Bacteria influence mountain pine beetle brood development through interactions with symbiotic and antagonistic fungi: implications for climate-driven host range expansion

Bark beetles are associated with diverse communities of symbionts. Although fungi have received significant attention, we know little about how bacteria, and in particular their interactions with fungi, affect bark beetle reproduction. We tested how interactions between four bacterial associates, two symbiotic fungi, and two opportunistic fungi affect performance of mountain pine beetles (Dendroctonus ponderosae) in host tissue. We compared beetle performance in phloem of its historical host, lodgepole pine (Pinus contorta), and its novel host recently accessed through warming climate, jack pine (Pinus banksiana). Overall, beetles produced more larvae, and established longer ovipositional and larval galleries in host tissue predominantly colonized by the symbiotic fungi, Grosmannia clavigera, or Ophiostoma montium than by the opportunistic colonizer Aspergillus and to a lesser extent, Trichoderma. This occurred in both historical and naïve hosts. Impacts of bacteria on beetle reproduction depended on particular fungus–bacterium combinations and host species. Some bacteria, e.g., Pseudomonas sp. D4–22 and Hy4T4 in P. contorta and Pseudomonas sp. Hy4T4 and Stenotrophomonas in P. banksiana, reduced antagonistic effects by Aspergillus and Trichoderma resulting in more larvae and longer ovipositional and larval galleries. These effects were not selective, as bacteria also reduced beneficial effects by symbionts in both host species. Interestingly, Bacillus enhanced antagonistic effects by Aspergillus in both hosts. These results demonstrate that bacteria influence brood development of bark beetles in host tissue. They also suggest that climate-driven range expansion of D. ponderosae through the boreal forest will not be significantly constrained by requirements of, or interactions among, its microbial associates.