Joanne Whalen, Extension IPM Specialist;jwhalen@udel.edu
Soybeans
We continue to find a variety of defoliators present in full season and double crop fields. In some cases, we are finding that fungal diseases are starting to help reduce green cloverworm populations. As indicated in past newsletters, defoliation thresholds should be used to make a treatment decision for the complex of defoliators present in a field.
Corn earworm populations still remain low and spotty but can be found in full season and double crop fields in Kent and Sussex counties. Since population levels vary from field to field, the only way to know if you have an economic level will be to scout all fields. When looking at foliage feeding by corn earworm, you will need to use defoliation thresholds to make a treatment decision. Once pods are present, the best approach to making a decision on what threshold to use for corn earworm is to access the Corn Earworm Calculator developed at Virginia Tech (http://www.ipm.vt.edu/cew/) which estimates a threshold based on the actual treatment cost and bushel value you enter.
Be sure to continue to scout for stinkbugs in fields that are in the pod development and pod fill stages. Economic damage is most likely to occur during these stages and a combination of species can be found in fields throughout the state. You will need to sample for both adults and nymphs when making a treatment decision.
We are also getting reports of an increase in soybean aphid populations in a few fields throughout the state. The recent cooler weather pattern favors an increase in populations. The economic threshold for soybean aphid established in the Midwest is 250 aphids per plant. Populations should be increasing and most of the plants should be infested (>80 percent) in order to justify an application. This threshold is appropriate until plants reach mid-seed set (R5.5). Spraying at full seed set (R6) has not produced a consistent yield response in the Midwest. We are also seeing a high number of beneficial insects in fields with soybean aphids (lady beetles and parasitized aphids) so you should also consider beneficial insect activity before making a treatment decision. Most products labeled for soybean aphid will provide effective control.