Acetic acid stimulates burrowing and colonization success of the ambrosia beetle, Xylosandrus germanus

The ambrosia beetle Xylosandrus germanus preferentially burrows into the stems of living but weakened trees for establishing its nutritional fungal symbiont Ambrosiella grosmanniae and rearing offspring. Ethanol is induced in trees in response to a variety of physiological stressors and represents a long range attractant for X. germanus. The presence of ethanol within stems also benefits the growth of X. germanus' fungal symbiont and promotes brood production. Acetic acid is a metabolite of ethanol and thereby present within stems and emitted from the outer bark. Since acetic acid can influence fungal growth, we hypothesized the presence of acetic acid within stem tissues would influence colonization by X. germanus. Stem sections (i.e., bolts) of Cornus florida were soaked for 24 hrs in 0.1% to 10% acetic acid, 5% ethanol, and water. Small cages were then used to confine individual X. germanus with no-choice to the bolts. To assess burrowing activity, the amount of sawdust within each cage was weighed after 21 d. To assess colonization success, bolts were then dissected after 21 d to quantify the number of eggs, larvae, pupae, and adults within the host galleries. Similar to ethanol, acetic acid stimulated extensive burrowing and brood production by X. germanus as compared to the water-soaked bolts. Acetic acid, ethanol, and pH were also quantified within stems of flood-stressed trees and soaked bolts to assess ecologically relevant concentrations. The enhanced brood production by X. germanus could be due to increased growth and/or nutritional quality of the nutritional fungal symbiont A. grosmanniae.