Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
Continue to sample for potato leafhoppers on a weekly basis. We continue to see both the adult and nymph stage. As indicated before, the nymphs can cause damage very quickly so sample fields on a weekly basis for both stages. Once plants are yellow, yield loss has already occurred. The treatment thresholds are 20 per 100 sweeps on alfalfa 3 inches or less in height, 50 per 100 sweeps in 4-6 inch tall alfalfa and 100 per 100 sweeps in 7-11 inch tall alfalfa.
Soybeans
Be sure to sample fields in the seedling stage for bean leaf beetles, slug damage, grasshoppers and thrips. In the earliest planted fields, we can also find green cloverworm activity so be sure to scout soybeans for all of these defoliators.
We have started to find very low levels of spider mites in the earliest planted soybean fields. Although you would not expect to find spider mites under the current weather conditions, it is not unusual for us to start finding mites this early, especially in no-till situations. As we all know from past experience, early detection and control is needed to achieve spider mite suppression. In addition to dimethoate and Lorsban, we now have Hero (zeta-cypermethrin + bifenthrin) as well as a number of stand alone bifenthrin products (not all are labeled so be sure to check the label) available for spider mite control in soybeans. All of these products need to be applied before mites explode. Be sure to read the labels for use rates and restrictions – there is a limit on the number of applications as well as the time between applications on all of the materials labeled for spider mite control.