There is growing evidence that some devastating biotic invasions are facilitated by microbial symbionts. The red turpentine beetle (RTB), an innocuous secondary insect attacking weakened trees in North America, has formed an invasive complex with the fungus Leptographium procerum in China, and this invasive beetle-fungus symbiotic complex is capable of attacking and killing healthy pines. A previous study demonstrated that three Chinese-resident fungi, newly acquired by RTB in China, induce high levels of a phenolic defensive chemical, naringenin, in pines and this invasive beetle-fungus complex is suppressed by elevated levels of naringenin while the beetle uses its gallery as an external detoxification system in which particular yeast-like fungi and bacterial species biodegrade naringenin. However, the functional roles of key microbial players in the symbiosis, contained within the microbiome of the bark beetle gallery, have not been well elucidated. In this report, the symbiotic naringenin-degrading microbiota were found to increase RTB survivorship in the presence of induced host defenses, and potential genes associated with degradation pathways were discovered. While fungi in the gallery microbiota had little involvement in naringenin degradation, bacterial community structure within the beetle gallery was highly correlated to naringenin degrading activity. Phylotypes of the Gram-negative bacterial genus Novosphingobium, which possessed genes involved in degradation pathways, were highly correlated to naringenin degradation activities and RTB associated with an isolated species of this genus acquired protection against naringenin and gained fitness. Our results demonstrated that symbiotic bacterial community of RTB galleries enhances the survivorship and overall fitness of invasive beetles by degrading the host phenolic naringenin, ultimately overcoming the tree defenses and facilitating the success of the invasive beetle-fungi complex. This dynamic interplay between the invasive insect pest and multipartite microbes suggests a putative mechanism in invasion ecology for mitigating biotic resistance to symbiotic invasion.

Insect-microbe interaction is a key area of research in multiplayer symbiosis, yet little is known about the role of microbe-microbe interactions in insect-microbe symbioses. The red turpentine beetle (RTB) has destroyed millions of healthy pines in China and forms context-dependent relationships with associated fungi. The adult-associated fungusLeptographium procerumhave played key roles in RTB colonization. However, common fungal associates (L. procerumandOphiostoma minus) with RTB larvae compete for carbohydrates. Here, we report that dominant bacteria associated with RTB larvae buffer the competition by inhibiting the growth and D-glucose consumption ofO. minus. However, they didn’t inhibit the growth ofL. procerumand forced this fungus to consume D-pinitol before consuming D-glucose, even though D-glucose was available and a better carbon source not only forL. procerumbut also for RTB larvae and associated bacteria. This suggests the most frequently isolated bacteria associated with RTB larvae could affect fungal growth and the sequence of carbohydrate consumption. Thus, this regulates carbohydrate allocation in the RTB larva-microbe community, which may in turn benefit RTB larvae development. We also discuss the mechanism of carbohydrate allocation in the RTB larva-microbe community and its potential contribution to the maintenance of a symbiotic community.

The exposure to plant defense chemicals has negative effects on insect feeding activity and modifies insect gut microbial community composition. Dendroctonus valens is a very destructive forest pest in China, and harbors a large diversity and abundance of gut microorganisms. Host pine defensive chemicals can protect the pines from attack by the holobiont. In this study, boring length of D. valens feeding on 0 mg/g α-pinene and 9 mg/g α-pinene concentration in phloem media for 6 and 48 h were recorded, and their gut bacterial communities were analyzed in parallel. Nine milligram per gram α-pinene concentration significantly inhibited boring length of D. valens and altered its gut microbial community structure after 6 h. The inhibition of boring length from 9 mg/g α-pinene in diets ceased after 48 h. No significant differences of the bacterial communities were observed between the beetles in 0 and 9 mg/g α-pinene concentration in phloem media after 48 h. Our results showed that the inhibition of the feeding behavior of D. valens and the disturbance to its gut bacterial communities in 9 mg/g α-pinene concentration in phloem media after 6 h were eliminated after 48 h. The resilience of gut bacterial community of D. valens may help the beetle catabolize pine defense chemical.