Imported Longhorned Weevil

Encyclopedia Article


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.

imported longhorned weevil
Figure 1. Imported longhorned weevil is a rare pest of Iowa soybean. Photo Marlin E. Rice.

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).

imported longhorned weevil defoliation on soybean
Figure 2. Soybeans defoliated by imported longhorned weevils. Photo by Marlin E. Rice.
imported longhorned weevils
Figure 3. A hand-full of weevils collected from soybean with a sweep net. Photo by Marlin E. Rice.

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.

imported longhorned weevil injury on soybean
Figure 4. Imported longhorned weevil injury to soybean near Mapleton, Iowa. Photo by Marlin E. Rice.
imported longhorned weevil injury on soybean
Figure 5. Imported longhorned weevil injury on soybean. Photo by Marlin E. Rice.
imported longhorned weevil defoliation
Figure 6. Imported longhorned weevil defoliation to soybean leaves: 1% [left], 5% [center], and 15% [right] tissue removal is shown. Weevils make a distinct irregular pattern of leaf injury on soybean leaves as seen here. Photo Marlin E. Rice.

Originally prepared by Marlin E. Rice. Updated by Erin Hodgson in 2017.