Temporal and species variation in cold hardiness among invasive rhizophagous weevils (Coleoptera: Curculionidae) in a northern hardwood forest.
A complex of invasive rhizophagous weevils has established in North American northern hardwood forests. Little is known regarding the overwintering biology of these weevils and thus how cold hardiness and weather affect population dynamics. Field data from winter 2006-2007 showed a decline in larval abundance but an increase in larval weight of the surviving individuals. During winter 2008-2009, we examined several aspects of overwintering biology of Phyllobius oblongus (L.), Polydrusus sericeus (Schaller), and Barypeithes pellucidus (Boheman). Larvae were collected in the Upper Peninsula of Michigan and transported in bulk field soil to the University of Notre Dame, South Bend, IN, for laboratory assays. Supercooling points (SCPs) of P. oblongus and B. pellucidus larvae not in contact with ice were highest in October and lowest in March, but SCPs of larvae that were in contact with ice did not differ among sampling dates. Larval cold tolerance increased over the winter, with 11% of P. oblongus and 40% of P. sericeus surviving 24 h at -12°C. Few B. pellucidus or P. oblongus survived 30 d at temperatures of -3.3°C or lower. Body water content increased from January to March in P. sericeus and B. pellucidus. Larval hemolymph from all species showed some thermal hysteresis and hexagonal crystal formation, indicative of low levels of antifreeze proteins or glycolipids. These subterranean-dwelling larvae are buffered from ambient winter temperatures, but our data also suggest low levels of freeze avoidance. We discuss how these overwintering strategies may affect adult population dynamics.