Kansas soybean growers should closely monitor their fields over the next several weeks as bean leaf beetles and soybean podworms become more active, according to Kansas State University crop entomologist Jeff Whitworth.
August is one of the most important months for soybean production because plants are setting pods and building yield. Timely rainfall and effective pest management during this period can make a significant difference at harvest.
Insecticide applications should be based on field scouting rather than the calendar. Applying insecticide when pests are not present or have not reached treatment thresholds provides little benefit.
Bean leaf beetles currently are found primarily in immature stages beneath the soil surface, where larvae feed on soybean roots. Growers who notice weak or stunted areas in their fields should dig up plants and inspect the root systems.
The larvae resemble corn rootworms and feed on root hairs and larger roots, disrupting the movement of water and nutrients through the plant. Continued feeding can cause soybean plants to wilt.
As soybeans begin setting pods, adult bean leaf beetles shift their feeding to the pods and remain active until green pods are no longer available later in the season. They also will feed on leaves when pods are unavailable.
Podworms, including corn earworms, armyworms and fall armyworms, cause different types of damage. The caterpillars climb soybean plants and feed directly on developing beans inside the pods, leaving holes where seeds have been consumed.
Growers scouting fields after pod setting should look for holes in pods where beans are present or have been consumed. Podworm larvae feed for about 10 days to two weeks before moving into the soil to pupate, making insecticide timing critical.
Applications made before podworm larvae are present or after they have completed their feeding period are unlikely to provide effective control.
Frequent scouting throughout August is particularly important as soybean pods develop. Treatment thresholds can help determine whether insecticide applications are justified. For podworms, the general guideline is about one larva per row foot because the insects feed directly on the portion of the crop that will be harvested.
Bean leaf beetles spend the fall feeding in soybean fields before moving to overwintering sites such as alfalfa. Podworms attack a broader range of crops, including corn, soybeans, sorghum, cotton and, in some cases, wheat.
Fall armyworms and corn earworms do not survive Kansas winters, while armyworms can remain active long enough to threaten early-planted wheat.
Regular scouting and treatment decisions based on economic thresholds can help Kansas soybean growers protect yield potential during the critical pod-setting period.
Source: Kansas State University, "Scouting soybean fields now can prevent costly insect damage, K-State expert says"