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The effect of Plagiorchis elegans cercariae on the vertical distribution of second, third, and fourth instar larvae of Aedes aegypti in the water column was studied in the laboratory. Under daytime conditions and in the absence of the parasite, larvae divided their time between the top and bottom of the water column. At night larvae spent the majority of their time at rest at the top of the column. In the presence of P. elegans cercariae larvae spent more time at rest at the surface under daytime conditions but increased their active time at the bottom under nighttime conditions. The presence of cercariae elicited escape and grooming responses and tended to reduce and fragment feeding activity.

The effects of food availability and the presence of the parasite, Plagiorchis elegans, on the probability of Aedes aegypti preimagos to transform from one immature stage to the next over consecutive 24- or 48-hour periods of time were determined. Under conditions of low food availability, 24- and 48-hr transition probabilities of all larval instars, to the next were reduced. Increases in food availability reversed this process, causing significant decreases in same-stage transition probabilities. However, as transition to successive stages increased among first, second, third and fourth instars, there was concomitant significant increase in first and fourth preimago mortality. Addition of Plagiorchis elegans cercariae caused significant decreases in the 24 and 48-hour same-state probabilities for all pre-imago stages. Successive stage transitions for first, second and third instars were significantly increased while those of fourth instars and pupae were significantly decreased following exposure to the parasites. The mortality of all preimago stages significantly increased with exposure to parasite, but was highest among fourth instars and pupae. There were strong food-by-parasite interaction effects among first, second and third instars. Differences in transition probabilities increased with increasing food levels between controlled and parasite-exposed groups. The opposite was true for fourth instars and pupae. While there was no significant interactive effect between food and the presence of the parasite on pupae regardless of food concentration, this was not true for mortality probabilities among fourth instars. Significantly more fourth instars died in response to parasite exposure and there were significant interactive effects of food and parasites on the probability of fourth instars transformation to pupae over 48 hrs.

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.

The effects of food availability and selective control of late instar larvae of Aedes aegypti on preimago population structure, mortality, adult abundance, sex ratios, and size of females were examined. Food availability i an important factor in determining preimago population structure and mortality. At optimal food availability more adults emerged, females were larger and in higher proportions than at suboptimal food availability. The effects of selective control on preimago population structure and post treatment larval mortality differed with food availability. At suboptimal food availability the type of control applied had a significant effect. Simulated pathogen-induced mortality resulted in a shift of the population to late instars and temporarily reduced density dependent mortality. The findings suggest that although both pathogens and predators reduce intraspecific competition by directly killing competitors, pathogens also provide an influx of food. Compensatory mortality may result from a reduction of intraspecific competition caused by biocontrol agents.

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The impact of a primary infection with the incompatible digenean Plagiorchis elegans to groups composed of young, juvenile and adult Biomphalaria glabrata and a challenge infection with the compatible parasite Schistosoma mansoni was examined. Egg production of young B. glabrata was significantly reduced by P. elegans at the higher exposures of 16 eggs per snail, while egg production of adult B. glabrata was reduced at exposures of 8 and 16 eggs per snail. Egg production of juvenile B. glabrata snails was not significantly affected by any of the levels of P. elegans exposures. The survivorship of juvenile B. glabrata snails was significantly lowered by an exposure to 16 P. elegans eggs per snail. The survivorship of adult snails was significantly reduced by exposures to both 8 and 16 P. elegans eggs per snail. Interestingly enough, the survivorship of young B. glabrata snails was not significantly affected by P. elegans exposure. Snails harboring small P. elegans infections acquired a resistance to Schistosoma mansoni infection. Infection with P. elegans did not have any significant effect on the egg production of the young and adult sympatric snails, Helisoma trivolvis trivolvis. The survivorship of adult H. trivolvis trivolvis was not significantly affected by exposure to P. elegans. This being said, young H. t. trivolvis that were exposed to P. elegans had a higher survivorship than did control snails. Exposed Bulinus truncatus snails laid up to 50% fewer eggs for the first five weeks due to P. elegans infection. There was however no effect on the survivorship of the snails.