected October 2 9. In 1955 the first adults were caught on April 28, the population reached a peak the first week 28 of September, and the last adults were caught on October 21. The total catches of the light traps for each year are given in Table II. The catch, and possibly the popula tion, has greatly increased with each year. Interpretation of these data is limited by differences in the number of traps used, changes in the location of traps in 19$k-, and possible differences in effectiveness of mosquito control from year to year. Since a particular light trap will attract adults from its immediate vicinity only, any changes in irrigation in the area around a trap could be responsible for large changes in the catch of that trap from year to year. Relationship Between Environmental Temperature and Size of the Adult Population Temperature and population size data are compared graphically in Figures 8, 10, and 11, A water temperature of 26°0 seems to be the approximate threshold for development of an adult mosquito population. Figure 8 compares the mean daily water temperature of flooded lanes within the study area with the daily catch of the one light trap operated in the study area. From June l5 to July 11, the mean daily water temperature was 29 TABLE II DATA ON LIGHT TRAP CATCHES FOR THE YEARS 1952-1955% Year Number of traps operated Total adults Female captured Male 1952 5 800 13 1953 5 3,395 363 1954 7 62,788 10,171 1955 8 38,057 8,286 -%Data from the records of the Coachella Valley Mosquito Abatement District. Temperature summations were calculated from data in records of United States Date Gardens, Indio, California. 30 % »o « v > s? ï< «X III k * x V e > o o o • o CO % ? Vw I > Î Î 14 1 I OQ a « T 5 a Og O - I * « k e e e o VO OO VO : 11 V» ffi 3^ 26.1°C. During this time light trap catches were quite low. On July 19 the first conspicuous increase in trap catches occurred, simultaneously with a sharp increase in water temperatures. Large trap catches were recorded almost every day from July 15 through September 11, and the mean daily water temperature for this period was 28.1^0. Figure 9 shows records of water temperatures for 2^--hour periods during the period of high P. confinnis populations in the study area. Figure 10 compares mean weekly air temperature (data of United States Date Gardens, Indio) with the weekly catch of eight light traps operated in the valley (seven operated by C.V.M.A.D. plus one in study area); while Figure 11 compares the mean weekly maximum air temperatures with light trap catch. No adult P. confinnis were caught in the valley in the period January 1, 1955, to April 21 (one week before the first captures). During this period the mean weekly air temperature averaged 14.9^C and the mean weekly maximum air temperature averaged only 24.h°C. Adults were contin uously present from April 28 through October 20, and during this time weekly air temperatures averaged 28.2^0 and max imum air temperature averaged 36.9^C. From June 21 through October 6, when the largest catches occurred, the weekly 35 air temperatures averaged 30.5^0 and the weekly maximum temperature averaged 38.8°G, The occurrence of 26®C as the minimum water temper ature at which a population of adult P. confinnis can develop in the Coachella Valley depends upon the inter action of rate of aquatic development of P. confinnis and time of persistence of irrigation water. Horsfall (1914-2) showed the P. confinnis requires 7.0 i 0.1 days to complete its aquatic development at 26®C in the laboratory, and that development in the field required 5 to 6 days when water temperatures ranged between 30 and 35^0. In the present writer*s study area, irrigation water persisted on an average 5.1 days, and occasionally 7 to 8 days. From June l5 through July 11, when the water temperature averaged 26.2^0, the hatching of some few adults was possible, presumably in lanes remaining flooded 7 or 8 days. Large populations were made possible only when the water temperature had increased to the point where development could be completed in 5 days or less, the usual duration of irrigation water in the date ranch lanes. (As indicated later In the study, some development may occur after surface water has disappeared but while the soil is still moist.) Effect of Habitat on Adult Density As summarized in Tables III and IV, the highest 36 adult P. confinnis densities were found over wet or damp soil; density was approximately 50 per cent lower over moist soil, while very low densities occurred over dry soil. Females collected over wet soil were largely unfed, newly-hatched individuals, while those collected over moist or dry soil were largely engorged. The presence of grasses apparently makes an area more attractive to adult P. confinnis. As shown in Table V, densities over grass-covered lanes were 300 per cent greater than those over bare soil. Adult mosquitoes were observed clinging to the leaves and stems of grasses during the daytime. P. confinnis rarely fly during the daytime. Five sets of sweeps made at a height of 120 cm. captured only one adult. Circular sweeps made around the collector's body for a period of one half hour on each of five days netted no specimens. Adult Density and Presence of Livestock During August and the first half of September 1955, adult mosquito densities were measured at 20 ranches keep ing livestock and 20 ranches having no livestock. These ranches were scattered throughout the valley (see map, Figure 12). All ranches practiced irrigation. At each ranch adult density was measured by one set of sweeps at 37 TABLE III THE RELATION OF SOIL MOISTURE TO THE DENSITY OF ADULT PSOROPHORA CONFINNIS IN THE STUDY AREA Type of soil Number of sets of sweepings Number of adults collected Average density Wet and damp ^1 2,319 56.5 Moist 19 283 20.1 Dry 28 117 If.2 *One set consists of sweepings of an insect net over the particular type of soil (see methods, page 21). 