Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
Continue to scout fields on a weekly basis for leafhoppers. We have also received reports of increased levels of pea aphids and thrips in some alfalfa fields. As a general guideline for pea aphid management, you should consider a treatment in alfalfa less than 10 inches tall if you find 40 aphids per stem. The treatment threshold for alfalfa 10 inches or taller in height is 75-100 per stem. Beneficial insects can help to crash aphid populations; however, as a general rule, you need one beneficial insect per every 50-100 aphids to help crash populations.
In past years, we have seen increases in thrips during hot, dry weather conditions. Hopefully the recent rains will help plants grow ahead of damage. 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 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, blister beetles, Japanese beetles, bean leaf beetles and green cloverworm, etc.) present in full season and double crop beans. As 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 in full season bean fields so be sure to watch for this insect as the earliest maturing fields begin to set pods. Economic damage 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. We are currently following the same guidelines that are being used in Virginia. Thresholds are also based on numbers of large nymphs and adults, as those are the stages most capable of damaging pods. 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 we have only seen low levels of soybean aphids populations in a few fields in New Castle County, be sure to watch for this insect in full season beans. This aphid is favored by cooler temperatures so we could see population increases if we start to experience more moderate temperatures. The treatment threshold established in the mid-west is 250 aphids per plant from R1 through R5 stage of growth. The following links from the University of Wisconsin provides good information in sampling, stages of soybean growth and development, thresholds and treatment guidelines http://www.plantpath.wisc.edu/soyhealth/aglycine.htm http://www.plantpath.wisc.edu/soyhealth/pdf/aphid_thresholds.pdf