BROOKINGS – Small green caterpillars are being seen in soybean fields throughout South Dakota.
"Currently, these caterpillars aren't causing too much defoliation due to their small size, but as they grow their feeding may become more noticeable," said Adam Varenhorst, assistant professor and SDSU Extension field crop entomologist.
Green, cabbage/soybean looper or alfalfa caterpillars?
The three most commonly observed green caterpillars in soybean are the green cloverworm, cabbage/ soybean looper and alfalfa caterpillars .
"In some fields, two or three of these species may be observed simultaneously," Varenhorst said.
Although these caterpillars are generally not severe pests to South Dakota soybeans, Varenhorst said their presence is important when other defoliators may also be present within the field.
"If only one defoliator species is present in high populations it may be possible to determine thresholds for it, however, when multiple defoliator species occur in a field we recommend looking at the average cumulative defoliation within the field," he said.
How to ID the caterpillars
The three caterpillar species are all very similar in appearance. "That is, all three species are green and have white stripes that run the length of their bodies," Varenhorst said.
He explained that the easiest way to discern the green cloverworm, soybean/cabbage looper and alfalfa caterpillars from one another is to look at the number of abdominal prolegs they have.
Not familiar with abdominal prolegs? Varenhorst explained that all caterpillars will have three pairs of true legs, which are present near their heads.
In addition, caterpillars have one pair of legs at the end of their bodies, which are referred to as the anal prolegs.
The remaining legs in the middle of their bodies are the abdominal prolegs.
"The number of pairs of abdominal prolegs differs depending on the species of caterpillar," he said.
The green cloverworm caterpillars have three pairs of abdominal prolegs. The soybean looper and cabbage looper caterpillars each have two pairs of abdominal prolegs. The alfalfa caterpillars have four pairs of abdominal prolegs.
"Of the three species, the green cloverworms have the greatest potential for being serious defoliators in South Dakota, but all three species may be present and will feed on leaf tissue," Varenhorst said.
Thresholds and management
Because there is the potential for numerous defoliators to be present in soybean in the late summer, SDSU Extension staff recommend looking at the average cumulative defoliation that is present within a field.
To obtain an average, look at 10 plants on each leg of a "Z" walking pattern.
"Remember, the total defoliation of the plant should be recorded and not individual trifoliates," Varenhorst said.
For soybean during the reproductive stages (i.e., after flowering) 20 percent defoliation is the recommended threshold.
Defoliation that exceeds this will result in a 3 to 7 percent yield loss per acre.
For reference of defoliation caused by caterpillars and the recommended threshold.
If the 20 percent threshold is exceeded please refer to the current edition of South Dakota Pest Management Guide: Soybean for a list of insecticides that are labeled for caterpillar pests. This guide can be found on iGrow.
