Soybeans in the pod-fill stage should be inspected for bean leaf beetles, a South Dakota State University specialist said.
Extension Entomologist Mike Catangui said bean leaf beetles can be very destructive to soybeans because they feed directly on the pods. The pests can hurt yields and quality by eating pods that are developing, and in addition may clip pods and cause them to fall to the ground.
Bean leaf beetles are about one-quarter inch in length, yellowish in color with four dark spots on the back, and have chewing mouth parts.
The economic thresholds of bean leaf beetles on pod-fill soybeans can be calculated using a formula that takes into account the chemical-plus-application cost, predicted yield, and the expected market value of soybeans.
For example, Catangui said, a soybean field planted in 30-inch rows should have an average of six bean leaf beetles per-foot-of-row for spraying to be cost-effective. This scenario assumes a yield of 45 bushels per acre, expected soybean market value of $5 per bushel, and a chemical-plus-application cost of $8 per acre. If using an insect net, the economic threshold is four beetles per sweep.
Details on how to calculate economic thresholds, a current list of labeled insecticides, and other biological information can be found online at:
Bean leaf beetles will continue to feed on soybeans until the pods turn yellow. Scouting must be continued accordingly. Beetles on the field right now will overwinter in soil litter and shelterbelts, and will feed again on next year’s soybeans.
Catangui added that growers considering treatment later in the season need to be aware of the pre-harvest interval (PHI) of whatever product they intend to use. Read the label carefully because some products have a PHI of up to 60 days.