Scout soybeans almost until harvest
By Tom J. Bechman
The biggest threat left to soybean potential, at least form insects, is feeding by bean leaf beetles. Some areas had considerable bean leaf feeding on leaves earlier. The only way to know if they’re chewing up profit by gnawing on pods is to scout.
Here’s the question that the Indiana Certified Crop Advisers addresses this month.
Question: Bean leaf beetles are present in a couple fields and chewing pods. The beans haven’t started turning yellow yet. Should I spray? If so, what do I spray?
Betsy Bower, Ceres Solutions, agronomist, Terre Haute: The economic threshold for spraying bean leaf beetles during podding is when 10% pod damage occurs and there are 10 or more beetles per foot of row. Determine the threshold accuracy by randomly selecting two plants in each of five areas of the field. Count total number of pods, then number of pods showing feeding scars and calculate percentage damaged pods. To determine number of beetles per foot of row, use an insect sweep net and take five sets of 20 sweeps. Estimate the distance traveled in 20 sweeps. Count the number of bean leaf beetles and divide by 20. Then divide by the number of feet you traveled per sweep. There are several insecticides labeled to control bean leaf beetles. Consult your local crops specialist.
Steve Dlugosz, Heartland Co-op, agronomist, east-central Indiana: I have always felt late season pod feeding by bean leaf beetles reduces yields when beetle populations are high. Unfortunately many growers don’t venture out into tall beans to check pod feeding. It’s easy to spot damage.
Pull up several plants and look for signs of feeding on pods and/or shrunken or discolored beans. It may be difficult to find the actual insect since they tend to drop off the plant as you approach. Treatment thresholds are surprisingly low. Cost of control is usually around $4 per acre plus application costs. Even at today’s lower soybean prices compared to past years, it takes very few injured soybeans per foot of row to justify treatment.
Dan Ritter, Brodbeck Seeds, agronomist, Rensselaer: Based on the Purdue University publication E-77-W on this insect, one should treat for bean leaf beetles when 5% of the pods show feeding and there are 10 or more live beetles per foot of row. There are a multitude of viable products to use (for spraying bean leaf beetles.) Select one that is effective, economical and that has the shortest pre-harvest interval. (This refers to the length of time between application and when you can harvest the field.) When we begin discussing pods and (lack of) leaf drop in this question, I am persuaded to highly consider that the pre-harvest interval will be safe.
Advice on when to pull trigger to spray based on pod damage varies, but all agree threshold is low
Soybean bean leaf beetles can reduce yields if there are enough of them and they feed on pods
Make sure you know how long you must wait to harvest after spraying for bean leaf beetles