Soybeans, especially late-planted or late-maturing beans, should be monitored for bean leaf beetle feeding damage to pods. Pest managers have been noting bean leaf beetle feeding on foliage and are concerned about pods, especially with current prices. Bean leaf beetles scar the surface of pods, but only occasionally feed through the pod to the developing beans. During pod maturation, this scar may crack leaving an entry hole for plant pathogens that may cause discolored, moldy, shriveled, or diseased beans.
Pest managers would like to be able to predict whether economic damage will occur based on the numbers of beetles that are present and the stage of pod development (i.e., green, yellow, yellow-brown, or brown pods). Because the beetles are so mobile, there isn’t a correlation from leaf feeding to pod damage – one does not always lead to another. Part of the problem is that beetles may leave a senescing field for a neighboring one that is still green. The timing of harvest in relation to frosts also plays a role, although less so this year with early planting and harvest forecasts. Once the pods turn yellow to yellow-brown, they become less attractive and less susceptible to damage. Control is normally not warranted from this point on (see the following table).
Randomly select 2 plants in each of 5 areas of the field (minimum, more is better) and count the number of pods per plant and the number that show damage (10 total plants). Calculate the percentage of damaged pods per plant for the field as a whole. Note if the pods are green, beginning to turn yellow, or are yellow/brown.
At the same time, determine the number of beetles per sweep using an insect sweep net. Take 5 sets of 20 sweeps in the field. Determine the number of bean leaf beetles per sweep. Additionally, note whether beetles are still actively feeding while surveying the field.
Use the following table to determine when an insecticide treatment may be necessary. There are many compounds registered that will provide effective control of bean leaf beetle, but always remember to keep the pre-harvest interval in mind, many of these were listed in last week’s Pest&Crop. The original values of this table have been modified to reflect today’s higher commodity prices. These new, adjusted values are NOT time-tested, but certainly should be in the ballpark.
Erin Hodgson, Extension Entomologist at Iowa State University developed a dynamic calculator (Excel spreadsheet) which can be customized with commodity price and cost of treatment. It is based on beetles/sweep or row of foot. This too extrapolates from threshold data developed many years ago. You can download the calculator here: http://www.extension.iastate.edu/CropNews/2011/0815hodgson.htm.