Microbial mutualism suppression by Trichoderma and Bacillus species for controlling the invasive ambrosia beetle Xylosandrus compactus

Xylosandrus compactus (Eichhoff) (Coleoptera: Curculionidae: Scolytinae) is among the most invasive ambrosia beetles causing severe damage to host plants belonging to various ecosystems. Foundress females excavate galleries into the plant xylem introducing mutualistic ambrosia fungi which are the exclusive food source for their offspring. The cryptic habits of this fungus-farming insect and its wide host range facilitated its rapid spreading worldwide. Despite several advances in deepening the bio-ecology of this beetle, there is still a general lack of information about effective control strategies. In this context, microbials, such as antagonistic bacteria and mycoparasitic fungi, can have the potential to act as alternative management tools. Here, we evaluated, for the first time, the potential of commercially available microbial agents for suppressing the growth of the main beetle mutualist, Ambrosiella xylebori, and consequently to hamper the beetle progeny development. The antagonistic activity of five Trichoderma and two Bacillus-based biofungicides was first evaluated in vitro by means of dual-culture experiments in Petri dishes. Then, beetle bioassays were conducted by exposing carob twigs, previously treated with different concentrations of each biofungicide, to beetle foundresses. Tested microbial antagonists significantly suppressed the growth of A. xylebori on Petri dishes. A significant reduction of the beetle progeny was observed in galleries of treated twigs compared to controls. The higher reduction in mutualist growth and brood production was obtained within galleries of T. atroviride treated twigs. Our results show the potential of the tested microbials to suppress the beetle mutualist growth within galleries and consequently to affect the development of its progeny.