38 TABLE IV THE RELATION OF SOIL MOISTURE TO THE NUMBER AND SEX OF ADULT PSOROPHORA CONFINNIS IN THE STUDY AREA Type of soil Number of days after disap pearance of standing water Number of sets of sweeps Specimens Number of males collected Number of females Damp soil 2 i f Ih 192 Nearly damp 6 h 8 94 Moist 11 If 1 46 Nearly dry 13 If 1 22 39 TABLE V THE RELATION OF SOIL COVER TO DENSITY OF ADULT PSQROPHORA CQNFINNIS IN THE STUDY AREA Type of habitat Number of sets of sweeps Total number of adults collected Average density m2 Soil***’covered with grasses 10 321 32.1 8oil#bare, without grasses 10 82 8.2 w Soil was moist. ho FIGURE 12 MAP OP COACHELLA VALLEY # Light trap. 4» Ranch without livestock. Ranch with livestock. ^ Release point for adult-range study. E3 Location of the study area. j s « w i M r "S 5 J S t â * 5 ^ f f l jTw u - ^ ■ ^ 1 ^ a « * w . u J S JJSUOSliiDM i s owng n i y v JS uosjtaof js a ^ u ü o w J C U O S .ip O W jjj U O S i»Jl^ 0 3 S u lB p t/ & U |U S 1 H \ 1 + 2 each of four locations. Each set of sweeps was taken at an approximate height of 15 cm. through grasses growing on moist soil. (Exceptions: at one ranch only three sets of sweeps were taken; at one livestock ranch and one non livestock ranch all sweeps were over damp rather than moist soil). Seventy-nine sets of sweeps on ranches with live stock captured 770 adult P. confinnis. giving a gross density of 9.76 individuals per square meter. No adults were captured at three ranches. There was no apparent reason for the absence of adults at two of these ranches; one ranch had very low dikes and sandy soil that may not have retained water long enough for completion of the aquatic development of P. confinais. Eighty sets of sweeps on ranches without livestock netted only 6l P. confinnis. for a gross density of 0.76 individuals per square meter. Forty-nine of these 6l individuals were caught on one ranch, while seven other ranches yielded a total of only 12 mosquitoes. No mosqui toes were netted on 12 of the non-livestock ranches. The P. confinnis-livestock relationship is undoubt edly a predator-prey relationship. Whitehead (195D has shown that cattle are the preferred host of P. confinnis. An immediate blood meal plus at least one more feeding ^3 later on is necessary for survival of the female until the time of egg laying (Horsfall, 1942). II. ADULT FLIGHT RANGE At 4:00 P.M. September 1, 1955, more than 2,000 newly hatched adult P. confinnis marked with Rhodamine-B dust were released on one of the ranches in the study area. The soil of the ranch was dry at the time of release so that egg-laying sites were not available there, but live stock were present as a food source for the female mosquitoes. From September 1 to September 16, the catches of nine light traps located in the valley were examined in a fashion suitable for detection of the marked adults (see methods section, page 24). The locations of the light traps are shown on the map of Figure 12, page 4i* Two traps were within the study area, 0.7 and 1.0 mile south west of the point of release of marked mosquitoes. These traps were emptied daily. The other traps were located in various directions at distances of 4.0, 5*5, 6.0, 8,0 and 12.5 miles from the release point. These traps were collected twice weekly. Five marked adults were recovered at one of the traps 0.7 mile to the southwest, one male and one female on the second day after release, one male and two females 44 on the third day. No other marked individuals were captured. The writer believes it is quite possible that the marked adults captured may have been carried to this site in his car. At the time of release the car was parked about 20 meters from the release point; after the release, the car was driven to the investigator's house where it was parked quite close to the trap at which all marked adults were recovered. These results do not confirm those of Horsfall ( 1942) who found an 8.8 mile flight range in Arkansan rice fields. The Coachella Valley populations may be more restricted, or 2,000 marked individuals may be insufficient to establish the flight range with the few traps used. III. ADULT HABITS Daytime Activity During the day adult P. confinnis cling to the stems and leaves of grasses, usually within 20 cm. of the ground. Their coloration blends into that of their surroundings. Adults photographed in nature are shown in Figure 13. Adults do not leave the grasses unless they are agitated by an animal moving through the area. Air movement or shadows seemed to have no effect in disturbing the mosqui toes. When disturbed, P. confinnis fly at a height usually 45 Figure 13. Male (upper) and female (lower) Psorophora confinnis photographed at rest on grass stems during the day time. 46 no greater than one meter from the ground and return quickly to the grass cover after covering a linear distance of three meters or less. Some mosquitoes may cling to the lower parts of the legs of the disturbing animal and a few may reach the upper parts of the body. While sweeping for adults over a linear distance of 10 meters, the writer would find up to 30 adults on his clothing, depending on the density of mosquitoes in the particular area. In seven counts a total of 33 females were distributed as follows: 30 below the knees and one each on the thigh, waist, and shoulder. This demonstrates the feasibility of working on the ranches during the day time with the upper part of the body uncovered, a common practice of the farm laborers. Night Activity P. confinnis is very active at night. Circular sweeping of the net after dark in August 1955 captured an average of 1.3 adults for each five minutes of sweeping. Quantitative changes in night activity were measured by frequent collections at two light traps located in the study area. One light trap was operated from July 20 to August 3, with collections being made at 7, 9 and 11 P.M. and 5 A.m . Data for this trap are summarized in Table VI. The second trap, operated August 29 to September 12, was emptied at hourly intervals from 6 P.M. to 1 A.M. and H > S EH CO < ! > à 1 — 1 c d m c\j ir \ + D s - iT N H n O e u C M m O c O S rnmiTNON curoON \o o- n û l>- ( D ir\ lr\ F « 4 O H • r4 g A C D r4 C M ooo o . mCNjVNj- j - ONH IT V H c d HrOoOCM C M C M O. 1 — 1 S r4 C D Q H c d O J-O NH O O C O r4 lf\ C X ) mvO m A H C M m rorO C M I>-n O H 0 \\0 C N < D H U n H H H m O 0 + > 1 — 1 O-lfNO C M roso C M O C M IfN C M C d r o 1 —I l —1 CM O o \ S H 0 • H cd iCNOrOO O N s O C M v O [>- IfNrH m te-rO C M lC N CMro^EN. U > . O CM O N A 0 1 — 1 Un O N O N m O 0 4-> 1 — 1 + C J N lT N rn O'O C ^ C M \0 \0 o \ o- U N iH H H m 1 rH C M 0 Cl tH i O O O O OOO O H C M H J* A 0 k H O 0 +> H M 0 O O O H OOO O O O O 1 — 1 ITN S A 0 A A 0 _ A O rH C M m vo c^co O O H m H ,£ î Q) C M C M C M C M C M C M C M C M m m cd o A •P P P 0 >» cü >» • O 0 o 0 Q f P s s t : — — z — — z ^ EH o ^3 Tj p g h) < t! 47 48 again at 5 A.M. Data for these collections are summarized in Table VII. Results show that both males and females are attracted to light, and that some active adults are present at all hours of the night. Activity usually does not begin until after sunset. For July 20 to August 3 the average sunset time is 7:01 P.M., and in 11 days of trapping during this period only 5 £• confinnis were captured in the 5 to 7 P.M. trapping interval. In ten days of trapping between August 29 and September 12, having an average sunset time of 6:20 P.M., only four Pj^ confinnis were captured in the 5 to 6 P.M. trapping interval. Activity reaches a maximum two to three hours after sunset and then decreases grad ually until dawn. Difference in the total catch of the two traps ( 2,257 vs. 6,836 individuals) reflects the different sizes of the population during the two trapping periods (see graph. Figure 8, page 30). Feeding Habits Observations were made on biting and feeding in the field and laboratory. A few minutes after emergence the newly hatched female shows a tendency to bite when placed on a human arm. The proboscis is moved toward the skin in TABLE VII NIGHT ACTIVITY OF ADULT PSQROPHORA CONFINNIS AS INDICATED BY LIGHT TRAP CATCHES AUGUST TO SEPTEMBER 12, 1955 (Sunset time ranging from 6:29 to 6:11 p.m.) 1 + 9 Date 5 -6 M pm F 6 -7 M pm F 7-1 M 3 pm F 8 -9 M pm F 9 -10 pm M F 10-11 pm M F 1 1-12 pm M F 12. M -1 am F 1-1 M 5am F Aug. 29 0 0 7 8 17 122 18 110 3 144 9 73 6 58 8 27 14 25 " 30 0 0 7 8 21 86 4 59 8 71 4 38 5 35 7 64 3 14 " 31 0 0 If 31 10 308 7 187 4 61 3 94 2 49 6 57 8 80 S ept. 2 0 0 183 108 210 295 79 296 24 76 10 57 3 27 5 33 13 53 3 0 1 + 105 66 244 246 45 118 28 136 20 76 21 63 10 24 12 57 " 1 + 0 0 35 72 60 91 11 85 22 85 3 25 1 27 2 12 9 50 " 6 0 0 20 70 25 79 19 70 6 79 5 43 6 27 2 25 3 63 " 7 0 0 22 86 19 114 1 68 1 43 5 47 1 34 4 21 17 74 8 0 0 19 77 12 52 9 37 6 19 5 42 4 34 0 17 4 44 " 12 0 0 If IfO 1 15 4 18 2 11 1 8 3 6 3 7 15 19 T o ta l 0 If If06 53^ 719 1208 197 1048 104 725 65 503 52 360 47 287 98 479 Average c a tc h / h o u r/ day H L 1 9^ 193 125 83 57 41 33 14 50 a manner similar to that of an older female. The proboscis may also be moved to several adjacent sites along the arm, but no puncture is accomplished, presumably due to the weakness of the body of the newly hatched mosquito. Of six females tested, only one was able to bite and engorge with blood within the first four hours after emergence. When retested fifteen hours after emergence, the female that had previously fed did not bite again; of the others, two bit vigorously soon after touching the arm and became fully engorged, another bit at six adjacent locations but did not become fully engorged, and the remaining two did not bite. When a female tries to feed, it may probe many locations before inserting the proboscis. The proboscis is inserted at different angles by different individuals, from very acute angles to right angles. Part or all of the proboscis is inserted. When the entire proboscis is in serted the maxillary palps can be seen bent to the outside. As blood is sucked into the body a watery fluid is discharged in small drops from the tip of the abdomen. This discharge takes place at about five second intervals. The female sucks blood for about five minutes at each feeding. Ten freshly engorged females weighed collectively 100 mg., indicating that roughly 8 mg. of blood is sucked 51 at one feeding. Fully engorged females are unable to fly and cannot cling to a surface if it is agitated slightly. Some idea of the size of engorged individuals is given in Figure 1%+. Reaction of the host to the bite varies. Nearly all persons interviewed reported that they suffered no impor tant reaction. The writer, however, showed an allergic response in which the bitten area enlarged in a matter of minutes to an irregular hard nodule one centimeter wide. This further enlarged to 5 centimeters after two to three hours, became reddish in color and finally softened. The nodule disappeared after a few hours, but the area some times remained itchy and irritated and discharged a watery fluid when pressed. Seven months later a general itchiness developed on the arms and to a lesser degree on shoulders, back, and face, and was diagnosed by a dermatologist as a secondary response to "insect bites." Egg-laying Habits Fgg-laying habits were studied by the methods described on page 22. Engorged females captured in the field were placed in the experimental cage and after four to five days the petri dishes within the cage were examined for eggs. Eggs were found mostly in small holes or clefts in the soil or behind small bits of vegetation. Some eggs 52 L I X ! A Figure It. Engorged adult female of P. con finnis . In lower picture legs have been removed to show the full abdomen. 53 were laid between the edge of the mud and the glass wall of the dish, and some eggs were laid in groups of 2 to 20; individual eggs were separate from each other, not glued together in "rafts" as in Gulex. The results, as summar ized in Table VIII, clearly show the preference for moist soil, and for moist soil with a covering of grasses. The preference for moist soil agrees with the observations of Carpenter and LaOasse (1955). Field results also indicated a preference for grassy lanes. In two adjacent lanes, one grassy and one bare of vegetation, larval densities were 20/dipper and less than l/dipper respectively (see Figure 16), IV, ADULT LONGEVITY Longevity was measured under laboratory and field conditions by confining newly hatched, unfed male and female P. confinnis in wire-mesh cages. Two experiments were carried out in the laboratory at air temperatures of 2lj_ to 29^0 and relative humidities of 29 to i j . 