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Time to Watch for Defoliators in Soybeans

July 28, 2000

Telephone calls, e-mail messages, and observations from several areas in Illinois suggest that it's time to watch for several defoliators in soybeans. Although we are not aware of excessive defoliation being caused by any one of the insects discussed in this article (with the possible exception of the Japanese beetle), when soybeans bloom and pods begin to fill out, defoliation by insects can result in noticeable yield loss. While the plants are growing and producing new leaves, and again after the seeds are completely filled, soybeans can tolerate considerable defoliation without yield loss. But during the early part of the reproductive stage, the plants become more sensitive to defoliation. They are most sensitive during pod development. Even at this stage (R4 to R6), however, soybean plants normally can lose 20% to 25% of their leaf area before yield is affected. Keep in mind that a static threshold may not be appropriate during a year when soybean prices are low. Adjust the percentage defoliation threshold upward to accommodate low prices.

Color and pattern variations of bean leaf beetle adults.

Several observations of bean leaf beetles indicate that the adults from the first generation this year have emerged and will be feeding and laying eggs for the next generation. The second-generation adults will emerge sometime in August and September, and these adults will seek overwintering quarters.

Bean leaf beetle adults are about 1/4 inch long, with considerable variation in color. The background color of most bean leaf beetles is light yellow to tan; however, some bean leaf beetles are green, and others are red. Their wing covers usually have four main black spots and stripes along the edges, but these markings may be absent. A black triangle is always present behind the "necklike" prothorax. This latter feature may be especially important to look for this year. Mike Gray, during a recent rootworm-plot evaluation tour in northwestern and northern Illinois, spent some time walking through soybeans looking for the insects present. He observed tan bean leaf beetles, many of which had no spots or stripes. He also observed grape colaspis adults and a few northern corn rootworm adults. Grape colaspis adults are tan, and newly emerged northern corn rootworm adults may be tan before their exo-skeleton hardens and turns green. However, neither of these species has the black triangle behind the prothorax that is characteristic of bean leaf beetles.

Injury to soybean leaf caused by feeding by bean leaf beetle adult.

Defoliation by bean leaf beetles in soybeans appears as small, round holes in the leaves, quite different in appearance from the ragged, edge-of-the-leaf defoliation caused by grasshoppers, green cloverworms, and woollybear caterpillars, and also quite different in appearance from the lacy defoliation caused by Japanese beetles. Leaf-feeding injury caused by grape colaspis adults resembles injury caused by bean leaf beetles. Although the grape colaspis rarely caused economic damage by itself, it may contribute to the overall level of defoliation within a field.

The Japanese beetle probably causes more concern related to defoliation in soybeans than almost all other defoliators in Illinois. We have received reports that some fields are being sprayed to control Japanese beetles, and some of the applications may be warranted. However, some of the concern about Japanese beetles is unjustified because many people don't venture beyond field margins to examine the entire field. Japanese beetle adults move from cornfields where they emerged to soybeans in July and August, so much of the defoliation injury occurs near soybean field margins. People who walk only into the margins of soybean fields may overestimate the amount of defoliation for the entire field. I have received several reports that defoliation at the field margins is almost always worse than defoliation throughout the field. Therefore, it's very important to scout the field to get a reliable estimate of defoliation.

Redlegged grasshopper on soybean pod.

Green cloverworm on soybean leaf.

Yellow woollybear caterpillar on soybean leaf.

Other potential defoliators that may be common in soybean fields in Illinois during this time of year include grasshoppers, green cloverworm, and yellow woollybear caterpillar. However, we have not received any alarming reports about any of these insects this year. Nevertheless, it's a good idea to watch for them now and for the next few weeks. Injury caused by these insects, when combined with defoliation caused by any of the previously mentioned pests, could exceed static economic thresholds.

When scouting a soybean field for evidence of defoliation caused by any of the insects discussed in this article, some general guidelines apply:

  1. Without looking at the plants, stretch out your arm and collect at random 20 leaflets each from the top, middle, and bottom thirds of scattered plants in the field, for a total of 60 leaflets. At this time of year, you probably can focus on the top and middle portions of the plants.
  2. Compare the leaflets with the set of diagrams in Figure 1, which illustrate insect-produced defoliation at six increments.
  3. Record your estimates of the percentage defoliation for each of the 60 leaflets, and determine the mean (add up the estimates and divide the total by 60). The result is the overall level of defoliation in the field.

Although defoliation thresholds accommodate virtually all defoliators, it's important to identify the pests accurately to choose the right insecticide, if warranted. Different insecticides may be labeled for different insects, and the rates of application vary as well.

Western corn rootworm adults feeding on a soybean leaf.

One final remark is that while Mike Gray was looking for insects in soybean fields near Monmouth, Illinois, he noted a complete absence of western corn rootworm adults. We still have not verified the occurrence in western Illinois of the variant of western corn rootworm that deposits eggs in soybeans. Thus far, the lack of western corn rootworm adults in soybeans in Warren County suggests that western counties in Illinois are still free from this problem. Nevertheless, we will keep watching, and we ask you to help us watch for the problem that seems to be spreading slowly westward and northward in Illinois.--Kevin Steffey

Author: Kevin Steffey

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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