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 increases in thrips during hot, dry weather conditions. Reports from other areas of the country indicate that thrips feeding on developing leaf tissue can cause the leaves to distort as they emerge. Leaves may also be curled, with a cupped or puckered appearance. Since there are no thresholds for thrips in alfalfa and we have limited experience with this pest in Delaware, the following information from other areas of the country may be helpful when considering the need for thrips management: “(a) high populations ofbean 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, blister beetles, Japanese beetles, bean leaf beetles and green cloverworm, etc.) present in full season and double crop 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 stink bug 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. So far, very few brown marmorated stink bugs have been found and in most cases they have only been found 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. This year we will again use the same thresholds for stink bugs that are being used in Virginia and were developed in states to our south. 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.
Spider mites continue to be found in fields throughout the state. The rain last Friday may in some cases help beans to grow and allow treatments to be more effective. If egg populations are high at the time of application, a second application will most likely be needed. Be sure to read all labels carefully since there are restrictions on the number of applications as well as the minimum number of days needed between applications.