Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Soybeans
As the potential for late season insect control increases, be sure to check all labels for the days from last application to harvest as well as other restrictions
Continue to scout for soybean aphids, especially if the weather turns cooler. We continue to find very low levels in fields in New Castle County. Although our survey efforts have been concentrated in full season soybeans, you should scout both full season and double crop soybeans for aphids. As a general guideline, treatment is needed through the R-5 stage (seed is 1/8 inch long in the pod of one of the four uppermost nodes on the main stem) of soybean development if economic levels are present. It may also be beneficial to spray through R-6 stage (pods containing a green seed that fills the pod cavity at one of the four uppermost nodes on the main stem) — reports vary as to the benefit of spraying once plants reach the R-6 but in some years and some situations there has been an economic return. Spraying after R-6 stage has not been documented to increase yield in the Midwest. The suggested treatment threshold from the Midwest is still 250 aphids per plant with 80% of the plants infested with aphids. You can also consider using speed scouting to make a treatment decision. Information on how to use speed scouting can be found at:
http://www.nwroc.umn.edu/Cropping_issues/2007/Issue9/07_17_07_no4.htm
or
http://breeze.ag.vt.edu/speedscouting
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. 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. We continue to use the same threshold as in past years. These thresholds are based on numbers of large nymphs and adults since they are the stages most capable of damaging pods. As a general guideline, current thresholds are set at 2.5 per 15 sweeps in narrow-row beans, or 3.5 per 15 sweeps in wide-row beans. Low levels of Brown Marmorated Stink Bugs have been found in soybeans in New Castle County in the Middletown area. This is the first detection in soybeans in Delaware. We will need to watch how this pest develops since it has the potential to be a more aggressive stink bug and more difficult to manage.
As far as green cloverworm, we are starting to see populations begin to crash due to fungal pathogens that attack the larvae. So before making a decision to treat, be sure to look for sign of the disease. However, be sure to watch for other defoliators including grasshoppers, blister beetles and bean leaf beetles. 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 corn earworms (CEW) as we enter the pod set and pod fill stages. We continue to find low levels of corn earworm in full season and now double crop soybeans, so be sure to scout fields on a routine basis. We are starting to receive reports of fields with CEW either just at or above threshold levels in southern Maryland and on Virginia’s eastern shore so you will need to begin sampling fields during the next week. As trap catches increase, open canopy blooming fields will be the most attractive to egg laying moths. However, all fields should be scouted (full season and double crop) because as corn dries down all fields can be attractive to egg laying moths. 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.