The imported longhorned weevil, Calomycterus setarius, is an occasional pest of soybean in Iowa, but they have been reported here since 1943. This species is originally from Japan and was first recorded in the United States in 1929.
Identification
Adult: Adult imported longhorned weevils are small beetles (about 4 mm long) that are mottled gray and covered with short stiff hairs. Like other weevils, they have "elbowed" antennae (Photo 1).
Biology and Ecology
The imported longhorned weevil is a parthenogenetic insect, with only female forms. Adults do not fly; 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 observed moving into soybeans from adjacent bromegrass waterways or fields (Photos 2 and 3).
Injury and Management
Imported longhorned weevils have, on rare occasions, caused injury in Iowa (Photo 4). They can occasionally cause significant foliar injury in seedling to V4 soybean. Injury has been reported from this pest in Iowa from late June to early July. Typically, this insect is concentrated at the borders 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 (Photos 5 and 6). Plants that are completed defoliated may not produce any seeds. 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.