Webworm Problems In Double-Crop Soybean

Published: August 4, 2009

During the past two weeks, many fields of double-crop and late planted soybean have experienced problems with webworms. These fields have covered a relatively large area including western and southwest Missouri. It is unknown which webworm species reached the economic threshold in these fields, although several webworms do attack soybean plants. Webworms eggs are laid on seedling soybean plants by small moths. These eggs quickly hatch with larvae moving to plant terminals to wrap plant tissues in pockets of webbing in which the developing larvae feed on foliage. The economic threshold for webworms in soybean is to treat when 10 to 12% or more of plants have heavy webbing on plant terminals and defoliation has reached 30% prebloom and 20% from bloom to pod fill. Webworms rapidly move through their life cycle with numerous small dull colored moths often seen in fields following completion of larval development. These moths generally move to other fields or hosts to lay eggs for the next generation.