A seasonal shift in egg-laying behaviour in response to cues of future competition in a treehole mosquito

1. We tested a set of hypotheses to predict egg-laying behaviour in Aedes tr.isevintus in the field in Massachusetts. These hypotheses reflect three main themes: females avoid competitive or predatory threats to their larvae; feinales judge larvae or eggs as cues to future perinaileilce or productivity within a treehole; and females are either opportunistic ovipositors, or hedge their bets by scattering their eggs in multiple habitats. We predicted that ~nosquitoes might vary in their response depeilding on whether their offspring will hatch immediately or enter diapause and hatch the next year. 2. We stocked artificial treeholes with larvae at four densities ranging froin 0 to 90 larvae per habitat for four trials in 1992 and eight trials in 1994. In 1992, we added predatory Anopheles barbevi larvae to half the replicates of each density level. 111 1994, we tested for an interactio~l between egg and larval density by stocking additional traps with eggs in batches of 0, 20 and 200, and larvae in densities as in the previous trials. 3. The population revealed a significant interaction between time of year and larval density. More eggs accumulated in traps stocked with fewer larvae (0 or 15) early in the season, but fenlales showed a slight positive response to higher densities (45 or 90) later on, after their offspring were 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. Fenlales did not respoild 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 fe~nales allowed to oviposit under confinenlent in laboratory vials, suggesting that bet-hedging by scattering of eggs anlong multiple habitats did not occur. 5. An unexpected result, a positive response to higher egg densities, is discussed in light of studies on other insects. This result conflicts with our predictions based 011 fe~nales choosing less competitive environn~ents for their larvae and also co~lflicts with previously published accounts of avoidance of eggs in this species. Preference for habitats with conspecific eggs inight be explained if eggs reflect habitat stability or if larvae gain an advantage f ro~n being in an aggregation. Neither hypothesis is resolved by the present study.