Lingering effects of a hard childhood: larval competition and adult immunity in Aedes mosquitoes

For insects, stress on immatures produces effects on adults. Physiological debilitation may result when larvae suffer food shortage due to competition for limited resources, and resulting adults may have impaired immune function. Container-dwelling Aedes are an excellent system for investigating cross-stage effects. Detritus input to containers determines nutrient content and, with density, determines per capita food availability. We focus on effects of density stress and associated food shortage on adult immunity and susceptibility to parasites. Larval competition prolongs development, increases larval mortality, and reduces adult size, fecundity, and longevity in Aedes mosquitoes. Because competitively stressed Aedes may be more susceptible than non-stressed individuals to dengue virus infection and dissemination, I postulated that larval competition also renders stressed individuals more susceptible to vector-borne metazoan parasites via inhibition of pathogen-induced immune responses. When adult Aedes aegypti from larvae raised at multiple experimental densities were challenged with mixed bacterial inoculation, Cecropin A (a broad-spectrum antimicrobial and antiparasitic peptide) transcription was greater in density-stressed than in non-stressed individuals (P<0.006), suggesting a compensatory immune response in stressed females. A similar response was absent for transcription of the immune peptide Defensin A. Using A. aegypti (Liverpool) and Brugia pahangi as a model system, we tested whether different larval densities yield adults differing in susceptibility to B. pahangi infection. We found no effect of larval density stress on intensity of B. pahangi infection of adults. These preliminary results suggest that effects of competition among larvae on immunity and infection may be complex.