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. Since the weather has turned warmer and dry, you may need to reduce treatment thresholds by one-third to one half.
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. As far as stinkbugs, we have no thresholds for our area; however, the following is information from Georgia:
“Corn is most sensitive to stinkbug injury during ear elongation before pollen shed. The treatment threshold at this stage is 1 bug per 4 plants (25% infested plants). Once pollination occurs, feeding though the husk causes damage to individual kernels. Kernels are susceptible to damage up until the milk stage (R3) and possibly early dough stage (R4). The threshold at this time is 1 bug per 2 plants (50% infested plants).”
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 cannot tolerate as much defoliation as full season beans so, be sure to watch newly emerged fields carefully.
We are starting to find spider mites in full season soybeans and economic levels are being found in an occasional field. Early detection and control is needed to achieve spider mite suppression. With the current warmer, dry weather, we could see an increase in populations. In addition to dimethoate and Lorsban, we now have Hero (zeta-cypermethrin + bifenthrin) as well as a number of stand alone bifenthrin products (not all are labeled so be sure to check the label) available for spider mite control in soybeans. All of these products need to be applied before mites explode. Be sure to read the labels for use rates and restrictions – there is a limit on the number of applications as well as the time between applications on all of the materials labeled for spider mite control. As a reminder, dimethoate is very susceptible to alkaline hydrolysis (chemical breakdown from high pH water) so pH of water is an important factor to consider with dimethoate applications.
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 so we could see population increases, especially if we continue to experience moderate temperatures. Be sure to be sampling for this pest on a weekly basis. 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 and http://www.plantpath.wisc.edu/soyhealth/pdf/aphid_thresholds.pdf.