Quantifying effect of Xylosandrus sp. attacks on mortality of apple trees

Invasive ambrosia beetles such as Xylosandrus crassiusculus and X. germanus are attracted to and found to attack weakened and dying trees. There is strong observational evidence associating Xylosandrus ambrosia beetles with dieback and tree mortality that has motivated research into mitigating their impact in fruit, nut, and ornamental production. However, a causal effect on tree mortality has not been conclusively demonstrated for these beetles. The nutritional symbionts of X. crassiusculus and X. germanus, Ambrosiella roperi and A. grossmaniae, have not been shown to be pathogenic on host trees to date. Xylosandrus beetles will only invade trees that are under stress and are releasing ethanol signals. It is therefore difficult to disentangle the observations of tree mortality from abiotic stressors and beetle attack. Current recommendations for ambrosia beetle control include high application rates of broad-spectrum persistent insecticides.

In this study we artificially stressed potted 3-to-4 year old potted apple trees by flooding with water to cover roots or drenching with 3% ethanol to elicit beetle attacks naturally, and by caging beetles to trees. A control treatment consisted of trees that were wrapped in insecticide-impregnated netting to exclude beetles from attacking the lower trunk. The health and survival of trees were tracked to compare effect of excluding beetle attack. Flood stress caused high mortality independent to beetle exposure. Tree mortality was increased by beetle attack where flood stress was severe. These findings suggest that abiotic stress mitigation is likely more important than beetle monitoring and control for preventing tree mortality.