Functional investigation of monoterpenes for improved understanding of the relationship between hosts and bark beetles
Jiaxing Fang, Sufang Zhang, Fu Liu, Bin Cheng, Zhen Zhang, Qing‐He Zhang, Xiangbo Kong
Abstract
Abstract Spruce bark beetle ( Ips typographus L.) is the most destructive insect pest of spruce forests in Eurasia. However, contact toxicity, in vivo metabolism, and ecological functions of host monoterpenes are poorly understood at the spruce tree–bark beetle–predator tritrophic level. Spruce monoterpenes including S ‐(–)‐α‐pinene, R ‐(+)‐α‐pinene, and myrcene showed contact toxicity to I. typographus , with LD 50 values ranging from 22–32 μg/mg. When topically treated with S ‐(–)‐α‐pinene or R ‐(+)‐α‐pinene, the amounts of volatile metabolites, including 4 S ‐(–)‐ cis ‐verbenol, 4 S ‐(+)‐/4 R ‐(–)‐ trans ‐verbenol, R ‐(+)‐/ S ‐(–)‐verbenone and 1 R ‐(–)‐/1 S ‐(+)‐myrtenol, in the hindgut extracts of I. typographus varied significantly between sexes, and their quality (enantiomeric composition) varied significantly with the chirality of α‐pinene. More importantly, S ‐(–)‐α‐pinene induced male adults to produce large amounts of 4 S ‐(–)‐ cis ‐verbenol and S ‐(–)‐verbenone. When topically treated with myrcene, the expected semiochemicals such as E ‐myrcenol, ipsenol and ipsdienol were not detected in the beetle hindguts, indicating that the pheromone biosynthetic system of I. typographus does not participate in the metabolism of host myrcene. In trap tests, S ‐(–)‐α‐pinene and R ‐(+)‐α‐pinene increased the catches of I. typographus and its predator Thanasimus substriatus in pheromone‐baited traps, whereas myrcene exhibited a strong repellent (or inhibitory) effect on I. typographus but not on its predator. I. typographus seems to adopt different ecological strategies (e.g. avoidance to myrcene and preference for α‐pinene) to adapt to and tolerate different host monoterpenes. Extensive investigation of these monoterpenes will help us understand their roles in manipulating the arms race between host trees and bark beetles, and potentially improve the efficacy of controlling I. typographus via the push‐pull strategy using host kairomones.