2 per cent. In one experiment involving 16 males and 9 females, all males and 7 females died within 21^ hours after emergence, while the other 2 females died within I j . 8 hours. In the second experiment involving 16 males and I j . females, all died within 2 I } . hours, except one female that 54 TABLE VIII RESULTS OF EXPERIMENTS TESTING PREFERENCE OF EGG-LAYING SITE Number of eggs laid on; Exper iment number Dish with bare moist soil Dish with bare dry soil Dish with water Dish with grassy moist soil 1 290 0 0 2 238 0 130 3 355 1 29 1+ 8 117 55 survived 72 hours. In two field experiments, adults were kept in cages placed on a table in the shade, or placed directly over the moist soil of a recently irrigated lane. Field conditions were: air temperature 20 to 27^0, relative humidity 34 to 39 per cent. All 93 individuals tested died within 2k hours. Of ten adults that were kept in a one-quart jar after emergence, 2 males and 3 females survived kQ hours, but all were dead after 120 hours. Feeding within the first 2^- hours is necessary for the survival of newly-emerged P. confinnis. The females tolerate starvation somewhat better than males. The short life span of the males undoubtedly accounts for their rapid disappearance after irrigation. V. THE AQUATIC POPULATION One Generation per Irrigation Because of the nature of the P. confinnis life cycle and the relatively short duration of standing irriga tion water, only one generation can develop following a particular irrigation. Flooding initiates the hatching of eggs that are present in the soil of a lane, these eggs having been laid in the soil while it was moist from the 56 previous irrigation. Since flooding of all parts of a lane is accomplished in a relatively short time, hatching and development of larvae will occur almost simultaneously in all parts of a particular lane. Only one stage will be dominant in a particular lane at a particular time. This was repeatedly observed in the field. For example, near the end of aquatic development many pupae would be present, some fourth-instar larvae would be found, but no earlier instars. Adults emerging from a particular lane could lay their eggs in the moist soil of the same lane, but develop ment of these eggs would be delayed until the next flooding of the soil. The density of aquatic P. confinnis in a particular lane varies with the changing area of standing water and the time after hatching. Density increases as water area decreases following hatching, and then density rapidly decreases as the adults emerge. Pupal densities in excess of 200 per dipper were observed in very small pools in rapidly drying lanes (see Figure 15). Table IX summarizes data on two lanes showing changes in density with water area and time. Types of Larval Habitats The writer visited nearly all the types of habitats in which Psorophora confinnis could possibly develop. This 57 m w m Figure 15. Extreme pupal density in a small pool of water left from irrigation. Pupae are the small round objects in the open are% at the end of the palm frond. 58 TABLE IX LARVAL-PUPAL DENSITIES IN RELATION TO WATER AREA AND TIME AFTER IRRIGATION# Days after Irrigation Water area, square meters Gross density## Predominant aquatic stage 2 350 5.2 (12) 1st,2nd instar 3 350 6.0 (12) k 350 6 .5 (12) l | . t h instar 5 234 7.3 (12) pupal 6 175 9.7 ( 7) pupal 7 0 k 350 2.0 (6) 1 4. th instar 5 350 3.8 (7) pupal 6 175 1.3 (7) pupal 7 120 0.1 (6) pupal 6 90 0.1 (8) pupal 9 0 ............ 4Upper values are for a lane studied August 10 to 15, 1955. Lower values are for the period July 12 to 17. ##Gro88 density is based on 6 dippings at each of several locations within a lane; the value in parentheses is the number of locations measured on a particular day. ##-:c-Three hundred fifty meters is the surface area of a fully flooded lane measuring 7 x 50 meters. 59 covered a wide range of situations from small spots of water resulting from water-faucet drips or drippings from evaporative coolers to large flooded pastures and flooded lanes of date ranches. Apparently date ranch lanes are exceptionally suitable; they were often the sole habitat containing P. Confinnis in the different ranches visited. Larvae were sometimes found in water patches formed by leaking irrigation pipes, and in roadside ditches where irrigation run-off water flows. Larvae were always found in the flooded areas of a local duck club, even in areas where there were very thick growths of tall plants. Larvae were found in the narrow ditches of a broom corn field, both in harvested and unharvested fields. These various habitats are illustrated in Figures 16 through 25. Special attention was given to irrigation stand pipes. Usually the open ends of these upright pipes are well above the surface of the lane into which their water flows; of 189 such pipes examined, 46 contained water, but none of these contained larvae. A lesser number of stand pipes had their upper end below the surface of the lane so that they formed a low spot in the lane, favorable for long persistence of water (see Figure 25). Forty-one such pipes were found, of which 21 contained water and 8 contained larvae or pupae with a gross density of 4.4/dipper (range 60 Figure 16. Flooded date ranch lanes. Such lanes provide the best conditions for develop ment of aquatic stages of P. confinnis. Higher densities were found in the grass-grown lane to the right than in the bare lane at the left. ' Figure 17. Good larval habitat. Date ranch lane covered with grass and prunings from date trees. 61 Figure 18. Exceptionally good P. confinnis habitat in grassy date ranch lane. Dipper is placed in area of highest density within lane. Figure 19. P. confinnis larvae were found abundantly in the small water patches in this grass-grown lane. Figure 20. Larval density was high along the grassy margin of this flooded pasture. Figure 21. High larval densities were found in the low spots retaining water in this flooded pasture. 