Plants produce a mixture of volatile organic compounds (VOCs) in response to herbivore feeding damage. These volatiles may serve as a defense mechanism by attracting parasitoids of the insects that feed upon them, and act as feeding/oviposition deterrents for herbivores. The majority of the VOCs released after herbivore damage in conifers are made up of monoterpenes, which have been shown to play an intrinsic role in plant defense. However, in addition to biotic stress, changes in abiotic factors can lead to variable concentrations of defense compounds, and can have strong effects on monoterpene emission rates. Due to the complex effects of both abiotic and biotic factors on plant physiology, our understanding of these combined controls over monoterpene production and emission is lacking. In this study, we examined the feedbacks between herbivore feeding, seasonal perturbations, and plant defense strategies as well as the effects these conditions may have in mediating herbivore/parasitoid interactions. From June to September 2008, a pinyon pine (Pinus edulis)-tiger moth (Lophocampa ingens)-parasitic wasp (Meteorus euschausiae) tritrophic system in southern Colorado was investigated. Volatile and foliar monoterpene concentrations were measured from herbivore damaged and undamaged pines (n=8) using a dynamic headspace sampling technique. In addition, C:N ratios, photosynthesis rates, as well as other related environmental variables were collected.
Results/Conclusions
Under drought stress during June and July, we observed a reduction in total monoterpene emission rates for both damaged and undamaged trees. However, with the onset of late summer monsoon rains in August, an increase in emission rates in both types of trees was seen. In addition, a significant difference observed in emissions between treatments, with damaged trees exhibiting higher amounts of total monoterpenes. α-pinene dominated the emission profiles in each month; however, emission rates were only significantly different between treatments during the month of August. Additionally, damaged pines were observed to have a significantly lower C:N ratio than those not exposed to herbivore damage. Furthermore, the differences in total monoterpene emissions, particularly α-pinene, observed between treatment groups in August, coincide with the time at which L. ingens is searching for oviposition sites. Our results are consistent with a significant interaction between drought stress and tiger moth herbivory, which uniquely affects the emission of specific types of monoterpenes and conveys useful information to the moths and parasitoids when searching for suitable hosts.
