Webworms (soybean)

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Authors: Buyung Hadi, Jan Knodel, Ken Ostlie, Robert J. Whitworth, J. P. Michaud and Phillip E. Sloderbeck

At least two species of webworms have been reported on soybean: the garden webworm, Achyra rantalis, and the alfalfa webworm, Loxostege cerealis. Both webworms are larvae of moths from family Pyralidae native to North America.


The appearance of both webworms is somewhat similar. The webworms are about 1 in. (25.4 mm) when fully grown, slender, green to brown in color with a light stripe running down the middle of the back three and three hairy dark spots arranged in triangular pattern on each side of the body segments (six spots per segment). The adult moth of garden webworm has about 0.75 in. (19.1 mm) wingspan, yellowish brown in color with irregular grayish markings. The adult moth of alfalfa webworm is similar in appearance with a slightly larger wingspan, measuring between 1-1.25 in. (25.4-31.7 mm).

Life Cycle and Seasonal History

In the Northern Plains, the webworms overwinter as pupae in the soil. In warmer areas such as the gulf coast states, the webworms may survive the winter as larvae. Adults emerge in the spring and mate. Mated females deposit egg in masses (ranging between 2-50 eggs per egg mass) on host plant leaves. The eggs take between 3 to 7 days to hatch. The hatched webworms feed on the underside of the leaves for about three weeks, forming a silk webbing on the leaves, before they begin to pupate. Pupation lasts between 1 to 3 weeks and occurs within silk burrows. There maybe more than 2 generations per year of webworms in the Northern Plains.


Plant Injury and Damage

Webworms defoliate soybean plants although their populations do not usually develop to economically damaging levels in the Northern Plains. Alfalfa appeared to be a preferred host, thus soybean planted nearby alfalfa fields may stand higher risk of webworms infestation.

Management Approaches

Scouting and Threshold

The control of webworms should be considered together as a part of soybean defoliators complex and management decision is taken based on the severity of defoliation in a given field. Webworms may attack soybean in early vegetative phases, thus scouting may have to begin early in the season. Generally, management action is recommended if defoliation reaches 40% pre-bloom, 20% during bloom and pod-fill and 35% from pod-fill to harvest.

Other Online Resources

Kansas State University

North Dakota State University

University of Minnesota

For information regarding labels of chemical control options, please visit Agrian.com