Agronomic Crop Insects – July 8, 2011

Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu

Alfalfa
If you have not been scouting fields, be sure to keep a close watch for leafhoppers. Remember, the nymphs can quickly cause damage and once yellowing is present significant damage has already occurred both in season as well as to the long term health of the stand. With the hot, dry weather, you should consider reducing treatment thresholds by at least one third.

Field Corn
We are starting to see an increase in beetles feeding on corn silks. Although beetles feeding on silks can potentially interfere with pollination, research from the Midwest indicates that silk feeding does not reduce pollination if they cut the corn silks after pollination has already taken place. As a general guideline, an insecticide treatment may be needed if two or more Japanese beetles or corn rootworm beetles are present per ear and silks are clipped to less than 1/2 inch prior to pollen shed.

Soybeans
We continue to see a wide variety of defoliators present in full season soybeans. The best way to make a treatment decision in full season soybeans is to estimate defoliation. Before bloom, the defoliation threshold is 30%. As full season beans enter the reproductive stages, the threshold drops to 15% defoliation. Remember that double crop soybeans can not tolerate as much defoliation as full season beans so be sure to watch newly emerged fields carefully.

Continue to scout for spider mites in full season and double crop soybeans. With the current hot, dry weather, economic populations are being found field wide so be sure to scout the entire field because edge treatments may not be effective.

As full season beans enter the flowering stage, we are starting to see low levels of stink bugs (mostly green). The first brown marmorated stink bugs have also been detected by consultants on field edges. 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. This year we will again use the same thresholds for stink bugs that are being used in Virginia and were developed in states to our south. 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.

This year we are participating in a regional project to identify the distribution of another new potential pest of soybeans in the U.S., the Trochanter Mealybug. This insect feeds on the roots of soybean plants. In the few states where it has been found, the fields looked like they had potassium deficiency-like symptoms (e.g., yellowed leaf margins and stunted plants). See attached photos 1 and 2 of damage symptoms and mealybugs on roots from Ron Hammond, OSU.

Photo 1. Leaf symptoms associated with Trochanter mealybug

Photo 2. Trochanter mealybug on roots

If you see plants with these symptoms, you should carefully dig up the roots and look for the presence of mealybugs. If you find a suspect field, please contact Joanne Whalen (jwhalen@udel.edu) since we would like to verify presence and distribution in the state.

 

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