A SEASONAL SHIFT IN EGG-LAYING BEHAVIOR IN RESPONSE TO CUES OF FUTURECOMPETITION IN A TREEHOLE MOSQUITO

1. We tested a set of hypotheses to predict egg-laying behaviour in Ae des triseriatus in the field in Massachusetts. These hypotheses reflec t three main themes: females avoid competitive or predatory threats to their larvae; females judge larvae or eggs as cues to future permanen ce or productivity within a treehole; and females are either opportuni stic ovipositors, or hedge their bets by scattering their eggs in mult iple habitats. We predicted that mosquitoes might vary in their respon se depending on whether their offspring will hatch immediately or ente r diapause and hatch the next year. 2. We stocked artificial treeholes with larvae at four densities ranging from 0 to 90 larvae per habitat for four trials in 1992 and eight trials in 1994. In 1992, we added p redatory Anopheles barberi larvae to half the replicates of each densi ty level. In 1994, we tested for an interaction between egg and larval density by stocking additional traps with eggs in batches of 0, 20 an d 200, and larvae in densities as in the previous trials. 3. The popul ation revealed a significant interaction between time of year and larv al density. More eggs accumulated in traps stocked with fewer larvae ( 0 or 15) early in the season, but females showed a slight positive res ponse to higher densities (45 or 90) later on, after their offspring w ere all likely to enter diapause. These results suggest that potential ambiguities of larval cues,with indications of habitat permanence as well as adversity of future competition, are resolved by females. Fema les did not respond differentially to the predatory Anopheles barberi larvae. 4. The estimated batch size obtained in oviposition traps did not differ from the fecundity of field-caught females allowed to ovipo sit under confinement in laboratory vials, suggesting that bet-hedging by scattering of eggs among multiple habitats did not occur. 5. An un expected result, a positive response to higher egg densities, is discu ssed in light of studies on other insects. This result conflicts with our predictions based on females choosing less competitive environment s for their larvae and also conflicts with previously published accoun ts of avoidance of eggs in this species. Preference for habitats with conspecific eggs might be explained if eggs reflect habitat stability or if larvae gain an advantage from being in an aggregation. Neither h ypothesis is resolved by the present study.