The adult imported longhorned weevil (Fig. 1), Calomycterus setarius, is a small beetle (about 4 mm long) that is mottled gray, covered with short stiff hairs, and has "elbowed" antennae. The imported longhorned weevil is rare in Iowa soybean, but they have been reported here since 1943. This species is originally from Japan and was first recorded in the United States in 1929.
Biology and Ecology
The imported longhorned weevil is a parthenogenetic insect, with only female forms. Adults do not fly and therefore movement is limited to walking short distances. Adults could be transported by farm equipment, baled hay or horticultural stock. This allows for explosive populations to build up when conditions are right (e.g., observed in Rice and Pilcher in 1998). Larvae feed on the roots of alfalfa, clover and several species of grasses. Very large populations of adults in Iowa have been observing moving into soybeans from adjacent bromegrass waterways or fields (Figs. 2-3).
Injury and Management
Imported longhorned weevils have, on rare occasions, caused injury in Iowa (Fig. 4). However, they have been reported here since the 1930s. They can occasionally cause significant foliar injury in seedling to V4-stage soybean. Injury has been reported from this pest in Iowa from late June to early July. Typically, this insect is concentrated at the boarders of fields and moves inward as the population grows. However, even under extremely large populations, significant defoliation is confined to border rows and most plants grow faster than the advancing weevils can defoliate them (Figs. 5-6). Plants that are completed defoliated may not produce any beans. There are no treatment recommendations for imported longhorned weevil in soybean because they are not known to cause economic injury.
Originally prepared by Marlin E. Rice. Updated by Erin Hodgson in 2017.