Evidence for a trade-off between host-range breadth and host-use efficiency in aphid parasitoids

"The jack of all trades is a master of none" describes the widely held belief that engaging in many tasks comes at the cost of being unable to do those tasks well. However, empirical evidence for generalist fitness costs remains scarce. We used published data from a long-term field survey of aphid parasitoids to determine if relative specialists are more abundant than generalists on their shared hosts, a pattern that would be expected if generalists suffer a tradeoff between host-range breadth and host-use efficiency. Relative specialists were more abundant than generalists on their shared hosts, but only when we used a measure of specialization that accounts for the taxonomic differences among parasitoids' hosts. These results suggest that a generalist-specialist tradeoff exists within this group of parasitoids, and that the generalist fitness cost depends on the taxonomic breadth, rather than the number, of host species that are used.