Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
We have seen an increase in potato leafhopper populations this past week so be sure to sample fields for adults and nymphs. 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.
Soybeans
We can find a number of defoliators in full season soybean fields including grasshoppers, green cloverworm, painted ladies, oriental beetles and bean leaf beetles. The predominate defoliator in many fields is the 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 could 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.
We are also seeing an increase in thrips and leafhopper populations in seedling stage soybeans. Although no specific thresholds are available, information from the Midwest indicates that controls may be needed for thrips if you find 8 per leaflet and plant damage is occurring. You will want to reduce this threshold to 4 per leaflet if plants are stressed. 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.
We have also started our soybean aphid survey in all three counties. No aphids have been detected so far. In states where aphids overwinter in the Midwest (remember they are still considered a migratory pest for us) they are reporting levels that are similar to populations found during outbreak years. As a reminder, we see more problems with soybean aphids during cool growing seasons.