Microhabitat selection by overwintering alder bark beetles (Coleoptera: Curculionidae: Scolytinae) on red alder (Betulaceae)

The alder bark beetle, Alniphagus aspericollis (LeConte), is known to overwinter as larvae and adults in the main stems of red alder, Alnus rubra (Bongard). Observations of adult alder bark beetles in the branch and bud nodes of planted saplings during the winter led to the hypothesis that these microhabitats represent alternative overwintering sites for the beetle in mature trees. To test this hypothesis, we surveyed crown branches at three coastal British Columbia locations, sampling three trees in 2003 and 2004 and three additional trees (nine branches per tree) in 2018–2019. Adult beetles were found overwintering inside sites excavated in the branch and bud nodes throughout the crowns of mature trees. Branch type, defined by growth class, and tree affected the overall distribution and likelihood of overwintering site occupancy among the three trees sampled intensively in 2018–2019, whereas overwintering site age (old versus new (current year)) affected the frequency of site occupancy across all six trees. We suggest that overwintering in tree crowns evolved from late-summer maturation feeding in new shoots and that the majority of beetles abandon terminal shoots as temperatures fall, moving deeper into the crown to overwinter preferentially in older branch tissues that offer greater thermal protection.