By Jennifer Shike, University of Illinois
It's time to monitor soybeans for defoliation that can be caused by several insect pests such as bean leaf beetles, grasshoppers, green cloverworms and Japanese beetles, urged Mike Gray, University of Illinois Extension entomologist.
Earlier this week, Gray noted bean leaf beetle defoliation at 5 to 10% levels in Champaign County.
"When defoliation reaches 20%, and there are 16 or more beetles per foot of row during bloom to pod-fill stages, producers are encouraged to consider a rescue treatment," Gray said. "A 30-percent defoliation threshold is used prior to the bloom stage of development and when there are 5 or more beetles per foot of row."
He said it's important to consider treatment decisions for soybean insect defoliators and encouraged producers to ask the following questions as they evaluate their fields:
- What insects are causing the primary defoliation?
- Are insect densities of the primary defoliator increasing or decreasing?
- Have I examined plants for defoliation and presence of insects beyond field edges?
- Is the primary defoliation occurring within the upper canopy?
- Is insect injury also occurring to developing pods?
By determining the answers to these questions, producers can make a more informed treatment decision. If the defoliation is primarily occurring in the upper canopy (not just along field edges), and has reached economic threshold levels, Gray said to select an insecticide known to be effective against the primary target insect.
Factors such as cost, residual effectiveness and pre-harvest interval, need to be considered in the decision-making process. Under extremely hot conditions, pyrethroid insecticides may not prove as effective, he added.
"It's also important to note that the defoliation thresholds used for many years were based upon much lower commodity prices," he said. "Because the value of soybeans has increased significantly in recent times, lower levels of defoliation could translate into yield losses of economic importance."
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