Agronomic Crop Insects – July 19, 2013

July 19, 2013 in Agronomic Crops

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

Alfalfa
Continue to scout fields on a weekly basis for leafhoppers. In past years, we have also seen an increase in thrips when weather conditions turn hot and drier. So far populations are low but that could change if the recent hot weather continues. Reports from other areas of the country indicate that thrips feeding on the developing leaf tissue can cause the leaves to distort as they emerge. Leaves may also be curled, with a cupped or puckered appearance. Although there are no thresholds for thrips in alfalfa, the following information from other areas of the country may be helpful when considering the need for thrips management: “(a) high populations of bean or onion thrips may cause damage, especially in dryland conditions and (b) if a thrips treatment is contemplated, it is best to cut as soon as possible and treat the regrowth if the infestation persists. Thrips are very difficult to control in alfalfa, so excellent coverage is important and two applications may be required for satisfactory results.”

Soybeans
We continue to see a number of defoliators (grasshoppers, Japanese beetles, bean leaf beetles and green cloverworm) present in full season beans. As full season fields enter the bloom to pod fill stages, remember that the threshold drops to 15% defoliation. When it comes to bean leaf beetle, in areas of the state where we have seen problems in the past, we are seeing an increase in populations. Although first generation populations may not cause economic defoliation, they can be a useful predictor of the second generation. Bean leaf beetle feeding on soybean pods caused by the second generation can lead to significant reductions in seed quality. To help make treatment decisions easier for first and second generation bean leaf beetles, a dynamic Excel spreadsheet has been created by entomologists at Iowa State University that might be helpful in assisting with decisions in our area since no local thresholds are available. Please see the following link for more information –

http://www.extension.iastate.edu/CropNews/2011/0815hodgson.htm .

We are starting to see an increase in stinkbug populations (green and brown) in full season bean fields so be sure to watch for this insect as the earliest maturing fields begin to set pods. Very few brown marmorated stink bugs have been found but we have found them on field edges near woods. 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, current thresholds are set at 1 large nymph/adult (either brown or green stink bug) per row foot if using a beat sheet, or, 2.5 per 15 sweeps in narrow-row beans, or 3.5 per 15 sweeps in wide-row beans.

Although the wet weather has kept spider mites populations in check we do find an occasional mite in a field. Be sure that you continue to sample for mites in your routine sampling each week. Early detection and control before populations are exploded is necessary to achieve effective control.

Lastly, although we have not seen soybean aphid problems in recent years be sure to watch for this insect while checking for other insect pests. Reports from the Midwest indicate that aphids were found earlier this year and weather conditions are more favorable for population increases this year.