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SignificanceAmbrosia beetles are among the true fungus-farming insects and cultivate fungal gardens on which the larvae and adults feed. After invading new habitats, some species destructively attack living or weakened trees growing in managed and unmanaged settings. Ambrosia beetles adapted to weakened trees tunnel into stem tissues containing ethanol to farm their symbiotic fungi, even though ethanol is a potent antimicrobial agent that inhibits the growth of various fungi, yeasts, and bacteria. Here we demonstrate that ambrosia beetles rely on ethanol for host tree colonization because it promotes the growth of their fungal gardens while inhibiting the growth of “weedy” fungal competitors. We propose that ambrosia beetles use ethanol to optimize their food production.

1 Xylosandrus germanus (Blandford) is a key pest of ornamental nursery trees. Ethanol is the most attractive semiochemical known for X. germanus, and its emission from trees represents a primary host-selection cue. Ethanol production is induced by a variety of abiotic and biotic stressors, which could thereby predispose trees to attack by ethanol-responsive ambrosia beetles. 2 To better understand X. germanus host-selection behaviour within ornamental nurseries, a series of experiments examined the influence of flood-stress on the attractiveness and susceptibility of flowering dogwood Cornus florida L. Under field conditions, more X. germanus were attracted to experimentally flood-stressed dogwoods than neighbouring nonflooded controls in 2009, 2010 and 2011. Floodstressed dogwoods were also preferentially attacked in 2009-2011, although no attacks occurred on any of the neighbouring nonflooded trees. 3 Solid-phase microextraction-gas chromatography-mass spectrometry detected ethanol in stem tissue from flooded dogwoods but not nonflooded trees. Acetaldehyde, acetic acid and ethanol were also emitted from the outer bark of flooded dogwoods but not nonflooded trees. 4 These results demonstrate that X. germanus preferentially lands on and attacks physiologically-stressed hosts, and further support the role of ethanol in mediating this interaction. 5 Attacks by X. germanus have previously been suspected to occur on trees viewed as 'apparently-healthy', although the possibility of such trees being in apparentlystressed at the time of attack cannot be ruled out given the results obtained in the present study. Minimizing the impact of stressors known to induce the production of ethanol should be the primary foundation of a management plan for X. germanus and other ethanol-responsive ambrosia beetles.include aesthetic damage to the bark, wilting foliage, branch dieback and profuse basal sprouts. The host range of X. germanus includes more than 200 species worldwide, although deciduous hosts are preferred over coniferous species (Weber & McPherson, 1983). Despite a broad host range, the quality of the host appears to play a critical role in mediating hostselection (Weber & McPherson, 1984;Ranger et al., 2010Ranger et al., , 2012. Previous studies have demonstrated that X. germanus exhibits an efficient olfactory mechanism for distinguishing among differing host qualities and locating hosts based on the