Forest trees face threats from many insect pest species, underscoring the importance of understanding their defense mechanisms for survival. In a North American conifer species Picea glauca, white spruce, a defense-related gene, βglu1, is responsible for releasing phenolic compounds (acetophenones) to defend against its insect defoliator, Choristoneura fumiferana, the eastern spruce budworm. βglu1 is also expressed in a Eurasian conifer species Picea abies, Norway spruce, although no major insect defoliator is present within the species’ natural range. We compared range-wide variation of βglu1 transcript levels from foliage samples of P. glauca in North America and P. abies in Europe using RT-qPCR and targeted transcriptome sequencing. βglu1 transcript levels were highly correlated between the two methods, with wide ranges of variation being detected within and between populations in both species. We found a significant longitudinal gradient in βglu1 transcript levels in P. glauca, with one βglu1 gene form being differentially expressed across populations, but not in P. abies. The expression level differences in P. glauca are consistent with the historically higher C. fumiferana outbreak frequency and severity in eastern compared to western populations, with C. fumiferana defoliation severity being a significant explanatory variable for βglu1 transcript levels. Climate per se was not a significant explanatory factor in either species. Overall, these results enhance our understanding of potential adaptive variation in acetophenone defenses in P. glauca, while the factors influencing βglu1 transcript variation in P. abies require further investigation.

How carbohydrate reserves change in conifers during drought and bark beetle attacks are poorly understood. We investigated changes in carbohydrate reserves and carbon-dependent terpene defenses in ponderosa pine trees 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 trenched-beetle attacked trees depleted carbohydrates and died within the first year of bark beetle attacks. While live trees contained higher carbohydrates than dying trees, amounts of constitutive and induced terpenes produced did not vary between live and beetle-attacked dying trees, respectively. Based on these results we propose that reallocation of carbohydrates to terpenes 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 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 beetle symbionts as nutrients.