Pacific Northwest Pest Alert

Published online: Apr 24, 2012
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Heavy infestations of cutworms have been observed by Dave Scantlin (crop consultant, The Amalgamated Sugar Company) in sugar beet fields in Jerome, Wendell, and western Twin Falls County.

Cutworms generally are nocturnal, remaining by day just under the soil surface; often they are not seen until after the plant already has been damaged. Cutworm larvae are about 1 inch when mature and vary in color from light gray to dark brown, with faint stripes or fine mottles on their smooth, hairless, soft bodies. They curl into a motionless C-shape when disturbed. Subterranean species feed on roots and stems, cutting off plants at the soil surface. Climbing species hide during the day in soil and either cut off plants at the soil surface or feed in the crown on newest leaves and stems.

Scouting is easiest when done early in the morning. Lightly scrape the soil surface and look for larvae. No formal economic thresholds exist for cutworm insecticide treatment decisions in sugar beet. Infestations typically are very spotty, usually occurring near weedy patches or along field borders. Consider spot treating infested sites rather than the entire field.

Insecticide seed treatments may suppress cutworms, but will not control heavy infestations. For current information on registered insecticides, consult the Pacific Northwest Insect Management Handbook.