63 1 Figure 22. P. confinnis larvae were found in the narrow irrigation furrows in this field of broom corn. Unharvested field above^ harvested field below. 6 ^ -*^T * ; > i u' ':i \ " t ‘i ’- V ' ' " ' I: %' Figure 23* P. confinnis larvae were consist ently found in this meadow of a duck club. Top picture shows an area of Sesbania (Ses- bania macrocarpsa) plants, lower shows a dense grass area. 65 # î''‘ S l - ' S Figure 24. P. confinnis larvae were found in the flooded furrows of this plowed field. Figure 25* Aquatic dipper handle propped up in opening of standpipe located below surface of lane. Larvae can develop in water per sisting in such pipes. 66 0.1 to 11.7/dipper). Apparently eggs are not laid in above ground standpipe water, or such eggs do not successfully develop, and belov;-surface standpipes are not important breeding sites. Microhabitats Important variations of larval-pupal densities were found within a particular water area according to the microhabitat conditions. Much higher densities were found in grassy areas than in adjacent areas without vegetation. This is at least partly related to the preference of the female in laying eggs on moist grass-grown soil. Pruned fronds of palm trees were sometimes left in the lanes (see Figure 17, page 60) and these also fostered higher dens ities, possibly because of shelter or food relationships. Table X compares densities in grassy and non-grassy areas of the same water patch; the effect of palm fronds is also shown. Larval densities are also higher near cow droppings. In two different water areas, dippings near cow droppings showed an average larval density of 39-5 per dipper, while dippings made at least one meter from the droppings showed an average density of 5.1 per dipper. In another study two small patches of water having an area of two square meters were found connecting to a 67 TABLE X A COMPARISON OF LARVAL DENSITIES IN: (1) GRASSY VERSUS NON-GRASSY PARTS OF THE SAME PATCH OF WATER, AND (2) WATER WITH AND WITHOUT PRUNED LEAVES OF DATE PALM Number of the study Density* in grassy area Density in non-grassy area 1 9.1 0.8 2 16.1 1.1 3 4.7 0.0 4 6.5 1.2 5 4.2 1.2 6 5.0 0.0 7 65.0 5.0 8 36.3 0.3 Gross density 18.3 1.2 Density among Density pruned date in open leaves water 9 2.1 0.3 10 14.3 1.3 Gross density 7*5 0.4 *Larval density measured by the average of 6 dippings, expressed as average number larvae per dip. 68 larger water area of 50 square meters. One small patch that contained a cow dropping had a larval density of 119*3 per dipping, while the other small patch had a density of 73.8 per dipping. This indicates that there is possibly an attraction of larvae to the manured area, probably because of the micro-organisms flourishing there and providing food to the larvae. There is no evidence that egg-laying females are attracted to manure. Existence of livestock in the breeding area of P. confinnis is not only a necessity for the adult mosquitoes but is also beneficial for the larvae. Relationship Between Larval Size and Density During field studies it was noticed that all fourth- instar larvae were not necessarily of the same size. "Giant" larvae were generally found in dippings from areas of low larval density. In a study specifically directed to this point, several of the largest fourth-instar larvae were selected from a series of dippings in different areas and measured. A celluloid millimeter ruler was used to measure the length of the body (from tip of head to end of siphon), the widest part of the thorax, and the width of abdomen. The results, as summarized in Table XI, demon strate the inverse relationship between size and density. M X g •H np 4 - > H 5 w C O C O • C \ J • On ^ [>-C^sO ® A • ••• •••• •O O 0 •OJ "rH rHrHiHrH y4 ^ :s -P -P a -O -P -H 0 W P IT\ 0 0 W OJ lf\ if\ M > d f H 0 OnOnONOO [ > « . 0 0 00 O- O cO^ HQ H P * M - « V h * 0 o a • O- • . W) 0 • fH • H 0 ÆÎ a # # # * • • A P O •<HI . r4 . r4 r —1 r4 0 T) > -H < > 0 W * 0 O * M X 0 ^ 0 • H -OO iTwO f - i P A • ••• •••• 0 *0 O •CvJ .«H rHrHrHi—1 > *H Æ < ^P 0 * bO IfS 0 0 P i>^ir\ c \ i j- C V J O M 0^ a CJ\00 G\CO i > . [ > . O- > 0 H A 0 rH »H bO 0 0 m hun 0 > \ P h ^ • u> j- irM>. u> c v i vo NÛ 0 0 ( Q r —t 0 < —1 0~ CO > iH 0 <; 10 44 O 0 0 0 P 4 0 k 0 > 0 HvOHoo sOJ-^ ro 0 P 4 W a 0 0 P H 0 Î 2 Î a P 4 * * 0 >> ^ * 0 44 0 * * a o p i - 4 * B p Hcvim^ u^sO[>.oo ^ w 69 a • H < 1 > ® U o Q > rÛ w 0 A o 0 P o 0 ü m 0 0 0 H H G • H 0 a • 0 G • M 0 0 0 A > P 4 0 A # % 0 p 0 G P 0 w O 0 a 0 . G a 0 A A * H r H A 0 0 H w H 0 0 a A 0 0 O G G 0 0 0 w G 0 0 0 X 0 0 0 0 O A A G P G O p w OT 0 0 P 0 0 0 0 a 0 W 0 r v P r G a G P o O 0 G 0 G G 0 b û 0 0 0 J G 0 A 0 0 0 P 0 H P w • 3 w 0 G 0 w 0 0 0 0 0 A 0 A 0 0 G 0 G a 1 5 c o > 0 0 A 0 P 0 0 0 < t j * 0 s ; 0 a * * * * * * * * * 70 Larval Behavior In the field larvae typically move back and forth between the surface of the water and the bottom, spending a short time at each place. This was measured in many in dividual cases, and these data are summarized in Table XII. In the first eight studies, on third and fourth-instar larvae, about twice as much time was spent in each visit to the bottom (29.4 seconds) as in visits to the surface (16.6 seconds). In the last four studies just the reverse is true— 24.4 seconds on the bottom versus 50.1 seconds on the surface. The only possible explanation that can be offered for this difference is that the larvae in the last four studies could have been preparing to pupate. It was observed in the laboratory that just before pupation, larvae would spend most of their time at the surface and exhibit less activity than previously. Water temperature and depth had no observable effect on time spent at either surface or bottom. The following types of behavior were observed in the laboratory when larvae were kept in petri dishes, as well as in the field: 1. Quiet resting at the surface. The larva remain suspended at an angle of about 45 degrees, with no movement of either body or mouth brushes TABLE XII ACTIVITY OF THIRD AND FOURTH-INSTAR LARVAE OF PSQROPHORA CONFINNIS 71 Number of study Average time spent on the bottom Average time spent near the surface* 1 2 5. 