Costs and benefits of prey specialization in a generalist insect predator

1. There are very few studies on prey specialization in predatory insects. Of the prey that the larvae of the generalist ladybird beetle Adalia bipunctata regularly feed on in the field. some are more suitable as food than others. A laboratory experiment was undertaken to determine whether it is possible to select for improved performance of this insect predator on a 'poor quality' prey, and the cost, if any. of such specialization. 2. The ladybird performed better when reared on the pea aphid Acyrthosiphon pisum than on the black bean aphid Aphis fabae. Over the course of six generations of selection there was a significant increase in performance on both species of aphid, especially the black bean aphid. 3. Ladybirds previously selected for five generations for better performance on the black bean aphid performed significantly worse when reared on pea aphid compared with those reared continuously for six generations on pea aphids, and vice versa. That is. specialization on one species of aphid resulted in a poorer performance on another. 4. If, as reported here, the specialization on one kind of prey generally entails a trade-off in performance on another, then the interactions between insect predators and their prey are more homologous to those of herbivorous insects and plants than previously thought. The significance of this for prey specialization in ladybird beetles is discussed.