Ambrosia beetle response to ethanol concentration and host tree species

Ethanol emitted by stressed trees is an olfactory cue used by ambrosia beetles (Coleoptera; Curculionidae; Scolytinae) to locate susceptible hosts to colonize. In addition, ethanol enhances the growth of ambrosia beetle fungal symbionts, improving colonization. Whether host selection and colonization are affected also by the amount of ethanol produced by stressed trees and by tree species is unclear. To investigate these mechanisms, we induced attacks by ambrosia beetles in bolts of eight tree species by coring and filling them with either 5% or 90% ethanol solutions in water. For each ethanol concentration, bolts of the eight different tree species were replicated six times in a randomized complete block design. Entry holes were used as a proxy for host selection whereas gallery development stage was used as a proxy for colonization. Ethanol concentration differentially affected host selection of the three ambrosia beetles that were active during this study. Anisandrus dispar Fabricius preferentially attacked bolts with 90% ethanol concentration, Xylosandrus crassiusculus (Motschulsky) preferentially attacked bolts with 5% ethanol concentration, and Xyleborinus saxesenii (Ratzeburg) attacked bolts irrespective of ethanol concentration. Colonization of X. crassiusculus reflected the same pattern observed for entry holes. The effect of host tree species on host selection was most prominent for X. saxesenii, while X. crassiusculus established a higher number of developed galleries in Ostrya carpinifolia Scopoli bolts than on five of the other tested tree species. Our results suggest that ethanol concentration and host tree species may influence ecological niche partitioning among ambrosia beetle species.