0** 1 5. 7** 2 28.2 27.2 3 48.0 12.6 4 30.8 17.3 5 1 7 .0 12.1 6 19.9 8 .5 7 33.5 1 9 .0 8 33.4 2 0 .8 Average 29.4 16.6 8 14.9 31.1 10 30.0 84.4 11 40.6 67 .4 12 12.0 18.0 Average 24.4 50.1 ^Average time was calculated from observations of 10 to 20 larvae, individually examined. **As measured in seconds. 72 (see Figure 26, a). 2. Grazing at the surface. The larva hangs in a 45-degree position, but moves about and feeds without wriggling. 3. Grazing parallel to the surface. The larva brings its head to the surface, bends its body slightly and moves forward or backward as it feeds (Figure 26, d). 4. 8-shaped position. A larva may sometimes bend its body into an S-shape while remaining near the surface. Defecation may be observed imme diately or soon after this bending, and there may be some relationship (Figure 26, e). 5. Diving. The larva may dive to the bottom by diving slowly at an angle of 30^, without wriggling (Figure 26, f), or by wriggling. 6. Grazing at the bottom. The larva grazes with its head near the mud and its body at a 45- degree angle, moving smoothly without wriggling (Figure 26, g). 7. Movement under the water surface. While a larva is grazing it may change its site, moving hor izontally by a few wriggles. 8. Ascending to the surface is accomplished by a 73 FIGURE 26 DRAWINGS OF LARVAL ACTIVITY Drawing a. Drawing b.. Drawing e. Drawing d. Drawing e. Drawing f. Drawing g. Larva resting quietly at the surface of the water. (This phase may precede pupation.) Start of skin shedding during pupation. The respiratory trumpets protrude first, the larva assuming a position parallel to the water surface. After shedding of the larval skin. The abdomen of the new pupa is bent anteriorly and assumes a "comma" shape. Parallel position taken by the larva. Dorsal view of larva in characteristic S-shape. Diving larva. Larva grazing at the bottom of the water. 7^ C e 3 Rquhc 16 75 continuous series of wrigglings, or by wrig gling s interrupted by one or more periods of rest. 9. Response to disturbance. The movements of an other larva or agitation of the water will cause a larva to wriggle quickly away toward the bottom. Effect of Temperature A number of larvae were placed in a refrigerator and kept at 9°C for 12-t hours. When removed, all larvae were lying horizontally either at the bottom or surface; all larvae recovered normal movements after three hours when the water around them had warmed to 27.5^C. Only 5 per cent of larvae tested survived 15 days of 9^0 temperatures. Larvae placed in k-0 to ^1°C water remained inactive at the surface. Pupae in the same water remained active and behaved normally in response to agitation of the water. Phototropism Five studies were made of the response to light at night, as described on page 25. The flashlight used illuminated a circle of water 30 cm. in diameter; dippings were taken at five minute intervals directly below the light and one meter from the light. Both larvae and pupae 76 responded positively to the light. Larvae swam toward and concentrated immediately below the light; activity in the lighted area seemed to be the same as observed in the day time. The dipping data summarized in Table XIII definitely show this positive phototropism. The effect of the flash light illumination was shown strikingly by additional dips made in connection with study 5 in Table XIII: two dips made below the light before it was turned on captured an average of 10 larvae per dip; eight dips below the light while it was lit captured an average of 173 larvae (range 73 to 223); two dips made two minutes after the light was turned off captured 12 larvae per dip. Water pH The pH of water containing P. confinnis larvae was frequently measured with Phydrion paper. PH ranged from 5*5 to 8.0, but generally was between 6.4- to 7.0. No relationships were found between the larval population and pH of water. Pupation and Adult Emergence Many fourth-instar larvae were observed as they changed into pupae. Previous to shedding its skin, the larva hangs down from the surface without moving. Body 77 TABLE XIII DATA ON ATTRACTION OF P. CONFINNIS LARVAE AND PUPAE TO LIGHT Number of study Sunset P.S.T. Time at which the study was made Larval-pupal density* under the light bulb Larval-pupal density one meter distant from the light 1 7:09 pm 8:20-8:4-5 pm 6.3 0.2 2 7:09 pm 9:00-9:30 pm 3.0 1.1 3 7:09 pm 8:30-9:00 pm 9.0 1.1 4 - 7:12 pm 9:00-9:4-5 pm 1.1 0.0 5 6:51 pm 7:4-5-8:30 pm 173 11.5 Gross average density 38.5 2.7 ♦Average densities were based on four dippings in study number 2; six dippings in study numbers 1, 3, and 4-; and on eight dippings in study number 5. 78 stretching movements then occur and are followed by strong peristaltic movements. The larval skin splits in mid- thorax; the pupal respiratory trumpets protrude and are raised to the surface as the larval body swings parallel with the surface. Twenty-nine to thirty peristaltic move ments, from the posterior end forward, gradually push the larval skin to the tip of the abdomen where it usually falls off immediately with the siphon. The pupa then bends its abdomen downward and anteriorly, assuming the typical comma-shape (Figure 26, c). The pupa may remain motionless for some time, or it may immediately start moving about. The larval skin sometimes remains attached until the strong wrigglings of the pupa dislodge it. The total time taken for ecdysis, from the appear ance of the pupal respiratory trumpets to the final shed ding of the skin, is about one minute. When the adult is ready to emerge, the pupa hangs just below the surface with its trumpets open to the surface, A straight slit opens between the respiratory tnmpets and the mesonotum of the adult appears first and pushes out from the pupal skin. In a series of strong outward