Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
We continue to see an increase in potato leafhopper populations, both the adult and nymph stages. Remember, the nymphs can quickly cause damage and once yellowing has occurred you have already experienced yield loss. 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. In drought stressed fields, these numbers may need to be reduced by 1/3.
Soybeans
We can find a number of defoliators in full season soybean fields including grasshoppers, green cloverworm, Japanese beetles, oriental beetles and bean leaf beetles. Green cloverworm larvae are light green with three pairs of white stripes running the length of the body. In addition to the three pairs of legs near the head, they have three pairs of fleshy legs near the middle of the body, and one additional pair at the end of the body. Larvae wiggle vigorously when disturbed. Smaller larvae may drop from the leaf when disturbed. Young larvae skeletonize the underside of the leaf. Older larvae chew irregular shaped holes in the leaves and can eat all of the leaf except large veins. Although populations of green cloverworm generally increase in number from July through September, if the weather turns dry, we often see an earlier increase in numbers. Fungal pathogens often crash populations; however, under dry weather conditions this will not occur. As a reminder, double crop soybeans can not tolerate as much defoliation as full season beans so be sure to watch newly emerged fields carefully, especially for grasshoppers.
Some consultants are also seeing an increase in leafhopper populations in seedling stage soybeans. As a general guideline, a control may be needed for leafhoppers if you see plant damage and you find 4 leafhoppers per sweep in stressed fields and 8 per sweep in non-stressed fields.