Managing bean leaf beetle in soybeans
Bean leaf beetle is more common in the eastern part of our state.
There are generally two to three generations of the insect. Hence, we can see them while beans emerge all the way to when beans dry down.
Bean leaf beetle has not been incredibly abundant this year, but is present in our system, sometimes at economic levels.
Our threshold for bean leaf beetle is 30 percent defoliation throughout the plant two weeks prior to blooming (R1) and 15 percent defoliation two weeks prior to blooming until the pods have filled (R7-R8).
Use this article as a guide to assessing defoliation. Remember to assess defoliation throughout the entire canopy.
Later in the season (R5-R7), bean leaf beetle can sometimes scar pod walls, resulting in damage to the seed, or clip pods. This type of injury is possible in North Carolina, but it is rare that there are economic cases (that is cases where the cost of management is less than the damage caused by the bean leaf beetle).
An exception might be in seed production fields.
We do not have a threshold for pod scarring or clipping in North Carolina. However, pod thresholds have been developed and promoted in the Midwest.
Unfortunately, these pod thresholds are not adjusted for soybeans at $16 per bushel and are not adapted to our system. My best guess at a pod threshold for this insect in North Carolina for this year only is 20 bean leaf beetles per 15 sweeps with at least 5 percent injured/clipped pods in R5-R7 soybeans.
You would need more insects and more injury to justify a spray if beans were worth less. Seed producers should spray at half this amount.
A screening test was initiated in 2010 in the Tidewater region for bean leaf beetle (resultshere). This population had not been exposed directly to pyrethroids for several years because of declining efficacy.
The field had been treated previously in 2010 with two applications of acephate for insect pest management.
From the results, we inferred that bean leaf beetle tolerance to pyrethroids could be reversed, but that over-reliance on a single insecticidal chemistry is detrimental (i.e., acephate was now relatively ineffective against bean leaf beetle, while pyrethroids were effective). Note that all insecticides used in this experiment may not be registered.
Always consult a label before application.
General principles of IPM should be followed, which include using multiple management tactics and the judicious use of insecticides.