Responses of a native beetle to novel exotic plant species with varying invasion history

1. The impact of plant invaders on the fitness of native insects has received increasing attention, but it remains unclear how native insects that have a taxonomic conservatism in host-plant use respond to novel hosts. In this study, an experimental approach was taken to this issue by comparing the preference and performance of a native beetle, Cassida piperata Hope, on native hosts Chenopodium album and Alternanthera sessilis, and non-coevolved exotic hosts Alternanthera spinosus and Alternanthera philoxeroides of varying invasion history with choice and cross-rearing experiments. In host choice experiments, adult beetles preferred to oviposit on the older invader A. spinosus to the same degree as it did the native hosts, but generally avoided the newer invader A. philoxeroides. However, in rearing experiments, larval beetles developed more slowly on the two exotic hosts than on the native hosts. The varying responses of adult beetles to invaders might be explained by their differing invasive history, and suggest that the beetle has adapted to the older invader behaviourally. However, the slower development of the beetle on the two invaders suggests that the beetle has failed to adapt physiologically to either species of invasive plant. These results offer insights into the temporal dynamics of a native insect adapting to plant invaders, and suggest that when testing the impact of exotic plant invasion on native insect fitness, it is necessary to consider the duration of novel association between the insect and the novel plant species.