The invasion of a novel aphid species typically occurs in three phases. The initial epidemic phase, in which large populations cause widespread economic damage to host plants, is followed by an attenuation phase, in which aphid populations decline as natural enemies respond and become increasingly effective in exploiting the novel resource. Finally, the endemic phase is reached, in which the aphid remains present seasonally in low numbers, but populations rarely reach economic levels requiring control; geographic range may contract. However, the community of arthropods associated with the affected crop may be changed significantly as consequence of these events, with some changes transitory and others permanent. Just as herbivores may occasionally undergo shifts to new host plants, so predators and parasitoids may alter their associations with particular crop plants in response to encounters with novel, abundant prey species. It has long been known that arthropod predators and parasitoids can express intrinsic attraction to specific plant species, independent of the presence of prey, and usually mediated by olfaction. However, we understand very little about the heritability of such plant preferences, or how quickly they can evolve de novo. In this presentation, I review some historical data, and observations from the recent invasion of sugarcane aphid, Melanaphis sacchari, that converge in support of the rapid evolution of novel crop plant affinities by aphid predators following their response to an aphid invasion. I will argue that large populations of aphids associated with the epidemic stage of the invasion generate large populations of natural enemies, and that these large numbers alone create novel evolutionary opportunities for both the aphid and its most effective natural enemies. This is because the rate of evolution is limited by effective population size, which governs not only the range of standing variation in the population, but also the rate at which novel mutations can occur. Nevertheless, the speed with which novel crop affinities can appear in natural enemy populations seems to implicate epigenetic modes of inheritance of novel plant responses rather than changes in gene frequencies.
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