Employing evolutionary theory to improve biological pest control: Causes of non-adaptive sex allocation behavior in parasitoid wasps and implications

Models based on sex allocation theory predict that when the fitness gains from larger size differ between male and female offspring, mothers should produce the sex that will offer the greatest investment return. Behavioral studies on parasitoid wasps have confirmed predictions of models, which additionally have practical implications because of their relevance in biological control. We investigated how a parasitoid attacking a scale insect matches theoretical model predictions in a 2-year field study. As predicted by Charnov's host quality model, mothers laid female eggs in hosts above a threshold size. This threshold was absolute, i.e. independent of the host size distribution, independently of the sampling site and date. Further laboratory assays confirmed field results for at least one parasitoid generation and, moreover, excluded the possibility that the observed behavior was a consequence of immature mortality. By comparing the characteristics of our system with others, we hypothesize that this short-term absolute threshold might be favored in polyphagous parasitoids that attack multivoltine hosts. We propose three measures to mitigate the negative implications of this sex allocation behavior in classical and augmentative biological control programs.