All the interest in soybean aphids and twospotted spider mites may have caused some folks to overlook some of the other insect pests of soybean that typically occur at this time of year. Matt Montgomery, Sangamon/Menard Extension unit educator--crop systems, has observed relatively large numbers of bean leaf beetles and green cloverworms in soybean fields in his area. Both of these insects are defoliators that can cause significant loss of leaf tissue if they occur in large numbers. Other people also have noted the presence of relatively large numbers of bean leaf beetles in other areas of the state. In some fields, bean leaf beetles are beginning to feed on pods.|
Bean leaf beetle injury to pods.
Bean leaf beetle and injury to leaflet.
You usually can distinguish a difference in type of defoliation caused by bean leaf beetles and green cloverworms. Bean leaf beetles chew small, rounded holes in the leaves, usually between the veins. Green cloverworm larvae consume leaf tissue between the main veins, giving the leaves a tattered appearance. And obviously the insects are quite different in appearance. We've described the bean leaf beetles in a previous issue (no. 7, May 12, 2001) of the Bulletin. The green cloverworm is a green (I'm a master of overstatement) caterpillar that is about 1 to 1-1/4 inches long when fully grown. Two white stripes are apparent along each side of the body. They have four pairs of prolegs (three pairs of abdominal prolegs and one pair of anal prolegs) on the abdomen. Green cloverworms also wriggle violently when disturbed.
When soybeans are in their reproductive stages of growth, significant defoliation can cause yield loss. A standard defoliation threshold is 20% or more defoliation during pod set and pod fill. For green cloverworms, we add 12 or more half-grown larvae per foot of row as a density guideline. Defoliation thresholds should be increased when the value of soybeans is low. However, thresholds also can be decreased when soybean plants are undergoing other stresses (lack of water, etc.). Use your best judgment. For bean leaf beetles feeding on pods, treatment may be warranted when 5 to 10% of the pods are damaged, the leaves are green, and there are 10 or more beetles per foot of row.
Potato leafhopper injury to soybean leaves.
Potato leafhoppers also are fairly common in soybean fields, as they have been in alfalfa fields. In fact, in some areas of the state, leafhoppers are causing significant injury to alfalfa, especially where soil moisture is low. Leafhoppers typically cause more damage when the weather is hot and dry. In soybeans, however, potato leafhoppers usually don't cause significant economic damage, even though symptoms of their feeding injury are apparent. Feeding by potato leafhoppers in soybean fields causes leaves to turn yellow. Severely injured leaves often appear crinkly or burned around the margins. Although the economic threshold is relatively low (13 or more leafhoppers per plant), most people don't treat for potato leafhoppers in soybeans. Nevertheless, they are worthy of attention, especially considering the other stresses that soybeans are suffering right now.--Kevin Steffey