Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Field Corn
This week we found a couple of additional fields in New Castle County with high levels of brown marmorated stink bugs (3-4 per plant) on field edges near woods. Adults, nymphs and egg masses could all be found. Populations in field interiors were still generally low (3-5 per 100 plants). Although we can see the beginning of kernel damage, we have also started to find adults moving out of corn into nearby soybeans fields. Once nymphs develop, they too will start moving into nearby soybeans. An edge treatment, if practical, might be considered, if numbers remain high and you can find ear damage. As indicated in last week’s newsletter, demonstrations are being conducted in Maryland to look at the effectiveness of edge treatments so we will have more information after this season.
Soybeans
Continue to watch for defoliators as well as spider mites in both full season and double crop soybean fields. In full season soybeans in the pod fill stage, the defoliation threshold drops to 10-15% defoliation. Remember, double crop soybeans cannot tolerate as much defoliation since they often do not reach the leaf area index needed for maximum yields.
You should also scout for stink bugs and pods worms as we enter the pod set and pod fill stages.
Corn earworm – Open canopy blooming soybeans will be attractive to egg laying corn earworm moths, especially in drought stressed areas where corn is drying down early. Corn earworm trap catches continue to increase in our area as well as areas to our south. The following link provides information on the annual corn earworm survey conducted in Virginia (http://www.sripmc.org/Virginia/downloads/cewsurvey2011.pdf). Since corn is considered a nursery crop for earworm, their data (over 30 years) has shown that there is nearly a 1:1 relationship between the infestation level in corn and the amount of soybean acreage that gets treated with insecticide for corn earworm in Virginia. However, only time will tell if we will have a podworm outbreak in soybeans in our area. Although we are finding a few corn earworms in full season soybeans, this is not unusual for this time of year and only scouting on a routine basis will tell you if you have an economic problem. 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.
Stink bugs – We have started to see an increase in native green and brown stink bug levels in fields throughout the state. Economic damage from stink bugs is most likely to occur during the pod development and pod fill stages. Available thresholds are based on beans that are in the pod development and fill stages. As a general guideline, current thresholds for native stink bugs 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. We do not have a threshold for brown marmorated stink bugs (BMSB). In New Castle County and northern Kent County, we can find a mix of native green and brown stink bugs as well as BMSBs. In situations where BMSBs are being found mainly on field edges, an edge treatment may be considered.