Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
Continue to sample for potato leafhoppers on a weekly basis. We continue to see a few fields with yellowing. 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
As indicated in last week’s newsletter, the two most common insects being found in silk stage corn are Japanese beetles and brown stink bugs. 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 half an 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
Continue to watch for spider mites, thrips, potato leafhoppers, and defoliators (green cloverworm, bean leaf beetles, grass hoppers, etc.). Another group of defoliators, the Japanese beetle and other look-a-like beetles all in the same insect family, are also starting to show up in soybeans. One defoliator that has been present in more fields over the past two seasons is the silver spotted skipper. This insect caused significant losses in the 1980s when its host range shifted from leguminous shrubs and trees to soybeans. We have seen an occasional field with economic levels of defoliation from this insect in recent years so it should also be considered when sampling for defoliators. For more information and pictures of the silver spotted skipper please visit the following link:
Regardless of which defoliator is present, the treatment decision for defoliators is based on the amount of defoliation. Before bloom in full season soybeans, the threshold is 30% defoliation. Once fields reach the bloom stage, this threshold decreases to 15% defoliation.