Despite being the principal constituents of conifers, the role of monoterpenes as defensive compounds against defoliating insects remains controversial. Few studies have determined how herbivory alters foliar monoterpene levels in conifers, and less have asked how altered concentrations affect herbivores. Due to the volatility of these compounds, herbivory causes significant decreases in constitutive monoterpene pools. Greenhouse experiments using saplings of various pine species have shown induced monoterpene cyclase activity to replace the constitutive pool lost to volatilization, yet induction in pinyon pines has not been demonstrated. Furthermore, our field studies show mature herbivore-damaged pinyons to maintain lower total monoterpene levels throughout the growing season, suggesting a lack of induction. We assessed how individual and combined monoterpenes affect herbivore growth rate and immune response to parasitism by feeding tiger moths artificial diets with varying concentrations of α-pinene, β-pinene, β-myrcene, (-)-limonene, and mixtures of all four compounds that mimicked levels from undamaged and herbivore-damaged pinyons observed from field studies. Determining the importance of monoterpene quality, quantity, and synergistic interactions on energy trade-offs will aid in determining the importance of monoterpenes in pinyon defense as well as whether herbivory leads to induced susceptibility or resistance of herbivores to their natural enemies.
Results/Conclusions
Individual compounds had varying effects on tiger moth growth rate and immune response. While the differences in caterpillar growth rate and immune response were not significantly different between concentrations of β-pinene or (-)-limonene, there was a significant negative linear relationship among the means of each variable across concentrations (r (1) = -0.98, P<0.05; r (1) = -0.99, P<0.05, respectively). Both caterpillar relative growth rates and immune response decreased with increased concentrations of α-pinene, but this relationship was not significant. Caterpillars grown on “undamaged” diets exhibited a significant trade-off between growth and immune response (r (3) = -0.97, P<0.005), which may be primarily driven by (-)-limonene—the only compound in the diet that was significantly higher than “undamaged” diets. This relationship was not exhibited from caterpillars grown on “damaged” diets, yet overall, larvae grown on this diet demonstrated significantly higher melanization rates than those grown on “undamaged” diets. These data suggest (a) the importance of considering biologically active individual compounds in driving herbivore energy trade-offs when insects are exposed to multiple compounds, and (2) the possibility that lower foliar concentrations of monoterpenes resulting from herbivory (as opposed to higher concentrations from induction) may aid in a folivore’s resistance to parasitism.