pushes, followed by weak withdrawals, the head and proboscis and then the rest of the body emerge. The newly-emerged adult clings to a bit of grass or floating 79 debris until it is strong enough to fly. VI. PREDOMINANCE OF PSQROPHORA CONFINNIS IN TEMPORARY WATERS Usually only P. confinnis larvae and pupae were caught in dippings from standing irrigation water. Few individuals of other species were ever taken. Culex tar- salis was most frequently taken with Psoronhora confinnis. while Culex quinquefasciatus. C. pipiens. C. stigmatosoma and Aedes vexans were occasional associates of P. confinnis, The predominance of P. confinnis is undoubtedly due to its habit of laying eggs on moist soil subject to later irrigation, and also to its rapid aquatic development. Aedes vexans also lay eggs on moist soil that is exposed to reflooding. Culex species lay their eggs on water. The author is unable to explain satisfactorily why Aedes and Culex are not more abundant in the study area. P. confinnis is also predominant in flooded regions of the rice fields of Arkansas. Schwardt (1939) has reported that P. confinnis comprises 80 to 90 per cent of the mosquito fauna in that area. VII. RESISTANCE OF LARVAE AND PUPAE TO DESICCATION A fortunate field observation brought about the development of this subject. One of the C.V.M.A.D. 80 workers, in the course of spraying mosquito-breeding spots in the valley, omitted spraying several small patches of water left from irrigation, reasoning that the water would dry out before the aquatic cycle of the mosquito could be completed. The writer visited this area 24- hours after disappearance of surface water and found fourth-instar larvae and pupae in the moist mud. Active larvae and pupae were recovered by placing scrapings of the mud in a jar of water. Two sets of sweeps taken in the area captured 24- newly-hatched adults. This demonstrated the ability of P. confinnis to complete its development following the actual disappearance of irrigation water. This may be a frequent occurrence in disked lanes where surface water persists only two to three days. Laboratory experiments showed that the success of P. confinnis larvae in resisting desiccation and completing development depends upon their developmental age at the time the surface water disappears. Larvae and pupae were kept in the laboratory in covered petri dishes containing moistened filter paper. Air temperatures ranged from 24- to 310C. Third-instar larvae were not able to develop further when kept in petri dishes. Inability to feed is undoubt edly the critical factor. Of twenty-eight larvae kept in 81 one covered dish, II 4. survived l \ . 2 hours, but all were dead after 66 hours. Fifteen larvae accidentally left in each of four uncovered petri dishes were dead after 9 hours. Fourth-instar larvae have enough stored energy to enable them to complete pupation on a moist surface. Pupae, if kept moist, can complete their transformation to adults. Of 160 fourth-instar larvae placed in ten differ ent dishes, 1^0 pupated; from these pupae 70 adults suc cessfully emerged. Thus, Ij.3.7 per cent of the larvae were able to complete successfully their development. Pupation was complete within 10 to 20 hours; emergence of adults began within 32 hours and was complete in 1 { . 3 to \ \ S hours after the larvae were placed in the dishes. At refrigerator temperatures the survival of fourth- in star larvae on moist surfaces is possibly somewhat less than that of larvae kept in water. Of thirty larvae placed in dishes at 7°G, two survived 21 hours, while none sur vived for as long as 32 hours. Of ten larvae kept in water at 7°G, three survived 21 hours, one for 32 hours, none for 4-6 hours. Pupae showed the greatest success of development in the absence of surface water. This is to be expected since pupae, even in water, do not feed, but utilize stored energy for their transformation. From 4,02 pupae collected 82 in damp mud and placed, with the mud, in a petri dish, 365 adults emerged— a survival of 9O .8 per cent. This survival to the adult stage is about twice that for larvae kept in petri dishes. The longer the period of exposure to air, the lower the survival to the adult stage. CHAPTER V OBSERVATIONS ON THE EFFECTIVENESS OF TOXAPHENE AGAINST P. CONFINNIS In the course of field work the writer checked many patches of water after they had been sprayed with larvi- cide. Preliminary observations suggested that further study of the effectiveness of the spraying procedures of the C.V.M.A.D. would be worth while. The C.V.M.A.D. currently uses the larvicide Toxa- phene for all species of mosquitoes present in the valley. Three-quarters of a gallon of Toxaphene, dissolved in 50 gallons of water, is used per five acres. Application is by spraying from a jeep-borne sprayer. Experience has shown that Toxaphene in this concentration is highly effec* tive against first-, second-, and third-instar larvae of P. confinnis. Fourth-instar larvae tolerate the chemical rather well, while pupae show considerable resistance. This was confirmed by the present writer's experiments. 84 In three laboratory experiments larvae of various instars and pupae were collected from sprayed lanes, kept in water from these lanes, and observed for survival. Second and third-instar larvae taken from sprayed lanes invariably died about 22 hours after initial application of the spray. In one instance involving 30 fourth-instar larvae, 22 larvae survived to pupate and 17 adults emerged about 50 hours after spraying. In a second experiment 26 of 78 fourth-instar larvae pupated and 21 adults emerged within 50 hours. This indicates a survival of sprayed fourth-instar larvae ranging from 27 to 56 per cent. Twenty adults hatched from 23 pupae taken from sprayed water, a survival rate of 87 per cent; emergence occurred within 22 hours after pupae were taken into the laboratory. Measurements were made in the field of larval-pupal densities before and after spraying with Toxaphene. Two lanes having densities of 26 and 59 second- and third- instar larvae per dip one-half hour before