Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
Continue to sample for potato leafhoppers on a weekly basis. 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.
Field Corn
Although we continue to find Japanese beetles and brown stink bugs in silking corn, population levels are variable throughout the state. The following are general guidelines for management of these two insect pests in silk stage field corn:
(a) Japanese Beetle – Treatment may be needed if silks are clipped back to less than ½ inch before 50% pollination and beetles are present and actively feeding. Pollen shed for an individual tassel generally takes 2-7 days to complete and 1-2 weeks for an entire field (information from Bob Nielson, Purdue University).
(b)Stink Bugs – During the pollination to blister stages, stink bugs can feed through the husk and damage individual kernels. Although we do not have thresholds for our area, information developed in states to our south can be used to make a treatment decision. From the end of pollen shed to blister/milk stage, the threshold used in the south is one stink bug for every two plants (50% infested plants). Please refer to the following link for more information on stink bug management in field corn http://entomology.ces.ncsu.edu/2014/07/stink-bugs-in-corn/.
Soybeans
We continue to find a variety of defoliators in soybean fields including grasshoppers, green cloverworm, silver spotted skipper, oriental beetles and Japanese beetles. In general, a treatment decision should be based on percent defoliation. Before bloom, the defoliation threshold in full season soybeans is 30% defoliation. Once fields reach the bloom stage, this threshold decreases to 15% defoliation.
Low levels of spider mites and thrips are also present in fields. Although no precise thresholds are available for thrips, as a general guideline, treatment may be needed if you find 4-8 thrips per leaflet and plant damage is observed. Although spider mite populations can start on field edges, be sure to also watch for hot spots of activity in field interiors. Early detection and control is needed for spider mite management.
We are starting to see an increase in stinkbug populations (native green and brown); however, populations are still relatively low. 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