The effects of food availability on the population structure of Aedes aegypti in the presence and absence of Plagiorchis elegans cercariae were studied in the laboratory. Under conditions of low food availability and in the absence of the parasite, larval population levels cycled at 40 day intervals. Cycles were initiated by the periodic build-up and decline of fourth instar larvae. These major competitors for food resources appear to be responsible for the release of a cascade of events which allowed the intermittent flow through of larvae from the first to the third instar. Under both optimal and suboptimal conditions of food availability, the introduction of high densities of P. elegans cercariae resulted in a decrease in the production of adult mosquitoes. Although the parasite reduced competition by preferentially removing fourth instar larvae and hastened the development of surviving instars, such survivors were exposed to the effects of the parasite as soon as they reached the fourth instar.

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