Responses of egg-laying female mosquitoes to the density of
larvae (J. S. Edgerly, M. McFarland, P. Morgan, T. Livdahl,
1998)
We tested a set of hypotheses, based on previous ecological
studies, to predict egg-laying behaviour in
Ochlerotatus triseriatus in the field in
Massachusetts. These hypotheses were in three main
categories: females avoid immediate competitive or
predatory threats to their larvae; females judge larvae or
eggs as cues to future permanence, productivity, or
competition within a treehole; or females hedge their bets
by scattering their eggs in multiple habitats. We also
predicted that mosquitoes might vary in their response
depending on whether their offspring will hatch immediately
or enter diapause and hatch the next year. 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 predatory
Anopheles barberi larvae to half the replicates of
each density level. In 1994, we tested for an interaction
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.
The population showed a significant interaction between
time of year and larval density (top left). Eggs
accumulated more in oviposition traps stocked with lower
densities of larvae early in the season, but females showed
a preference for higher-density traps later on, after their
offspring were all likely to enter egg diapause (top right,
right). This may be the first report of a seasonal shift in
egg-laying decisions in an insect; the results suggest that
potential ambiguities of larval cues, with indications of
habitat permanence as well as adversity of future
competition, are resolved by females. Females did not
respond differentially to the presence or absence of
Anopheles barberi larvae. The estimated batch size
obtained in oviposition traps did not differ from the
fecundity of field-caught females allowed to oviposit in
laboratory vials (Below, left), suggesting that bet-hedging
by scattering of eggs among multiple habitats did not
occur. An unexpected positive influence of egg density was
found, (Below, right), in conflict with our predictions
based on females choosing less competitive environments for
their larvae and contrary to previously published accounts
of avoidance of eggs in this species.