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Green Cloverworm

September 16, 1998

Throughout Illinois, we are experiencing large numbers of green cloverworm moths. These moths are very dark brown and about one inch long. The head is elongated slightly into a pointed snout caused by protruding palps that are associated with the mouthparts. When sitting, the moths form a triangular or inverted V shape. Although these moths are not pests of landscapes, they are so numerous this year that landscapers, garden center operators, and other readers of this newsletter are probably being asked about them.

Green cloverworm feeds primarily on legumes including clover, alfalfa, soybeans, and garden beans. It also feeds on raspberries and strawberries. The caterpillar is slender and green with two thin white stripes on each side. The full-grown caterpillar is about one inch long. Most years, the caterpillars are decimated by a fungal disease that keeps their numbers low. They are also attacked by several parasitic insects.

They overwinter as adult moths, but do not survive the winter north of 41 degrees north latitude, which is slightly north of Galesburg and slightly south of Dwight. However, observations in Illinois indicate that they usually do not survive the winter north of the southern few counties of the state. Typically, moths fly from southern locales to populate the state in the spring. There are usually two generations per year in this part of the country, but three or four generations occur in southern areas of the United States.

Why are there so many green cloverworm moths this year? We don't know for sure, but we have a couple of ideas. One is that the mild winter of 1997-1998 allowed the moths to overwinter farther north throughout most or all of the state. This would mean more moths laying eggs earlier than usual. In addition, we had warm weather early this year, creating an "early spring." We are also about two weeks ahead in heat units. These conditions may have allowed the green cloverworm to have three generations in Illinois instead of the normal two. This extra generation naturally would result in many more moths being produced.

Another possibility is that the disease that normally kills many green cloverworm caterpillars was not a major factor in keeping down numbers this year. However, with the above-normal rainfall in spring and early summer, one would think that the fungus had good conditions for attack and growth. Perhaps the fungus was scarce for other reasons. Also, perhaps the parasitic insects that normally attack this insect were less common than normal.

The bottom line is that large numbers of moths are coming into residential areas where they are highly attracted to lights at night. They are numerous in heavily vegetated areas, where they might be searching for protected overwintering sites. They are a nuisance but are unlikely to cause any damage to landscape or garden plants. Ignore them or swat them with a fly swatter. No insecticidal control is recommended.

Author: Phil Nixon


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