Spatial and temporal distribution of Xylosandrus germanus and Anisandrus maiche (Coleoptera: Curculionidae: Scolytinae) in Michigan apple

Ambrosia beetles (Coleoptera: Curculionidae: Scolytinae) are significant pests of stressed trees within apple orchards, boring into trees and introduce symbiotic fungi that cause continued stress, potential yield loss, and dieback. The invasive Xylosandrus germanus (Blandford) has become increasingly problematic, while Anisandrus maiche (Kurentzov) is newly documented in this system. To optimize monitoring and understand beetle activity, we conducted trapping studies over 3 growing seasons in Michigan apple orchards. Bottle traps captured significantly more X. germanus and A. maiche than sticky v-traps and delta traps, and had the lowest proportion of non-target beetles (6.6%). Monitoring conducted from 2023 to 2025 showed X. germanus activity peaked between 102.4 and 273.3 (DD10ºC), while A. maiche peaked later between 961.2 and 1,102.9 DD10ºC. We found that trees with 6 or more boring holes from previous growing seasons were significantly more likely to experience continued infestation. Beetle captures were higher at orchard edges than in orchard interiors, and a similar pattern was observed for tree infestation intensity relative to distance from the nearest woodlot. These findings provide insight into the spatial and temporal dynamics of X. germanus and A. maiche in apple orchards and inform monitoring and management strategies.