Importance of leaf volatiles in the host selection behavior of the redbay ambrosia beetle Xyleborus glabratus

We have investigated the chemical ecology of the redbay ambrosia beetle Xyleborus glabratus. Xyleborus glabratus, is an invasive wood boring beetle that has become established in the southeastern United States and transmits a fungus, Raffaelea lauricola, that causes lethal laurel wilt. We developed a novel laboratory bioassay to test the response of ambrosia beetles to volatiles. This bioassay uses a 4-ways olfactometer where each arms are connected to a different odor source.

We first tested the hypothesis that leaf volatiles may cause differential beetle response and allowed redbay ambrosia beetle to differentiate hosts (redbay and swampbay) from non-hosts (oak). We found that redbay and swampbay volatiles were strongly attractant to ambrosia beetles, whereas non-host volatiles were not. Subsequently we explored whether leaf volatiles play a role in the occurrence of ‘escape trees’ – trees that remain living in forest where redbays were otherwise killed by the fungal pathogen transmitted by beetles. We compared leaf volatile profiles of resistant and susceptible redbay trees with GC-MS. We found that leaf volatile profiles of redbay were composed of 12 major volatiles and that the major peaks were α-pinene, dl-limonene, sabinene, eucalyptol and camphor. Interestingly, we did not find sesquiterpene such as α-copaene or calamenene, two compounds known to be attractant to ambrosia beetles. This indicates that attraction of ambrosia beetle to leaf volatiles is mediated by unidentified molecule(s).  We compared leaf volatiles profile on GC-MS of susceptible and resistant redbay and we observed that susceptible redbay were characterized by a higher release of sabinene. With our olfactometer bioassay we tested different concentrations of sabinene and found that beetles were indeed attracted to sabinene. It is possible therefore that resistance observed in the field may be mediated by leaf volatiles.