Synergistic Effects of Acetic Acid and Ethanol on Offspring Production and Gallery Expansion by Fungus-Farming Ambrosia Beetles

Wood-boring ambrosia beetles and their offspring obligately depend on cultivated fungi that they maintain within host trees. Stressed trees produce at least two potent antimicrobials, ethanol and acetic acid, which typically suppress the growth of most fungi. Ethanol facilitates the growth of ambrosia beetles' fungal mutualists and thus aids in host colonization; however, acetic acid's effects are unknown. Here, we evaluated the effects of acetic acid on tunneling and offspring production by two exotic ambrosia beetle species, Xylosandrus germanus and Anisandrus maiche. Cut wood stems (i.e., bolts) were infused with water alone, 5% ethanol, or mixtures of 5% ethanol with acetic acid at various dilutions. Infestations by X. germanus and A. maiche resulted in more frass, larvae, pupae, and adults in the combined 5% ethanol-acetic acid treatment, than in the 5% ethanol alone or water control treatments. X-ray micro-computed tomography revealed that bolts infused with this 5% ethanol-acetic acid mixture had larger insect gallery volumes than those with only 5% ethanol, which in turn were larger than the water control. A positive relationship was found between frass ejected by each beetle and gallery volume across all experiments. This study demonstrates that acetic acid and ethanol synergistically increase offspring production while facilitating gallery expansion by the larvae. Understanding how ambrosia beetles utilize stress-induced compounds within host trees to benefit their fungiculture could lead to novel management strategies for these invasive insects.