Volatiles of fungal cultivars act as cues for host-selection in the fungus-farming ambrosia beetle Xylosandrus germanus

Abstract

Many wood-boring insects use aggregation pheromones during mass colonization of host trees. Bark beetles (Curculionidae: Scolytinae) are a model system, but much less is known about the role of semiochemicals during host selection by ambrosia beetles. As an ecological clade within the bark beetles, ambrosia beetles are obligately dependent on fungal mutualists for their sole source of nutrition. Mass colonization of trees growing in horticultural settings by exotic ambrosia beetles can occur, but aggregation cues have remained enigmatic. To elucidate this mechanism, we first characterized the fungal associates of the exotic, mass-aggregating ambrosia beetleXylosandrus germanusin Southern Germany. Still-air olfactometer bioassays documented the attraction ofX. germanusto its primary nutritional mutualistAmbrosiella grosmanniaeand to a lesser extent another common fungal isolate (Acremoniumsp.). During two-choice bioassays,X. germanuswas preferentially attracted to branch sections (i.e., bolts) that were either pre-colonized by conspecifics or pre-inoculated withA. grosmanniae. Subsequent analyses identified microbial volatile organic compounds (MVOCs) that could potentially function as aggregation pheromones forX. germanus. To our knowledge, this is the first evidence for fungal volatiles as attractive cues during host selection byX. germanus. Adaptive benefits of responding to fungal cues associated with an infestation of conspecifics could be a function of locating a suitable substrate for cultivating fungal symbionts and/or increasing the likelihood of mating opportunities with the flightless males. However, this requires solutions for evolutionary conflict arising due to potential mixing of vertically transmitted and horizontally acquired symbiont strains, which are discussed.