At 20C air temperature, female Culex pipiens L. laid the greatest number of egg rafts at water temperatures between 20 and 25C. They laid very few at 15 or 35C even when given no alternative site. The possibility is discussed of manipulating temperature in a mosquito control programme

Belton, P.

Gillespie, B. I.

At 20C air temperature, female &lt;i&gt;Culex pipiens&lt;/i&gt; L. laid the greatest number of egg rafts at water temperatures between 20 and 25C. They laid very few at 15 or 35C even when given no alternative site. The possibility is discussed of manipulating temperature in a mosquito control programme

Journal of the Entomological Society of British Columbia

34 J. E NTOMOL. Soc. BRIT. COLUMBIA 77 (1980) , DEC. 31, 1980 OVIPOSITION OF CULEX PIPIENS IN WATER AT DIFFERENT TEMPERATURES B. I. GILLESPIE AND P. BELTON Department of Biological Sciences Simon Fraser University Burnaby, B.C., Canada V5A IS6 ABSTRACT At 20C air temperature, female Culex pipiens L. laid the greatest number of egg rafts at water temperatures between 20 and 25C. They laid very few at 15 or 35C even when given no alternative site. The possibility is discussed of manipulating temperature in a mosquito control programme. INTRODUCTION 1 t was once thought that mosquitoes sca t · tered their eggs indiscriminately but that in a given habitat , only certain species were able to survive. Field studies have failed to support this idea ; in fact, larvae in a given breeding site correspond to the eggs found there (Bates 1940). It is more likely that the restriction of given species to a certain type of larval habi tat results from selective oviposition (Wallis 1954). Cx. pipiens L ., from a recently colonised local population, was used in this study. The species was probably introduced into the Vancouver area early in this century. It now lays egg raft s in stagnant drainage ditches, sewage lagoons and a variety of temporary pool s and artificial containers and it can be a pest in late summer when it enters houses and bi tes . MATERIALS AND METHODS Eggs were obtained as needed from a colony of Cx .. pipiens maintained at Simon Fraser University since 1974. The colony was started fr om larvae collected at Richmond, B.C .. by Angerilli (1977 ) and reared continuously since, using McLintock 's method (1960). When adults were needed for experiments, egg rafts were taken from the main colonies and the larvae were reared in 2000ml of distilled water in an enamel tray , 40 x 25 x 4.5 cm deep. F inely·ground Tetramin and ground baker 's yeast was provided, using the qua ntities sug· gested by Gerberg et al. (1969). The pupae were placed in emergence cages 18 x 12 x 16cm high . Adults were provided wi th a 10% sucrose solution and t he RH was mainta ined at 60 to 65% by immersing a paper towel wick in a beaker of water. Oviposition experiments were conducted in a windowless plywood cage of 182 x 61 x 122 cm. The two oviposition sites were at opposit e ends of the cage. Each consisted of a bowl , 17 cm in diam and 8.5 cm deep, containing 1000 ml of distilled water, a heater with a thermos· tat, a thermometer, and a temperature probe leading to a chart recorder. The bowl rested on a cold water circulating chamber. With this arrangement we were able to set the tempera-ture of the water above or below that of the room and could regulate it within IC . For each experiment 200 to 300 adults of each sex were introduced into the plywood cage. A 100 ml Erlenmeyer flask in front of each ovipo-sition site contained 10% sucrose solution and was stoppered with cotton wicks. After 1 week a restrained, shaved guinea pig was placed in the cage on . 4 consecutive llights so that the female mosquitoes could obtain blood meals. Egg rafts were collected over the next 10 day s. Each raft was counted as an oviposition event but the eggs in the rafts were not individually counted as their number was found by Wallis (1954) to be independent of the suit-ability of the water at the breeding site. A set of experiments was run with both sites at constant water temperatures of 15, 20, 25, 30 and 35C, one experiment at each tempera-ture. In another set , duplicate tests were run with each of the following different pairs of temperatures, 15 & 20. 15 & 25, 20 & 25, 20 & 30 and 25 & 30. The data were changed to proportions of total rafts laid and were transformed using t he arc sine square root , before t he data were s ubjected to analysis of variance followed by a Student-Newman-Keuls mul tiple range tes t . RESULTS When the temperature of the water a t the t wo ovipos ition sites was the same, their posi-tion had no statis tically significant effect on the proportion of egg rafts laid . The total num-bers of egg rafts laid at the 5 temperatures were: Degrees C 15 20 25 30 35 Numbers 4 44 68 37 10 When the two oviposition sites were at dif-ferent temperatures there was a similar trend for more rafts to be laid in the middle range of temperatures. To allow for differences in the total numbers of rafts laid in the replicated experiments, proportions of the rafts found at J . ENTOMOL. Soc. BRIT. COLUMBIA 77 (1980), DEC. 31 , 1980 35 I II III r-~ -~ .75 -LL « .50 0: - -I- _~ --CJ .25 CJ -w r -, LL 15 20 20 25 20 0 -Z 0 .75 t-o: 0 0... .50 0 0: .25 0... 