Adaptation of pests and their natural enemies to Eucalyptus, an introduced wood crop

Introduced host plant species may be attacked by endemic herbivorous insects, but it is unlikely that these insects will possess the right adaptations to thrive on their new host. Natural selection will favour genotypes coding for the right adaptations, but their effect will strongly depend on the connectedness of the populations developing on the new host with those on the original host. Adaptations to a new host are expected to evolve especially when performance on the new host goes at the expense of performance on the original host. Such a trade-off will reduce gene flow between populations, but little experimental evidence has been found for the existence of such trade-offs. We suggest an alternative mechanism that may limit gene flow, i.e. the absence of natural enemies on the new host plant. Such an absence will lead to relaxation of the selection pressure on traits associated with the avoidance or reduction of predation, resulting in a decrease of defensive capacity of the herbivores. Upon returning to the native host, herbivorous insects may face increased predation risks relative to the original population, and this could well reduce gene flow between populations on novel and native hosts. We aim to study these mechanisms in insects attacking Eucalyptus forests in Brazil. Since its introduction, this tree is attacked by a variety of herbivores from native Brazilian trees, whereas their enemies have remained ineffective in controlling them on Eucalyptus. Studying the adaptation of herbivores and their natural enemies to this novel host will yield insights into the effect of natural enemies on gene flow between populations on novel and native hosts and may contribute to the development of sustainable forest protection.