Recent mountain pine beetle outbreaks in western North America killed millions of lodgepole pine trees, leaving few survivors. However, the mechanism underlying the ability of trees to survive bark beetle outbreaks is unknown, but likely involve phytochemicals such as monoterpenes and fatty acids that can drive beetle aggregation and colonization on their hosts. Thus, we conducted a field survey of beetle-resistant lodgepole pine (Pinus contorta) trees to retrospectively deduce whether these phytochemicals underlie their survival by comparing their chemistry to that of non-attacked trees in the same stands. We also compared beetle attack characteristics between resistant and beetle-killed trees. Beetle-killed trees had more beetle attacks and longer ovipositional galleries than resistant trees, which also lacked the larval establishment found in beetle-killed trees. Resistant trees contained high amounts of toxic and attraction-inhibitive compounds and low amounts of pheromone-precursor and synergist compounds. During beetle host aggregation and colonization, these compounds likely served three critical roles in tree survival. First, low amounts of pheromone-precursor (α-pinene) and synergist (mycrene, terpinolene) compounds reduced or prevented beetles from attracting conspecifics to residual trees. Second, high amounts of 4-allyanisole further inhibited beetle attraction to its pheromone. Finally, high amounts of toxic limonene, 3-carene, 4-allyanisole, α-linolenic acid, and linoleic acid inhibited beetle gallery establishment and oviposition. We conclude that the variation of chemotypic expression of local plant populations can have profound ecological consequences including survival during insect outbreaks.

How carbohydrate reserves in conifers respond to drought and bark beetle attacks are poorly understood. We investigated changes in carbohydrate reserves and carbon‐dependent diterpene defences in ponderosa pine trees that were experimentally subjected to two levels of drought stress (via root trenching) and two types of biotic challenge treatments (pheromone‐induced bark beetle attacks or inoculations with crushed beetles that include beetle‐associated fungi) for two consecutive years. Our results showed that trenching did not influence carbohydrates, whereas both biotic challenges reduced amounts of starch and sugars of trees. However, only the combined trenched‐bark beetle attacked trees depleted carbohydrates and died during the first year of attacks. While live trees contained higher carbohydrates than dying trees, amounts of constitutive and induced diterpenes produced did not vary between live and beetle‐attacked dying trees, respectively. Based on these results we propose that reallocation of carbohydrates to diterpenes during the early stages of beetle attacks is limited in drought‐stricken trees, and that the combination of biotic and abiotic stress leads to tree death. The process of tree death is subsequently aggravated by beetle girdling of phloem, occlusion of vascular tissue by bark beetle‐vectored fungi, and potential exploitation of host carbohydrates by bark beetle symbionts as nutrients.

Fatty acids are major components of plant lipids and can affect growth and development of insect herbivores. Despite a large literature examining the roles of fatty acids in conifers, relatively few studies have tested the effects of fatty acids on insect herbivores and their microbial symbionts. Particularly, whether fatty acids can affect the suitability of conifers for insect herbivores has never been studied before. Thus, we evaluated if composition of fatty acids impede or facilitate colonization of jack pine (Pinus banksiana) by the invasive mountain pine beetle (Dendroctonus ponderosae) and its symbiotic fungus (Grosmannia clavigera). This is the first study to examine the effects of tree fatty acids on any bark beetle species and its symbiotic fungus. In a novel bioassay, we found that plant tissues (hosts and non-host) amended with synthetic fatty acids at concentrations representative of jack pine were compatible with beetle larvae. Likewise, G. clavigera grew in media amended with lipid fractions or synthetic fatty acids at concentrations present in jack pine. In contrast, fatty acids and lipid composition of a non-host were not suitable for the beetle larvae or the fungus. Apparently, concentrations of individual, rather than total, fatty acids determined the suitability of jack pine. Furthermore, sampling of host and non-host tree species across Canada demonstrated that the composition of jack pine fatty acids was similar to the different populations of beetle’s historical hosts. These results demonstrate that fatty acids composition compatible with insect herbivores and their microbial symbionts can be important factor defining host suitability to invasive insects.

Chemical induction can drive tree susceptibility to and host range expansions of attacking insects and fungi. Recently, mountain pine beetle (Dendroctonus ponderosae Hopkins; MPB) has expanded its host range from its historic host lodgepole pine (Pinus contorta var. latifolia Douglas ex Loudon) to jack pine (Pinus banksiana Lamb) in western Canada. Beetle success in jack pine forests likely depends upon the suitability of tree chemistry to MPB and its symbiotic phytopathogenic fungi. In particular, how rapid induced defenses of jack pine affect MPB colonization and the beetle's symbionts is unknown. In the field, we characterized and compared differences in rapid induced phloem monoterpenes between lodgepole and jack pines in response to various densities of Grosmannia clavigera (Robinson-Jeffery and Davidson)-a MPB symbiotic fungus used to simulate beetle attack-inoculations. Overall, lodgepole pine had higher limonene and myrcene, but lower α-pinene, concentrations than jack pine. However, myrcene concentrations in jack pine increased with inoculation density, while that in lodgepole pine did not respond to density treatments. We compared the growth and reproduction of MPB's symbiotic fungi, G. clavigera, Ophiostoma montium (Rumford) von Arx and Leptographium longiclavatum Lee, Kim and Breuil, grown on media amended with myrcene, α-pinene and limonene at concentrations reflecting two induction levels from each pine species. Myrcene and α-pinene amendments inhibited the growth but stimulated the reproduction of G. clavigera, whereas limonene stimulated its growth while inhibiting its reproduction. However, the growth and reproduction of the other fungi were generally stimulated by monoterpene amendments. Overall, our results suggest that jack pine rapid induction could promote MPB aggregation due to high levels of α-pinene (pheromone precursor), a positive feedback of myrcene (pheromone synergist) and low levels of limonene (resistance). Jack pine is likely as susceptible to MPB-vectored fungi as lodgepole pine, indicating that jack pine induction will likely not adversely affect symbiont activities enough to inhibit the invasion of MPB into jack pine forests.

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