Several reports from central and southwestern Iowa indicate that the green cloverworm is causing significant defoliation to soybeans. The moths migrate into Iowa each summer from southern states. There are two generations, one in July and the other in August, but rarely do the larvae develop into economically damaging populations. Major outbreaks did occur during 1966, 1968, and 1973 but it has been a long time since they have caused serious problems.
Larvae can be identified by a combination of characteristics. They are pale green with one or two white stripes extending down each side of the body, and three pairs of prolegs in the middle of the body. Larvae also wiggle violently when touched; no other caterpillar in soybeans exhibits this behavior.
The eggs of this insect hatch in 4 days and the larvae feed for 17-23 days. They develop through 6 instars (stages) and consume most of their food during stages 4-6. Young larvae will feed anywhere on the soybean plant, but older larvae confine their feeding in the upper one-third of the soybean canopy. Diseases frequently suppress green cloverworm populations, however, this may not occur until after a substantial amount of defoliation has occurred.
Larry Pedigo, research entomologist at Iowa State University, conducted a variety of studies on this insect during the 1970s and early 1980s. He was able to develop economic thresholds from this research for the green cloverworm. These are shown in the table and are expressed as the number of larvae per foot of row. Sampling should be done with a drop cloth (see photo in bean leaf beetle article on page 162). There are no thresholds for sampling with a sweep net.
If the green cloverworm population is below the economic threshold, but defoliation is approaching 20 percent, then control measures still should be considered. Several other insects, most notably bean leaf beetles and grasshoppers, also can contribute to the defoliation, even though the population of just one of these pests by themselves is not at the economic threshold.
Green cloverworm economic thresholds.
|Crop value ($ per bushel)
||Treatment cost per acre (insecticide + application)
||No. of larvae per foot of row
Common chemicals labeled for green cloverworm control.
||Amount per acre
||Harvest interval (days)
This article originally appeared on pages 165-166 of the IC-478(21) -- September 2, 1997 issue.