Causes and consequences of host expansion by Mnesampela privata

The autumn gum moth, Mnesampela privata, utilizes several species of Eucalyptus planted outside regions of endemism within Australia. We investigated whether foliar monoterpene composition influenced oviposition in the field on the natural primary host (E. globulus) and a novel host (E. rubida), both characterized by nonstructural epicuticular waxes. In the laboratory, oviposition preferences of females for species and families of known host, novel hosts, and non-hosts that were characterized by both nonstructural and structural waxes but also varied in foliar concentrations of the purportedly toxic plant secondary metabolite (sideroxylonal) were studied. Although M. privata laid as many eggs on trees of two families of E. rubida as they did on trees of two families of E. globulus, there were significant differences in the numbers of clutches of eggs laid. When combined with data for oviposition on another five families of E. globulus, we found a negative relationship between mean numbers of eggs and foliar concentration of alpha-pinene but a positive relationship between egg numbers and the concentration of alpha-terpineol. The field data suggest that female M. privata are just as willing to lay eggs on novel hosts with comparable foliar monoterpene compositions to those of the primary host, especially if they produce nonstructural epicuticular waxes. Oviposition assays in the laboratory endorse this mechanism of host plant hierarchy and support the long-held assumption of the host primacy of E. globulus. In laboratory assays, some larvae pupated on all hosts (except Corymbia eximia) but the number completing larval development was greater on hosts with softer leaves. Larval survival was also reduced on hosts with high concentrations of sideroxylonal but only if those hosts also had modest to high concentrations of monoterpenes. Larval survival was high on a host (E. macarthurii) with a high concentration of sideroxylonal but with virtually zero monoterpene content. This suggests that the monoterpene content of a host could antagonize the effect on M. privata larvae of its sideroxylonal content. The larval food plant most affected the fitness of female rather than male pupae. Of the known host expansion events, all have occurred in mixed species plantations. The co-occurrence in these plantations of either the primary host or other highly ranked species probably explains the eventual expansion onto the neighboring species of Eucalyptus and Corymbia.