To feed or not to feed? How drought induced shifts in monoterpene concentration may promote successful attack by a bark beetle-fungus complex

Michael Friedman, Forest and Wildlife Ecology, University of Wisconsin–Madison, Madison, WI and Amy Trowbridge, Entomology, University of Wisconsin-Madison, Madison, WI

Background/Question/Methods
Rapid climate change has increased mortality risk for forest ecosystems worldwide. Specifically, drought predisposes trees to attacks by biotic agents, leading to mortality and disrupting regeneration and succession patterns. Ips confusus, or pinyon engraver, is one such biotic agent. This bark beetle and its associated microbial complex infest drought stressed pinyon pine (Pinus edulis) across the southwestern United States. One of its vectored blue stain fungi, Ophiostoma ips, has been proposed to facilitate beetle attack success through nutrient acquisition, metabolism of tree chemical defenses, and weakening the tree by xylem occlusion (and subsequent disfunction). Notably, this insect-fungus complex does not infest healthy trees, suggesting that a level of drought stress may be required to alter the constitutive and/or induced chemical defenses of pinyon pines, making them suitable hosts. However, if and how drought-induced shifts in pinyon pine defenses increase host suitability for this beetle-fungi complex is unknown. Here, we assessed the effects of monoterpenes, the most abundant chemical defenses employed by pinyon pines, on the growth of O. ips through a series of assays. First, we grew the fungal isolate on media amended with a combined mixture of monoterpenes that showed significant differences between ambient and droughted trees during a long-term drought experiment. We then conducted growth assays on media amended with individual monoterpenes to determine the isolated effects of each compound. Lastly, compounds with strong effects were used in a dose assay to assess the relationship between concentration and fungal growth.
Results/Conclusions
O. ips growth was significantly reduced on media amended with higher levels of total monoterpenes from ambient trees compared to lower total concentrations characteristic of drought-stressed trees. In subsequent tests of single compounds, ambient concentrations of 3-carene (835 µg/g) reduced fungal growth 33% compared to drought concentrations (95 µg/g), but growth did not differ between amendment treatments of (-) α-pinene, terpinolene, or bornyl acetate. Finally, we observed a negative relationship (r = -0.71, P < 0.001) between 3-carene concentration and fungal growth, where concentrations of 1.5- and 3-times ambient levels lead to further decreases in growth of 4% and 13%, respectively. Together, these results suggest that constitutive monoterpene concentrations characteristic of ambient trees, specifically 3-carene, may be a limiting factor to successful attack by this beetle-fungus complex. Future work aims to assess variation in monoterpene concentrations along a drought continuum and to test whether pinyon pines are able to induce a defense response against these biotic agents.