Ubiquitous volatile compound facilitates efficient host location by a non-native ambrosia beetle

Xylosandrus germanus (Blandford), a species native to Asia but currently invading North American and European forests, exploits living, but weakened trees. In response to many sources of stress, trees emit ethanol, which represents an important host-location cue for X. germanus. Because stressed trees can be spatially and temporally variable over a landscape, we assessed the role of olfaction in aiding X. germanus to efficiently locate vulnerable trees during natural dispersal. We conducted a series of experiments and observed that attacks occurred on trees baited with ethanol, but immediately ceased upon removal of the ethanol cue. X. germanus also efficiently located and attacked ethanol-injected trees, but rarely landed on adjacent trees not emitting ethanol, and never attacked these neighboring trees. A spatial analysis of trees attacked by ambrosia beetles within diverse landscapes revealed that only certain host species or cultivars, and only certain individuals within these host species or cultivars, were attacked; ethanol was also detected in the attacked trees, but not in non-attacked trees. Thus, X. germanus uses an efficient olfactory mechanism while orienting among perceived non-hosts to specifically locate trees associated with ethanol. Combined with other attributes, we propose that the remarkable efficiency by which this non-native ambrosia beetle uses volatile cues to locate specific vulnerable hosts across a diverse landscape aids its successful establishment and population spread.