Role of volatile and non-volatile plant secondary metabolites in host tree selection by Christmas beetles.

Individual Eucalyptus trees in south-eastern Australia vary considerably in susceptibility to herbivores. On the one hand, studies with insect herbivores have suggested that variation in the concentrations of foliar monoterpenes is related to variation in susceptibility. On the other, studies with marsupial folivores have suggested that variation in the concentrations of sideroxylonals (a group of formylated phloroglucinol compounds) is responsible for variation in susceptibility. We examined relative importance of sideroxylonals and 1,8-cineole (a dominant monoterpene) in host tree selection by Christmas beetles (Anoplognathus species: Coleoptera: Scarabaeidae) by using no-choice experiments, choice/no-choice experiments, and manipulative experiments in which concentrations of sideroxylonals or 1,8-cineole were altered. We used two species of host Eucalyptus, one species of non-host Eucalyptus, and three species of non-host non-Eucalyptus trees. Leaf consumption by Christmas beetles was negatively correlated with the concentrations of sideroxylonals and 1,8-cineole. Artificial increases in the concentration of sideroxylonals or 1,8-cineole reduced leaf consumption by Christmas beetles. An artificial reduction in foliar monoterpenes had no effect on leaf consumption by the beetles when leaves contained high or very low concentrations of sideroxylonals. However, when the concentration of sideroxylonals was moderate, a reduction in the foliar monoterpenes increased leaf consumption by the beetles. Therefore, monoterpenes such as 1,8-cineole may be used as a negative cue by Christmas beetles. The pattern of food consumption on non-host Eucalyptus species and non-host non-Eucalyptus species suggest that both positive and negative cues may be used by Christmas beetles to select host trees.

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.

Effects of different leaf traits on growth rates of insect herbivores on willows.

We examined relative effects of traits of leaf quality of ten willow species (Salix: Salicaceae) on growth rates of five species of insect herbivores found in interior Alaska (a willow sawfly, Nematus calais; the tiger swallowtail butterfly, Papilio canadensis; and three species of chrysomelid beetles, Gonioctena occidentalis, Calligrapha verrucosa, and Chrysomela falsa). Leaf traits examined were water content, toughness, total nitrogen contnet, pubescence, and presence or absence of phenolic glycosides. Of ten Salix species, four species contain phenolic glycosides in their leaves. We examined relative effects of water content, toughness, and nitrogen content of the Salix leaves on larval growth rates at three different levels, i.e., on a single host species, between different host species, and between herbivore species. The within-host analyses showed that effects of water content, toughness and/or nitrogen content on herbivore growth rates were generally significant in early-season herbivores but not in late-season herbivores. For each herbivore species, differences in growth rates between hosts were not explained by differences in water content, toughness, or nitrogen content. The between-herbivore analysis showed that the interspecific difference in larval growth rates were related to difference in water and nitrogen content of the hosts. Pubescence of Salix leaves had little effects on herbivore growth rates. Presence of phenolic glycosides had a positive effects on growth rates of a specialist, N. calais, but no effect on the other specialist, Ch. falsa. Presence of phenolic glycosides had, in general, negative effects on growth rates of nonspecialists, G. occidentalis, C. verrucosa, and P. canadensis.