Late-planted and double-crop soybeans are at an increased risk of damage from webworm feeding.
Webworms skeletonize soybean foliage in a protected webbed area.
Infested fields should be treated with a labeled insecticide when thresholds are reached.
Webworm larvae cause damage to soybeans by feeding on foliage. Double-crop and late-planted soybeans tend to be at an increased risk of damage associated with webworms. They often feed on alfalfa or weeds, such as waterhemp and other pigweed species, before migrating into soybean fields. Beige webworm moths move into soybean fields and lay their eggs after alfalfa has been harvested, or weeds have been treated, and their food supply is diminished. Larvae are slender and green,sometimes brownish-orange, with three black spots or tubercles in a triangular pattern on each body segment. One or more stiff hairs extend from each tubercle (Figure 1).
July and August are usually the peak months for possible damage from webworm infestations in soybeans. Webworm populations are usually not widespread throughout the field; however, significant damage can occur quickly. Webworms can be controlled and damage can be minimized if populations are detected and treated early.
Webworms spin webs around the leaves they are feeding on, and grow up to an inch in length before feeding stops and they pupate in the soil. Larvae typically complete their development in 3 to 5 weeks. Webworm larvae quickly move back into their protective webbed area when disturbed. Damage occurs as larvae feed on soybean foliage, leaving a skeletonized appearance (Figures 2 and 3). Careful scouting is required since multiple generations can occur over the season and infestations often occur in localized areas within a field. Significant yield reduction can be anticipated if mid-summer defoliation reaches 60 percent or more. Treatment may be necessary when 10 to 12 percent of plants have heavy webbing in the top leaflets, or when defoliation levels reach 30 percent pre-bloom or 20 percent from bloom to pod fill.
There are several management options available for the control of webworms. Webworms are generally not difficult to control, and treatment options are often economical and effective. Insecticides containing a pyrethroid or organophosphate and that are labeled for the control of webworm in soybeans can be used to treat an infested field. Aerial application may be the best application method depending on the size of the plants. But remember, webworm infestations are often localized infestations and it may not be necessary to treat an entire field.
Stalcup, L. April 1, 2010. Watch for webworms: keep and eye out for webworm pests in your soybean fields. Corn and Soybean Digest. verified 7/17/17 ID 170714151211