Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
Continue to scout fields on a weekly basis for leafhoppers. It is also time to start watching for defoliators in alfalfa, including grasshoppers, corn earworm, webworms and beet armyworm.
Field Corn
Although we are aware that spider mite populations are heavy in a number of fields, including irrigated fields, we are extremely limited in available control options for our area; the control options available have rotational restrictions and REIs that limit their use in our area; we do not have experience with efficacy of labeled products on field corn in our area; and mites are found on the undersurface of the leaves so contacting them can also be an issue. Information from other areas of the country indicates that they can cause yield loss and controls may be effective if applied before mites move above the ear leaf. However, we do not have experience with controlling mites in field corn and I can not find any good recommendations/studies on mite management in field corn. At this point, maintaining a good irrigation schedule to reduce plant stress will be extremely critical.
I also have some folks asking about Western Bean Cutworm in Delaware. We are running 2 traps (one north and one south) and so far no moths have been detected. If you want more information on this pest, please follow the following link from Ohio: http://corn.osu.edu/c.o.r.n.-newsletter#2
Soybeans
We are starting to see an increase in the levels of bean leaf beetles and green cloverworm (some consultants are reporting a new hatch of small larvae) in full season soybeans. When green cloverworm are small, they produce damage that appears as a “window pane” on the leaves. In double crop soybeans, grasshoppers are the predominant defoliator present at this time. Remember, at the bloom to pod fill stage in full season soybeans, the defoliation threshold drops to 10-15% defoliation. Double crop soybeans can not handle as much defoliation as full season fields at the pre-bloom or pod-fill stages. In addition to defoliation, bean leaf beetles can also feed on pods. Bean leaf beetles can clip pods or plant diseases may enter the pod through their feeding sites. This can result in seeds that appear shrunken, discolored, and moldy resulting in a reduction in seed quality. Although we have not established thresholds for pod feeding in our area, the following link provides information that is used in the Midwest: http://www.ipm.iastate.edu/ipm/icm/2000/8-21-2000/lblroof.html. When possible, a material with residual control should be used for bean leaf beetle control.
Even after the recent rains, economic levels of spider mites continue to be found in both irrigated and dry land fields throughout the state. Heavy rains last week have removed a lot of stress on the plant but you need to make sure that ongoing spider mite problems don’t continue to rob yields. It is important to continue to scout the entire field for mites since in many cases we continue to find hot spots throughout fields and edge treatments will not be effective. As a reminder, under heavy mite pressure and extended hot, dry weather, it often takes an extended periods of free moisture on leaves, high humidity during the day and cool evening temperatures to get an increase in the fungal pathogens that can significantly reduce exploded mite populations. Although dimethoate is labeled for mite control in soybeans, growers and consultants are reporting poor control with dimethoate, even in irrigated fields. The bifenthrin products (such as Sniper and Brigade), Hero ( a combination of bifenthrin and zeta-cypermethrin) and Lorsban ( chlorpyrifos) have provided effective control/suppression. If egg populations are high at the time of application, a second application will mostly likely be needed. 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. Lastly, be sure to consult your crop insurance provider regarding their rulings this year regarding the need to make an attempt to control mites under drought stress conditions.
You should also scout for stinkbugs and pods worms as we enter the pod set and pod fill stages. As corn earworm trap catches increase, open canopy blooming fields will be attractive to egg laying earworm 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.
There have also been reports of economic levels of beet armyworm in soybeans fields in Virginia and consultants are starting to see a few in fields in Delaware. When making a decision for beet armyworm, you will need to look at defoliation. Although they can feed on pods when populations are extremely high, at this point the defoliation threshold should be used to make a treatment decision. If beet armyworm is present in the mix, be sure to select a material that will also provide BAW control. Remember, the pyrethroids have not provided effective BAW control in past years.