Ecology of mosquito larvae in the Winnipeg area and evaluation of insecticides for future use in mosquito control

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Dixon, Robert Donald
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A survey of mosquito developmental sites around Winnipeg during 1967 and 1968, showed that mosquitoes use less than thirty percent of the available water for development and the amount of water present on the land for mosquito development is in part, determined by the length of time between rainfalls. The distribution of rainfall over the city largely determined mosquito abundance in any particular region. The time between rainfalls determined whether a new generation could be produced. Aedes vexans populations increased in size throughout the summer, provided precipitation patterns were favourable. Pools were used repeatedly by the same species, but most pools were shared by several species of mosquitoes. Adult surveys by means of a light trap, indicated that mosquito species may vary in their response to light traps. Response to a light trap also depends upon the physiological state of the mosquito and atmospheric conditions. In regard to all species trapped, the adult population levels inside the city were smaller than those outside the city in both controlled and non-controlled areas. The ratio of inside/ outside populations in the non-controlled area was higher than in the controlled area. 0n a monthly basis, or on a whole season basis, light trap population figures can be meaningful. However, on a nightly or even a weekly basis, it was found that light traps mainly monitor adult activity. Weekly or bi-weekly peaks or lows do not necessarily indicate a change in the adult population, only activity. Natural repopulation of areas treated with insecticides did not indicate when insecticides were biologically inactive. Experimental plots were designed which enabled the determination of biological breakdown of an insecticide to the exact day. Residue analysis for DDT in the soil of treated areas (approximately 10 years at 1 lb./acre/year) within Winnipeg showed an average of 3.32 lbs. per acre. This amount will undoubtedly increase if the use of DDT is continued in the Winnipeg area. Tests with fuel oil and flit revealed that fuel oil would produce almost identical results to those of flit against larvae. Pupae were not tested...