Mark Carlton, extension specialist in crops, and Tracy Cameron, agronomist at Crestland Co-op in Creston, report loopers defoliating soybean in Lucas and Union counties, respectively, in southern Iowa. Both fields had been planted no-till into pasture. The field in Union County also had variegated cutworms that were chewing plants to the ground in the areas of thicker dead grass.
The presence of loopers in soybean early in the season is very rare for Iowa. I would not expect economic damage to occur in very many fields. The presence of dead grass or weeds may have been a common factor contributing to these two problem fields although another soybean field no-tilled into a bluegrass pasture just a couple of miles away from the Union County field had no loopers in the young soybean.
The most abundant looper in soybean is the soybean looper, although five other species may occur on the crop. In the June 4, 2001, ICM newsletter I noted  that the celery looper and bilobed looper were very common at lights around Ames so these two species may be causing some of the defoliation.
Loopers as a group can easily be identified by the two pairs of abdominal prolegs near the back of the body. All other crop caterpillars have either three or four pairs of abdominal prolegs. Loopers also have a very characteristic behavior when they walk, which gives them their name, looper. The larvae have six instars with the last instar reaching 35 mm in length. The last two instar stages eat 90 percent of all the leaf area consumed during their short life. The life cycle of the larvae lasts 13-14 days and there probably will be two more generations of this insect in Iowa this year.
|Soybean loopers defoliating soybean.|
Soybean plants are relatively tolerant of early-season defoliation. If only loopers are present in the field then an economic threshold would be 35 percent of the plants defoliated before treatment would be justified.
This article originally appeared on page 124 of the IC-486(15) -- June 25, 2001 issue.