Invasive bark beetle-associated microbes degrade a host defensive monoterpene

Conifers respond to herbivore attack with defensive chemicals, which are toxic to both insects and their associated microorganisms. Microorganisms associated with insects have been widely reported to metabolize toxic chemicals, which may help both microorganisms and host insects overcome host conifer defense. Dendroctonus valens LeConte, an introduced exotic pest from North America to China, has killed millions of healthy pines. Alpha-pinene is the most abundant defensive monoterpene in Chinese Pinus tabuliformis. Although microorganisms associated with D. valens have already been investigated, little is known about their bioactivities when encountering host defensive monoterpenes. In this study, we evaluated the influences of different concentrations of -pinene to D. valens and the three most frequently isolated yeasts and bacteria of D. valens, and further assayed microorganisms' capabilities to degrade -pinene. Results showed that the gallery lengths and body weight changes of bark beetles were significantly affected by 6 mg/mL and 12 mg/mL of -pinene applied in media compared to controls. The tolerance of experimental microorganisms to -pinene varied depending on the microbial species. Two out of three yeast strains and all three bacterial strains degraded 20%-50% of -pinene compared to controls in 24 h in vitro. The microorganisms capable of -pinene degradation in vitro and their tolerance to high levels of -pinene suggested that D. valens-associated microorganisms may help both microorganisms and the bark beetle overcome host -pinene defense.