Non-native ambrosia beetles as opportunistic exploiters of living but weakened trees

Authors: Ranger, Christopher; SCHULTZ, PETER - Virginia Polytechnic Institution & State University; FRANK, STEVEN - North Carolina State University; CHONG, JUANG - Clemson University; Reding, Michael - Mike

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/17/2015
Publication Date: 7/2/2015
Publication URL: http://handle.nal.usda.gov/10113/61398
Citation: Ranger, C.M., Schultz, P., Frank, S., Chong, J.H., Reding, M.E. 2015. Non-native ambrosia beetles as opportunistic exploiters of living but weakened trees. PLoS One. DOI: 10.1371/journal.pone.0131496.

Interpretive Summary: Non-native ambrosia beetles have been associated with sudden and extensive attacks on trees after their introduction into new ecosystems, but factors driving their host selection are poorly understood and critical for developing management tactics. The overall goal of this study was to characterize host location, preference, and colonization strategies by two successful and opportunistic invaders, Xylosandrus germanus and Xylosandrus crassiusculus. Field experiments demonstrated ethanol is a key attractant for both species. Ethanol is induced and emitted in response to a variety of stressors, particularly flooding, and flood-stressed trees are preferentially attacked. When beetles were given free-choice under field conditions among selected flooded and non-flooded tree species of varying intolerance to flooding, flood-intolerant tree species were preferentially attacked over more tolerant species. Higher concentrations of ethanol were also associated with flood stress intolerant species compared to tolerant and non-flooded species/individuals. When X. germanus and X. crassiusculus were confined to stems of flood stressed and non-flooded trees, eggs, larvae, and pupae were only associated with galleries created in flooded trees. These findings demonstrate X. germanus and X. crassiusculus specifically targeted certain trees based on their intolerance of physiological stress. Healthy trees were not preferred and poorly colonized, thus management practices should maximize host vigor. We propose that opportunistic ambrosia beetles exploiting living but weakened trees could therefore act as indicators of trees in an inconspicuous state of decline within natural and managed landscapes, particularly in response to extreme climatic events.

Technical Abstract: Exotic Xylosandrus spp. ambrosia beetles have been associated with sudden and extensive attacks on trees after their introduction into new ecosystems, but factors driving their host selection are poorly understood and critical for developing management tactics. The overall goal of this study was to characterize host location, preference, and colonization strategies by two successful and opportunistic invaders, Xylosandrus germanus and Xylosandrus crassiusculus. Field assays further demonstrated ethanol is a key attractant used during orientation and host location, but no evidence was observed for the production of beetle-derived aggregation pheromones. Ethanol is induced and emitted in response to a variety of stressors, particularly flooding, and flood-stressed trees are preferentially attacked. When beetles were given free-choice under field conditions among selected flooded and non-flooded tree species of varying intolerance to flooding, flood-intolerant tree species were preferentially attacked over more tolerant species. In particular, flood stressed flowering dogwood (Cornus florida) sustained more attacks than flood-tolerant species, such as silver maple (Acer saccharinum) and swamp white oak (Quercus bicolor). Higher concentrations of ethanol were also associated with flood stress intolerant species compared to tolerant and non-flooded species/individuals. When X. germanus and X. crassiusculus were confined in a no-choice test to stems of flood stressed and non-flooded C. florida, living foundresses, eggs, larvae, and pupae were only associated with galleries created in flooded trees. These findings demonstrate X. germanus and X. crassiusculus efficiently distinguished among varying host qualities and specifically targeted trees based on their intolerance of physiological stress. Healthy trees were not preferred and poorly colonized, thus management practices should maximize host vigor. We propose that opportunistic ambrosia beetles exploiting living but weakened trees could therefore act as indicators of trees in an inconspicuous state of decline within natural and managed landscapes, particularly in response to extreme climatic events.