Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
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. We continue to receive questions about leafhoppers in double crop fields. As a general guideline, a treatment may be needed of you find 4 leafhoppers per sweep. However, this is just a general guideline and the treatment threshold may need to be reduced if damage is present and plant growth is being held back by feeding. Also, there are no established thresholds for the number of leafhoppers per leaflet.
You should also scout for stinkbugs and pods worms as we enter the pod set and pod fill stages. Open canopy blooming soybeans will be attractive to egg laying moths, especially in drought stressed areas where corn will dry down early. Corn earworm trap catches have started to increase; however, only time will tell if this will translate into a major podworm outbreak in soybeans. 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. A treatment should be considered if you find 3 podworms per 25 sweeps in narrow row fields and 5 podworms per 25 sweeps in wide row fields (20 inches or greater).
We can now find soybean aphids in fields throughout the state. In general, populations are still low with the highest populations (an average of 40-50 per plant) in a few New Castle County fields. The treatment threshold established in the Midwest is 250 aphids per plant from R-1 (presence of first flowers) 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. 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.