Webworms in Soybean
Webworms occurring in soybean fields include alfalfa webworms (Loxostege cereralis), beet webworms (L. sticticalis), and garden webworms (Achyra rantalis). These insects are generally occasional pests, with activity varying by region across the United States, Canada, and Mexico.
Crop Damage
Webworms may attack soybeans at different growth stages. Early in the season, adult moths lay eggs at the base of seedlings. Upon hatching, larvae wrap themselves in emerging plant tissue, which can cause stand loss under severe infestations. During the middle to late season, larvae feed on weeds such as pigweed and ragweed before moving to soybean fields. They are also attracted to alfalfa and migrate to soybeans when nearby alfalfa is cut. Larvae create protective shelters by tying leaves together with silken webbing and feed within these areas. Feeding produces a skeletonized appearance; leaves eventually turn brown and net-like once green tissue is consumed.
Alfalfa webworm.
Whitney Cranshaw, Colorado State University, Bugwood.org
Pest Descriptions
Alfalfa Webworm: Full-grown larvae are approximately 1 inch long, light to dark brown or green, with a brown-tan dorsal stripe and six small spots per segment (less distinct than other webworms). Adults have tan, gray, and dark brown wing markings, with a prominent cream band near the forewing base.
Beet Webworm: Full-grown larvae are approximately 1.5 inches, olive to dark green, with a dark dorsal stripe and lighter lateral stripes. Six large spots per segment bear stiff hairs. Adults exhibit white, cream, tan, and dark brown wing patterns, with distinct cream and brown bands and white forewing fringes.
Garden Webworm: Full-grown larvae are approximately 1 inch, pale green to yellow-green, with tan to orange mottled heads, a light dorsal stripe, and six black spots per segment with stiff hairs. Adults are tan to brown with a distinct snout.
Webworm shelter (alfalfa webworm)
Whitney Cranshaw, Colorado State University, Bugwood.org
Scouting
Late-planted soybeans and areas adjacent to weedy or recently cut alfalfa fields. Check field edges and weedy areas; sample entire fields if outbreaks are suspected.
Management
Weed control and insecticide spot treatments may be warranted. Assess the combined impact of other defoliators before making control decisions. Defoliation thresholds are 30% for vegetative stages and 20% for reproductive stages. Severe early-season damage may require replanting.
Consult your local extension specialist for current management recommendations.
Prepared and reviewed by Carol Pilcher, Iowa State University, and Robert Wright, University of Nebraska-Lincoln, with support from the United Soybean Board.
Webworm shelter (alfalfa webworm). Whitney Cranshaw, Colorado State University, Bugwood.org
Alfalfa webworm. Black arrows indicate characteristic spots on each segment. Whitney Cranshaw, Colorado State University, Bugwood.org
Alfalfa webworm adult. Black arrow indicates cream band. Whitney Cranshaw, Colorado State University, Bugwood.org
Beet webworm-characteristic spots on each segment and hairs projecting from each spot. Frank Peairs, Colorado State University, Bugwood.org
Beet webworm adult. Black arrows indicate cream and brown bands with white fringe. Vladimir Petko, VN Sukachev Institute of Forest SB RAS, Bugwood.org
Garden webworm. Alton Sparks, University of Georgia, Bugwood.org
Garden webworm adult. Black arrow indicates distinct snout. Royal Tyler, Pro Pest and Lawn Store, Bugwood.org
Gallery images courtesy of Whitney Cranshaw, Colorado State University, Bugwood.org; Frank Peairs, Colorado State University, Bugwood.org; Vladimir Petko, VN Sukachev Institute of Forest SB RAS, Bugwood.org; and Royal Tylers, Pro Pest and Lawn Store, Bugwood.org.
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