spraying had densities of 0.0 and 0.7 larvae per dip 24 hours later. Another field study concerned the effectiveness of "prelarviciding," i.e., spraying the soil of the lanes within 24 hours before irrigation. Larval densities were measured in two lanes "prelarvicided" 12 hours before flooding and an adjacent untreated control lane. Gross 85 densities were determined on the basis of four dippings at each of 4 locations in each lane. On the second day after irrigation the prelarvicided lanes had densities of 7.4 and 74.5 larvae per dip, while the control lane had a dens ity of 10.3 larvae per dip. This indicates that prelarvi- ciding is probably ineffective. CHAPTER VI SUMMARY AND CONCLUSIONS The ecology of Psorophora confinnis in the Coachella Valley of Southern California was studied in the field in the summers of 1954 and 1955- It is believed this is the first ecological study of this species in this region. Results of these studies show that: 1. Good populations of P. confinnis develop only in irrigated areas of ranches that keep live stock as one aspect of their economy. These are principally date ranches. 2. Adult P. confinnis are present in numbers only from mid-June to the end of September when air temperatures and insolation are sufficient to warm irrigation water to 26^C or more. 3. Water temperatures in excess of 26^C are needed for successful establishment of an adult popula tion of P. confinnis. Above this temperature 87 P. confinnis can complete its aquatic stages within the average 5*1 days that irrigation water persists after flooding of date lanes. 4. Adults are strictly nocturnal. Flying begins shortly after sunset, reaches a peak about two to three hours after sunset, and then tapers off toward dawn. Both males and females are attracted to light. 5. Adult females are severe biters of man as well as of livestock, which makes them of economic importance as pests. Biting usually cannot be accomplished until about four hours after emer gence. Feeding within 24 hours of emergence is necessary for survival of the female. 6. Very few or no adults are found on irrigated ranches lacking livestock, indicating that live stock are the principal hosts of P. confinnis. 7. Female P. confinnis prefer to lay their eggs on damp or moist soil covered with grasses. Few or no eggs are laid on dry soil, bare moist soil, or water when these are offered as experimental choices. 8. Adult mosquitoes cling to grasses during the day. The greatest adult densities are found in 88 grasses over wet or damp soil. Adults fly only a few meters when disturbed, and rarely fly higher than a meter above the ground. They may cling persistently to an animal or man. 9. Mark-and-recapture studies failed to show any definite movement of marked adults from the area of their release. P.confinnis populations may be highly local. 10. P. confinnis larvae and pupae are restricted to temporary water areas because of the egg-laying habits of the adult. P. confinnis is by far the dominant mosquito in temporary waters. Aedes vexans and several species of Culex are in frequently found with P. confinnis. 11. There is only one generation of P. confinnis per irrigation. Development of all individuals within a particular flooded area is simultaneous, 12. Larval-pupal densities are higher in grass-grown parts of a flooded lane than in adjacent parts bare of vegetation. Larvae concentrate in the vicinity of cow droppings which may provide high concentrations of food organisms. 13. Larvae and pupae are attracted to light at night. 89 l i j . . The size attained by fourth-instar larvae tends to be inversely proportional to the larval density. l5* Fourth-instar larvae and pupae can successfully complete their development in absence of stand ing water, if kept on a moist surface. This allows completion of development in the field in moist muddy areas after irrigation water has disappeared from the surface of the soil. One such instance has been observed in the field. Third-instar larvae cannot develop further if kept on moist filter paper. 16. Toxaphene in concentrations of three-fourths gallon per 50 gallons of water per 5 acres will kill second- and third-instar P. confinnis, but is much less effective against fourth-instar larvae, and is largely ineffective against pupae. 17. Spraying the soil with Toxaphene before irriga tion will probably not prevent successful development of P. confinnis when the area is flooded. REFERENCES CITED REFERENCES CITED Aitken, T. H. G. 19i|-0. The genus Fsorophora in California (Diptera, Gulicidae). Rev. Ent., 11:67È-682. Ârribalzaga, Lynch P. I89I. Rev. Mus. La Plata, 2rllj.9. Bishop, P. E. 1933. Mosquitoes kill livestock. Science, 77:115-116. Carpenter, J. G. and ¥. J. LaCasse. 1955. Mosquitoes of North America. University of California Press, Berkeley. 360 pp. Coachella Valley County Water District. 1954. Map of Coachella Valley. Glimatological Data. California, Annual Summary, 194^. Climatological Data for the United States by Sections. W, S. Dept, of Commerce, Weather Bureau, 3p (l3) * 350-370. Hammon, ¥. M. and W. C. Reeves. 1943. Laboratory trans mission of St. Louis encephalitis virus by three generations of mosquitoes. Jour. Exp. Med., 78:241- Horsfall, W. R. 1942. Biology and Control of Mosquitoes in Rice Area. Arkansas Agri. Exp. Sta. Bull., 427: 1-46. Matheson, R. 1944- ^ Handbook of the Mosquitoes of North America. Comstock Pub. Assoc., Ithaca, New York. 341 pp. McParlane, N. L. and G. L. Winright. 1951. Desert Agriculture. Circular 176:1-14. Schwardt, H. H. 1939. Biologies of Arkansas rice field mosquitoes, Arkansas Agri. Exp. Sta, Bull., 377:1-22. 92 Shackleford, M. W. 1929• Animal communities of an Illinois prairie. Ecology. 10:126-154. Whitehead, F. E. 1951* Host preference of Psoroohora confinnis and P. discolor. Jour. Econ. Ent.. 4471919. îJfelverslty of Southern Calif oral*

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Core Title The ecology of Psorophora confinnis (the dark rice-field mosquito) in Coachella Valley, California

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