Soybean pests are in the field

Producers should be scouting fields for yield-reducing insects such as the corn ear worm.

The old saying goes “All it takes to be a good soybean farmer is some rain in August.” I’m not sure who said these words, but there is some truth to it. Soybeans are very resilient plants. However, as tough as they are, some insects can cause enough damage to the soybean that require some attention.

The first, and maybe most worrisome, insect pest in soybean is the corn earworm (Helicoverpa zea), which when in attacks soybean plants is called the podworm. These voracious eaters are also called tomato fruitworm and cotton bollworm and consume a wide variety of agronomic and horticultural crops. Technically, the “worm” is actually a caterpillar as the adult corn earworm is a moth. Scouting for this pest should begin when pods are being set. In fact, with recent calls about earworm moving to sorghum, the time to start scouting is already here. The larvae attack the developing seed within the pod and can cause significant yield loss. The recommended treatment level is when there is one earworm per foot of row. However, many would argue that this level is too low and spraying could be justified at a much lower level. A wide variety of insecticides are effective against earworm but, as always, read and follow label instructions when applying.

The next culprit in the soybean insect playbook is the bean leaf beetle (Cerotoma trifurcate). Bean leaf beetles closely resemble the size and shape of a ladybug, but their coloring differs. The bean leaf beetle has spots (six) near its midline with a black band around the border of its back.

The beetles chew oval holes in the leaf which truly sounds worse than it is. Early in the season, soybeans can tolerate up to 25 percent defoliation without harming potential yield. Later in the season, a chemical treatment may be warranted if pod-feeding is occurring, however. The treatment threshold is incredibly high at 50 or more beetles per foot of row. Once again, a wide variety of insecticides are effective on bean leaf beetle.  

Webworms (Achyra ra-ntalis) are also a problematic pest for soybeans in our area and have been seen this year. Once again, it is actually a caterpillar which becomes an adult moth. The caterpillar itself is slender and green with triangular spots along it side. This insect skeletonizes the leaf in a protected webbed pocket. This contributes to the difficulty in controlling the pest. Although treatment levels are not set in stone, but from 25 to 50 percent defoliation would require treatment. A wide variety of insecticides work on webworm as well.

Rarely has an insect been more accurately named than the green stinkbug (Acro-sternum hilare). The damage from this insect is interesting in that from the outside of the pod, very little can be seen. In fact, only a tiny black or brown spot appears on the pod itself. However, these pests pierce the pod targeting the developing seed and cause shrunken and shriveled seeds. The treatment level for green stinkbugs is when 10 bugs per 30 feet of row are found. Often, spot treatments may be effective at eliminating the pest and quite a few products eliminate stinkbug.

Finally, there have been a multitude of reports of blister beetle in our area this year. I witnessed a massive amount of striped blister beetles (Epicauta vittata) in a corn field a few weeks back.

There were hundreds (at least) in the field causing some localized damage. These beetles feed on the foliage, but rarely require more than spot treatment because of their nature. Early in plant development, up to 35 percent defoliation can be tolerated by the plant. However, when the plant is flowering and pods are forming, yield loss can occur at 20 percent defoliation. Once again, a wide variety of insecticides work on blister beetles.

Keep an eye on your fields. Even though soybeans are tough, some insect pests can impact yield. If you have questions or would like more information, please call me at the office 620-724-8233, or email me at jcoltrain@ksu.edu, or visit the Wildcat Extension District website at www.wildcatdistrict.ksu.edu. £

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