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Scout for Green Cloverworms in Soybeans and Dry Beans (08/29/19)

The most common insect pest problems in soybeans and dry beans reported the last couple of weeks are foliage-feeding caterpillars including a few late thistle caterpillars, and numerous green cloverworms.

The most common insect pest problems in soybeans and dry beans reported the last couple of weeks are foliage-feeding caterpillars including a few late thistle caterpillars, and numerous green cloverworms. Field reports have come in from the Red River Valley area of Fargo north to St. Thomas and east to Park Rapids, Minnesota.

Scouting:  Since green cloverworm larvae are green and blend in with the bean foliage, it is easier to find them by using a drop cloth placed between two rows of plants, or just vigorously shaking the plants to dislodge larvae from the plants onto the soil. Then, count the number of larvae to estimate the number per row feet. However, the Economic Threshold (E.T.) used for all foliage-feeding caterpillars is based on defoliation:

  • SOYBEANS - 20% defoliation from pod-set to maturity
  • DRY BEANS – 25-30% defoliation

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When estimating defoliation, remember to look at lower, middle, and upper canopy of foliage and calculate an average. Foliage-feeding caterpillars often cause the most damage only to the upper foliage of beans.

Green cloverworms are green with two narrow, white stripes down the side. When mature, larvae are 1¼ inches long. These caterpillars have only three pairs of fleshy prolegs on the abdomen, plus a pair of prolegs on the back segment. When moving, the worms move by arching the middle of the body, or “looping.” Young larvae scrape leaf tissue creating a transparent skin, or "window," on the leaf surface. Older cloverworms eat holes in the leaves.

When I went out to nearby soybeans and dry bean fields with crop consultants and growers last week, we found a few thistle caterpillars (most were already mature or dead from a viral disease) and many green cloverworms in all different stages - young to mature larvae. There were up to 30 green cloverworm larvae per square yard in some fields. However, we decided that the defoliation was below the E.T. of 20% defoliation for soybeans when you considered the lower, middle and upper canopies. Defoliation by green cloverworm was common only in the upper canopy of plants. Green cloverworm is less likely to feed on pods as it prefers foliage.

The other factor that we considered was maturity of beans. We decided that if the beans were fairly far along in seed set (R5 or R6 in soybeans) that the green cloverworms were less likely to affect yield than late-planted beans. For any late-planted soybeans or dry beans near the R3-R4 pod set stages, we recommended that it would be worthwhile to treat if defoliation is above the established E.T. These fields have a longer time to maturity, and it is more likely that the feeding injury from insect pests could negatively affect yield. Remember to watch for grasshoppers feeding on pods since grasshoppers will be moving out of cereal grain fields as they are harvested.

Also, the recent cooler temperatures are going to slow down insect feeding as low temperatures drop to 50F or below. Insects will be less active including grasshoppers and caterpillars. In fact, the preferred temperature range of grasshoppers is 86-112F. Grasshoppers need to warm up their body temperatures in the morning before they are capable of flying or other movements and food consumption.

 

 

Janet J. Knodel

Extension Entomologist

 

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