Agronomic Crop Insects – July 24, 2015

Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu

Alfalfa
Continue to scout fields on a weekly basis for leafhoppers as well as defoliators including corn earworm, webworms, fall armyworm and beet armyworm. Larvae must be small to achieve effective control. Defoliators can be destructive, especially during drought conditions. When defoliators are present, early harvest may eliminate the problem. Although there are no specific thresholds, as a general guideline if the crop is more than 2 weeks from cutting and 25 to 30 percent of the terminals are damaged, treatment is suggested.

Soybeans
We continue to see a mix of defoliators (grasshoppers, Japanese beetles, bean leaf beetles and green cloverworm) in full season and double crop soybeans. Before bloom, the defoliation threshold in full season soybeans is 30% defoliation. The defoliation threshold for full season fields in the bloom to pod fill stages drops to 15% defoliation. Double crop soybeans cannot handle as much defoliation as full season fields at the pre-bloom or pod-fill stages. As a general guideline, the defoliation thresholds for double crop soybeans are 20% prebloom, 10% flowering , and 15% at full seed.

It is also time to start checking for corn earworms in soybeans. We can find eggs and low levels of small larvae in few fields in Kent and Sussex counties. Although our corn earworm trap catches have been generally lower so far this season, we are starting to see an increase in a few locations. When populations are high, corn earworm larvae also feed on soybean leaves so you should include them in the mix when scouting for defoliators. The same defoliation thresholds used for other insects pre- bloom and during pod set would apply to corn earworm.

Although populations are still below threshold levels, we are starting to see an increase in stinkbug populations (native green and brown) in full season soybean fields. Be sure to watch for stink bugs as the earliest maturing fields begin to set and fill pods. Economic damage from stink bugs is most likely to occur during the pod development and pod fill 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. As a general guideline, we are using a new threshold in the MidAtlantic Region — 5 stink bugs per 15 sweeps. This is the threshold for soybeans produced for grain. If you are producing soybeans for seed, the threshold is still 2.5 per 15 sweeps.

We continue to survey for Kudzu Bug as well. At this point, we have not detected this pest in soybeans; however, low population levels have been detected in soybeans in Virginia and North Carolina. We will continue to survey for this insect and let you know if we detect any this season.

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