Joanne Whalen, Extension IPM Specialist;jwhalen@udel.edu
Alfalfa and Grass Hay Crops
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
We are starting to see and hear reports of an increase in the presence of defoliators (especially Japanese beetles, grasshoppers, silver spotted skipper, green cloverworm and isolated spots of fall, yellow and beet armyworm) in both full season and double crop soybeans. As a general guideline, treatment decisions for defoliators should be based on the following defoliation thresholds:
(a) Full Season Plantings – 30% defoliation pre-bloom; 15% defoliation from bloom through the end of pod fill; 35% – once fully developed seeds are present
(b) Double Crop Plantings (especially if growth is poor) – 20% defoliation pre-bloom, 10% defoliation from bloom through pod fill; 15% defoliation – once fully developed seeds are present.
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. Brown marmorated stink bug populations still remain low and are only being found along field edges that border woods in New Castle County.
As far as corn earworm, we continue to find very low levels of small larvae, mainly in double crop fields. The results of the annual corn earworm survey in field corn in Virginia, which has been used as an indicator of the potential for corn earworm in soybeans, show that the populations are similar to 2013 but lower compared to the 2009 through 2012 survey results. (http://blogs.ext.vt.edu/ag-pest-advisory/). We also continue to see lower corn earworm trap catches in many locations throughout the state. However, if the weather changes this could change as well. Therefore, it will be important to check fields for corn earworm on a weekly basis.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 and 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.