By Erin Hodgson, Mike McCarville and Adam Varenhorst, Department of Entomology
Imported longhorned weevils have been regularly collected in sweep net samples from soybean this year. A few reports of weevil damage in soybean are reported in Iowa annually since 1994. Damage is most likely to occur on border rows adjacent to grassy areas.
Imported longhorned weevil adults are about 1/4 inch long with a grey, pear-shaped body (Fig. 1). The forewings are covered with irregular tan markings and white scales. The antennae are long, prominent and elbowed. Adult weevils emerge from the soil in late June and become abundant during July and August. The adults are defoliators that create jagged leaf edges. Larvae are C-shaped and legless; they are root feeders with a wide host range.
Fig. 1. These small weevils can be easily collected with a sweep net. Photo by A. Varenhorst.
Imported longhorned weevils have one generation per year. Interestingly, this species only has females and can reproduce without males, similar to aphids in the summer. Furthermore, adults have wings but are incapable of flight, which limit their dispersal. They are often restricted to border rows because the adults are not very mobile (Fig. 2).
Fig. 2. Widespread defoliation from imported longhorned weevil damage is unusual, but sometimes damage can be detected along border rows. Photo by M. E. Rice.
Because they are considered a minor soybean pest that colonizes in field margins, an economic threshold has not be developed. Marlin Rice conducted insecticide efficacy evaluations for imported longhorn weevil in 1994. He found it was difficult to kill the adults with the low rate of insecticides and recommended border treatments with full-rate products if defoliation becomes significant. We were not able to find insecticides with imported longhorned weevil on the label.
Erin Hodgson is an assistant professor of entomology with extension and research responsibilities; contact at firstname.lastname@example.org or phone 515-294-2847. Mike McCarville and Adam Varenhorst are entomology graduate students working with Matt O'Neal, assistant professor of entomology.