15 25 25 30 -TEMPERATURE C PAIRS Fig. 1. Mean proportion of egg raf ts laid at paired sites held at the tempera t ures indicated . Means and 90% confidence intervals (vertical lines at the top of each bar ) were retransformed from arc sine square roots. Statistical analysis indica tes three different responses: I where significantly more rafts were laid at the higher temperature, II where there was no signifi-cant difference in their numbers and III where significantly more were laid at the lower tempera ture. each of the two temperatures were tes ted statistically . The results are shown in the bar graphs of figure 1. Differences between the pairs labelled I and III are all significant. When the lower temperature was 15C, the hi gher temperature had the most egg rafts (fig. 1, I). When the temperatures were 20 and 25C the proportion of rafts laid at the two sites showed differences no greater than t hose found in the experiments with both sites at the same temperature (Fig. 1, II) . When the higher temperature was 30C, the lower temperatures had the most rafts (Fig. 1, III). DISCUSSION Very few egg rafts were found in water at 15 and 35C. Wallis (1954) stated that Culex mos -quitoes "do not oviposi t in water that is unacceptable to them" and fou nd no Cx. pip-iens rafts in water with a salinity greater than 0.5%. Bellamy and Corbet (1974) found resorp-tion of eggs by Cx. tarsalis in unfavourable conditions . The simplest explanation for our results was a difference in the probability of oviposi-tion at different temperatures . Rafts were sel-dom laid at 15 or 35C ; the females evidently 36 J. ENTOMOL. Soc. BRIT. COLUMBIA 77 (1980), DEC. 31, 1980 resorbed their eggs or died without laying. Most rafts were laid at the intermediate tem-peratures of 20 and 25C . Because these trends were similar, whether the mosquitoes had a choice or not , and because some rafts were always laid in the water having . a less favourable temperature, it seems unlikely that the temperatures of the two oviposition sites were actually compared. As the air temperature was maintained at 20 ± 1C, the mosquitoes may have been com-paring air and water temperatures, laying most rafts at water temperatures up to 5C above air temperature. These results, however, do not rule out the possibility that they have an inter-nal absolute thermometer to measure water temperature directly. Tests at different air temperatures would be needed to resolve this question . It has been suggested that artificial pools could be used to determine the presence and abundance of disease-carrying mosquitoes (Belton 1967, Smith & Jones 1972). Oviposi-tion surveys using small containers already of-fer an economical, rapid and sensitive method for determining the presence of adult Aedes aegypti in the field (Fay & Eliason 1966). Knowledge of the favourable range of water temperatures shown here, could be used to in-crease oviposition in artificial pools. Heated pools have potential for use in an integrated control programme, either as a sensitive sur-veying method or perhaps 'as traps in which large numbers of immature stages could be eliminated with little hazard to the environment. REFERENCES Angerilli, N. P . D. 1977. Some influences of aquatic plants on the development and survival of mosquito populations. Ph.D. Thesis. Simon Fraser University . Bates, M. 1940. Oviposition experiments with Anopheline mosquitoes. Ann.J .trop.Med. 20: 569-583. Belton, P. 1967. The effect of illumination and pool brightness on oviposition by Culex restuans (Theo . ) in the field. Mosq .News 27:66-68. Fay , R. W. and D. A. Eliason. 1966. A preferred oviposition site as a surveillance method for Aedes aegypti. Mosq.News 26:531-535. Gerberg, E. J ., T . M. Hopkins and J. W. Gentry . 1969. Mass rearing of Culex pipiens L. Mosq. News 29:382-385. McLintock, J . 1960. Simplified method for maintaining Culex pipiens Linnaeus in the laboratory (Diptera: Culicidae). Mosq.News 20:27-29. Smith, W . W. and D. W. Jones. 1972. Use of artificial pools for determining presence, abundance and oviposition preferences of Culex nigripalpus Theobald in the field . Mosq.News 32: 244-245. Wallis, R. C. 1954. A study of oviposition activity of mosquitoes. Ann .J .Hyg. 60: 135-168. 

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Oviposition of Culex pipiens in water at different temperatures

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Oviposition of Culex pipiens in water at different temperatures

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