Agronomic Crop Insects – July 11, 2014

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
As fields begin to silk, the two most common insects being found 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 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. 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, we continue to find hot spots of activity in field interiors so be sure to scout the entire field to make a treatment decision. Early detection and control is needed for spider mite management.

At this time last year, we had found our first Kudzu bug adults in soybeans. So far this season, we have not found any Kudzu bugs on kudzu patches or in soybeans in our surveys. The preliminary early-season threshold used in the south is 5 bugs per seedling through mid-July. They still indicate that the most important time to control this insect is when the first nymphs are detected, so looking for egg masses will be important as well. In North Carolina and other states to our south, they are using the established threshold of one nymph per sweep (one swoosh of the net) once they reach mid-July. You should go to http://soybeans.ces.ncsu.edu/2014/06/early-season-kudzu-bug-management/ or http://www.kudzubug.org/ for more information on identification and management.

Print Friendly, PDF & Email