Egg Limitation in Parasitoids: A Review of the Evidence and a Case Study

Abstract

Life history theory predicts that a nontrivial proportion of parasitoids should exhaust their egg supply during their lifetime. We reviewed the literature on egg limitation in parasitoids and found partial support for this prediction. Most of the evidence in support of egg limitation is indirect and does not constitute absolute proof of egg limitation. However, a few direct studies in which field-collected parasitoids were dissected, gave unequivocal evidence of egg limitation. Egg limitation was detected both in proovigenic species, in which it signals the attainment of maximum fecundity, and in synovigenic species, where it is more likely to be temporary than permanent. We demonstrated substantial egg limitation in the synovigenic parasitoid Aphytis aonidiae. Although an unknown (and probably large) proportion of egg-limited individuals was apparently unable to mature eggs for unknown reasons, the egg load distribution indicated that even those individuals able to mature eggs could be at risk of becoming egg limited. Thus, egg limitation in this species can apparently occur through egg depletion or an inability to mature eggs. We investigated possible correlations between time of day, time of year, ambient temperature, parasitoid size, and host density on egg limitation in A. aonidiae. We found a slight tendency for egg load to drop during the course of the day. Although we could detect no effect of time of year or of temperature on egg load, larger parasitoids had higher egg loads than did smaller ones. Finally, we could detect no effect of host density on egg load. This result is paradoxical given recent theory predicting increased egg limitation with increased host availability. We suggest that already-documented state-dependent behavior in A. aonidiae may partly explain this unexpected result. Parasitoids may reduce the risk of egg limitation when the host encounter rate is high by exhibiting increasingly selective host-use patterns with declining egg load.