Composition and seasonal phenology of a nonindigenous root-feeding weevil (Coleoptera: Curculionidae) complex in northern hardwood forests in the Great Lakes region

Phyllobius oblongus (L.), Polydrusus sericeus (Schaller), and Sciaphilus asperatus (Bonsdorff) comprise a complex of nonindigenous root-feeding weevils in northern hardwood forests of the Great Lakes region. Little is known about their detailed biology, seasonality, relative abundance, and distribution patterns. We studied 10 sites over a 2-yr period. Two sites were in northeastern Wisconsin, and eight were in the neighboring southern upper peninsula of Michigan. Larval abundance was estimated by soil sampling, and adult abundance was estimated by sweep netting, emergence trapping, and beating samples. Sweep netting collected the most weevils overall (71.0%), whereas beating and emergence traps collected 22.1 and 6.9%, respectively. P. sericeus were the predominant larvae, representing 34.3% of total Curculionidae, whereas P. oblongus were the predominant adults, representing 66.4% of Curculionidae. Few S. asperatus and Trachyphloeus aristatus (Gyllenhal) larvae and adults were collected, with the latter being a new record for Wisconsin. Two additional species, Barypeithes pellucidus (Boheman) and an undetermined Polydrusus sp., were collected only as larvae. Six species of curculionids were collected overall, with at least five being confirmed as nonindigenous species. P. oblongus and P. sericeus adults were the most abundant. These did not coincide temporally. Over 63% of P. oblongus and P. sericeus were collected during single 4-wk intervals in mid-June and mid-July, respectively. Conversely, S. asperatus adults overlapped with both other species, occurring sparingly from 4 June through 28 August. One species was predominant at each site and generally accounted for greater than or equal to 80% of the total weevil population. P. oblongus larvae and adults predominated in five and eight sites, respectively, whereas P. sericeus and S. asperatus larvae and adults predominated in one site each. Adult and larval populations were generally clustered. We evaluated vertical stratification of P. sericeus larvae in the soil, and most were located within the top 10 cm.