Soybean Defoliators

Bryan Jensen, UW Department of Entomology and Division of Extension

A few insects to be thinking about in the very near future. First, Japanese beetles should soon be emerging or have already done so by the time you are read this article. At this point in time it is hard to tell how well they overwintered. My guess is well enough. Japanese beetles are showy and feed in groups on the upper part of soybean plants. Furthermore, damage is usually concentrated along field margins. All this makes Japanese beetles easy to spot. The economic threshold is not species specific and is applicable to all soybean defoliators. That threshold is 30% defoliation during pre-bloom and 20% defoliation on reproductive soybean. It does indicate that vegetative soybean are more resilient than soybeans in the reproductive stage of development. Be careful not to overestimate defoliation. Estimates should be made on a whole plant basis. Lower, undamaged leaves can compensate for damage on upper leaves. Therefore, % defoliation should be estimated based on the entire leaf area on each plant. This usually significantly lowers our defoliation estimates. Also, keep in mind that an insecticide application now will affect natural enemies of soybean aphids which could potentially make aphid establishment easier. Same goes for two-spotted spider mites if the weather ever dries out and warms up.

There are several “worms” commonly found in soybean at this time of year and a few which are uncommon. I would classify the latter as eye-catchers. Green cloverworm and soybean loopers are commonly found and occasionally exceed the defoliation threshold. The problem with these insects is they are less “showy” and found in the mid to upper canopy. If on the occasion the threshold is exceeded, please verify their presence. Don’t just base your recommendation on % defoliation. Like other insect pests we needed to consider the potential for future yield loss. Make sure the insects are present and are not at the end of their larval life stage.

Soybean attract a lot of insects. One of which was identified by PJ Liesch, Insect Diagnostician with the Department of Entomology. The silver-spotted skipper isn’t necessarily new to soybean production but is uncommon and unusual in its appearance and feeding habits. The adult is a butterfly and larvae can grow up to 1.5 inch and feeds on several legumes. Larvae are green-ish and have a large black head. A diagnostic feature for these larvae is a very prominent constriction behind the head making the head look even more oversized. Larvae tend to form their own shelter by rolling up leaves when feeding. This is usually what catches people’s attention.

Although I have not received a call regarding thistle caterpillars, I have seen several adults (painted lady butterfly) this spring and Iowa has reported some damage on soybean. If you recall this insect migrated into the Midwest and was common a few years ago. Larvae are usually dark colored but do vary in color . The will have bristly spines and will also roll leaves up to feed.

Green cloverworm 
Soybean Looper. Photo courtesy Clemson University – USDA Cooperative Extension Slide Series,
Thistle caterpillar
Silver-spotted skipper larvae. Note the neck constriction. Photo Courtesy PJ Liesch, UW Department of Entomology