Ambrosia beetle (Xylosandrus germanus) infestations and management trials in high-density apple orchards

Black stem borer (BSB), Xylosandrus germanus, an ambrosia beetle established in the US for over 80 years, is causing increasing damage in NY and other northeastern states. Normally a pest of hardwood trees, BSB has been documented in dozens of NY apple orchards since 2013, causing tree death and decline in mostly young dwarf plantings of trees ranging from 5–25 cm in trunk diameter. The tiny (~2 mm) adults overwinter in galleries at the base of infested trees, and emerge in spring to infest new sites after 2–3 days with max temps of 20°C, usually by mid-late April; a second flight takes place in August. The adult female drills a ~1 mm-diam hole and hollows out a channel into the heartwood of small trees, and starts to culture a fungal food source, Ambrosiella hartigii, laying eggs in the chamber. She lines the chambers with the ambrosia fungus for the larvae (3 instars) to feed on before pupating. The larvae feed on this fungus in the brood chambers, not the tree's tissues, but the tree reacts to the attack by shutting down its vascular system, which causes, wilting, yellowing, decline and death. Although these borers are known to attack physiologically stressed trees (e.g., from flood, drought, or cold injury) that are emitting ethanol volatiles, they also have been reported to attack apparently healthy trees. We used ethanol traps to document their occurrence and timing in over 50 NY orchards, and also tested different trunk insecticide sprays for BSB control in waterlogged potted apple trees placed in the orchard to stress them enough to produce ethanol.  Trunk and tree damage was assessed among the different treatments.