Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
If you have not been scouting fields, be sure to keep a close watch for leafhoppers. Remember, the nymphs can quickly cause damage and once yellowing is present significant damage has already occurred both in season as well as to the long term health of the stand. With the hot, dry weather, you should consider reducing treatment thresholds by at least one-third.
Field Corn
We are starting to see an increase in Japanese beetles and corn rootworm beetles feeding on corn silks. Both of these insects can potentially interfere with pollination. However, silk feeding by these beetles will not reduce pollination if they cut the corn silks after pollination has already taken place. As a general guideline, an insecticide treatment may be needed if two or more Japanese beetles or corn rootworm beetles are present per ear and silks are clipped to less than ½ inch prior to pollen shed.
Soybeans
We continue to see a wide variety of defoliators present in fields including Japanese beetles, green cloverworm, grasshoppers, painted lady caterpillars, blister beetles and silver spotted skipper. The best way to make a treatment decision in full season soybeans is to estimate defoliation. Before bloom, the defoliation threshold is 30%. As full season beans enter the reproductive stages, the threshold drops to 15% defoliation. Remember that double crop soybeans can not tolerate as much defoliation as full season beans so be sure to watch newly emerged fields carefully.
We continue to find economic levels spider mites in full season and double crop soybeans. With the current hot, dry weather, populations are often found field wide so be sure to scout the entire field because edge treatments may not be effective.
We have also found our first soybean aphids in a full season soybean field in New Castle County. Remember that this aphid is favored by cooler temperatures. The treatment threshold established in the Midwest is 250 aphids per plant from R1 through R5 stage of growth. The following links from the University of Wisconsin provides good information in sampling, stages of soybean growth and development, thresholds and treatment guidelines (http://www.plantpath.wisc.edu/soyhealth/aglycine.htm)
(http://www.plantpath.wisc.edu/soyhealth/pdf/aphid_thresholds.pdf)