The effect of an environmental change on mobile butterfly larvae and the nutritional quality of their hosts

1. Herbivores that forage on resources that change over time may be strongly affected by environmental factors that alter their temporal overlap with host plants. The magnitude of these effects may be mediated by the availability of alternative hosts and by behavioural adaptations for foraging on temporal food resources. 2. This study examines the temporal interaction of a butterfly species and its two host plants to determine how larvae utilize their host resources and are affected by conditions of accelerated host senescence. By changing host use over time, this butterfly may track temporal declines in host quality and buffer the impacts of environmental variation and change. At the same time, it is hypothesized that host declines and changes in the host environment affect larval survivorship and hence butterfly population size. 3. With three sets of field and greenhouse experiments, the following were examined: (i) larval host plant use and the dependence of larval diet on oviposition, (ii) nutritional differences between hosts, and (iii) the impact of conditions that accelerate host plant death (i. e. temperature) on larval survivorship and growth. 4. Larvae were observed to forage widely, vary their diet through time, and use hosts independently of their natal plant. Larvae tracked changes in host quality by steadily increasing their use of the longer-lasting, but nutritionally variable, host plant. Temperature conditions that accelerated host death actually conferred a survivorship advantage when larvae were able to utilize this host. 5. These results suggest that larval diet choice and movement may be an adaptive strategy for foraging on declining food resources. It also suggests that the net effect of environmental extremes on larvae is strongly mediated by host plant use. Differences in larval survivorship when one or both hosts are available suggest that the long-lasting host is essential to butterfly population persistence. By comparing this mobile taxa to species with fewer host-switching opportunities, we better understand the diversity of foraging strategies of food-limited insects and the effects of environmental change.