Agronomic Crop Insects – July 18, 2014

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 dry. 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.”

It is also time to start looking for defoliators in alfalfa. With the higher potential for fall armyworm moving north, be sure to watch for defoliation from fall armyworm, beet armyworm, webworms and corn earworm. No thresholds are available; however, controls should be applied before significant defoliation occurs.

Soybeans
We continue to see a number of defoliators (grasshoppers, Japanese beetles, Oriental beetles, silver spotted skipper, 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.

We are starting to see an increase in stinkbug populations (native green and brown); however, populations are still relatively low. Very few brown marmorated stink bugs (BMSB) have been found except for an occasional nymph on field edges near woods in New Castle County. 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 Mid-Atlantic 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.

Be sure that you continue to sample for mites since economic populations can still be found throughout the state. Early detection and control before populations are exploded is necessary to achieve effective control. In addition, be sure to sample the entire field since economic populations can be found throughout fields and in some cases higher populations and damage are being found in field interiors.

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