Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa and Grass Hay Crops
As indicated in last week’s newsletter, you need to continue to watch for defoliators in grass hay crops and alfalfa. Significant damage can occur in grass hay fields from true armyworm and fall armyworm. It is important to catch populations before significant damage has occurred and when larvae are small. In addition to checking labels for rates, be sure to check for all restrictions including but not limited to comments on control under high populations and size of larvae; days to harvest and forage/silage restrictions. No thresholds are available; however, controls should be applied before significant defoliation occurs.
Soybeans
Continue to watch for an increase in stinkbug populations. Economic damage from stink bugs is most likely to occur during the pod development and pod fill stages. You will need to sample for both adults and nymphs when making a treatment decision. Available thresholds are based on beans that are in the pod development and fill stages. As a general guideline, current thresholds are set at 1 large nymph/adult (either brown or green stink bug) per row foot if using a beat sheet, or, 2.5 per 15 sweeps in narrow-row beans, or 3.5 per 15 sweeps in wide-row beans. Although we do not have thresholds for brown marmorated stink bugs (BMSB), we continue to use this threshold for the stink bug complex including BMSB.
As far as corn earworm, corn surveys in states to our south continue to indicate that the wet season could result in lower populations in soybeans this year. However, if the weather changes this could change as well. So be sure to check fields for corn earworm. In the past, we have used the treatment threshold of 3 corn earworms per 25 sweeps in narrow fields and 5 corn earworms per 25 sweeps in wide row fields (20 inches or greater). These are static thresholds that were calculated for a 10-year average soybean bushel value of $6.28. A better approach to determining a threshold is to access the Corn Earworm Calculator (http://www.ipm.vt.edu/cew/) which estimates a threshold based on the actual treatment cost and bushel value you enter.