Within the last decade, pinyon-juniper woodlands throughout the southwestern U.S. have suffered large-scale mortality and shifts in ecosystem structure due to drought and associated insect outbreaks, with pinyon pine being the most sensitive species in the community. Plant secondary chemistry contributes to fluctuations in the population densities of herbivores, either by directly affecting defoliators through toxicity, or indirectly, by influencing an insect’s susceptibility or resistance to parasitism due to physiological costs of detoxification. Monoterpenes are the principal constituents of pinyon resin and the dominant biogenic volatile organic compound emitted by conifers; however, their defensive role against defoliating insects remains unclear. This study focuses on a specialist herbivore, the tiger moth (Lophocampa ingens: Arctiidae), which feeds on pinyon pine (Pinus edulis: Pinaceae) and whose population sizes are largely controlled by the parasitoid wasp Meteorus euschausiae: Braconidae. Because parasitoids determine densities of lepidopteran larvae causing major pinyon pine mortality, it is critical to determine (a) how environmental factors influence herbivore-induced monoterpene concentrations and (b) the effects of the resulting altered plant chemistry on the herbivore immune response to parasitism to gain a realistic perspective of current and predicted parasitism within a natural pinyon-herbivore-parasitoid system.
Results/Conclusions
Air and needle samples were collected from pinyons at two sites on either side of the Continental Divide that were experiencing tiger moth outbreaks. Results show that early season herbivory significantly increases total monoterpene emissions at both sites. Only on the Eastern Slope (with a pronounced late summer monsoon season) did previously damaged trees exhibit significantly large total monoterpene emission rates later in the season, suggesting a coupling of biotic and abiotic influences. Furthermore, a manipulative drought field experiment confirmed that monoterpene emissions do show a significant positive correlation with precipitation. This corroborates our hypothesis that environmental factors, particularly drought, can have strong interactive effects with herbivory on monoterpene emissions in natural systems. These effects are also seen in leaf tissue, as foliar monoterpene concentrations are significantly higher in undamaged trees throughout the growing season. Preliminary analyses demonstrate that higher concentrations of monoterpenes in the tiger moth diet negatively affect larval growth, which may subsequently affect larval immunocompetency, and that emissions resulting from caterpillar feeding influence parasitoid host-location. Thus, understanding the variables that affect monoterpene concentrations and their effects on tiger moth survival will allow us to better predict patterns of parasitism and recognize the determinants of insect population outbreaks in pinyon-juniper woodlands of the southwestern U.S.