Overlapping oviposition and chemical defense of eggs in two co-occurring species of ladybird predators of aphids

Oviposition behavior in two co-occurring species of predatory ladybirds of aphids were investigated. The two species of beetles often shared the same aphid colonies in bean plants in space and time and showed a similar oviposition pattern in response to aphid abundance. Overlapping oviposition presents potential risks of interspecific predation of eggs in unstable aphid colonies. Eggs in clusters by two ladybird species, Menochilus sexmaculatus and Coccinella transversalis, were defended from reciprocal predation and the defense was found to be chemical. Eggs occurring singly were found to be highly vulnerable to predation. Strength of chemical defense significantly increased from single eggs to eggs in mixed clusters and eggs in pure clusters. Results are not only consistent with the hypothesis that eggs in clusters are better protected from natural enemies but also extend the ambit of the hypothesis that coexisting ladybird predators have evolved to avoid the risks of heterospecific predation of eggs in a patch. Received: September 11, 2000 / Accepted: November 13, 2000