Gut Bacteria of Wood-Boring Streltzoviella insularis and Their Involvement in Pinoresinol Degradation of Ash Tree (Fraxinus sp.) Host

Insect gut microbiota have been widely reported to help the insects to overcome host tree defense. Streltzoviella insularis (Lepidoptera: Cossidae) is one of the most common wood borers in China, attacking various hosts, including ash trees (Fraxinus sp.), but little is known about its gut microbial associates and their involvement in host tree defense. We isolated gut bacteria of S. insularis larvae, analyzed their ability to degrade pinoresinol (a defense compound of ash trees) and cellulose, and identified pinoresinol degradation products. Larval mortality increased with increasing pinoresinol concentration (reflecting natural variation observed in the host trees). All the five detected gut bacteria isolates were able to degrade pinoresinol, two of which were also capable of cellulose degradation. Furthermore, gut bacteria were also shown to degrade pinoresinol via the gluconeogenesis pathway. These results suggest that S. insularis-associated microorganisms help to overcome host pinoresinol defense and possibly contribute to insects or gut microbial nutrition via carbohydrate synthesis.