Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
Continue to sample for potato leafhoppers on a weekly basis. 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 insect pests as well as spider mites feeding on soybeans. A number of defoliators can be found including grasshoppers, green cloverworm, velvet bean caterpillar and Japanese beetles. In some fields, growers and consultants are reporting significant defoliation from green cloverworm (over 30% in some fields). 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. There are also velvet bean caterpillars present in fields. This insect is a migratory pest and has four pairs of fleshy prolegs which will help to distinguish it from the green cloverworm. Large larvae can also consume the entire leaf, except for the leaf veins. Please follow this link for pictures of both larvae (http://www.ag.ndsu.nodak.edu/aginfo/entomology/ndsucpr/Years/2007/august/16/ent_16aug07.htm).
Although populations of green cloverworm generally increase in number from July through September, the dry weather may have resulted in earlier than normal populations. In addition, fungal pathogens often crash populations; however, under dry weather conditions this will not occur. We are just starting to see an increase in Japanese beetles so be sure to watch carefully for them in the next week. With the continued dry conditions, as well as increased value of soybeans, you should consider using a lower defoliation threshold to make a treatment decision. As a general guideline, you may want to reduce thresholds by at least 1/3. In addition, remember that double crop soybeans can not tolerate as much defoliation as full season beans so be sure to watch newly emerged fields carefully.
We continue to find mites in a number of fields and economic levels can now be detected. Control options include dimethoate, Lorsban (chlorpyrifos) and Hero. Remember, with all of these products early detection will be needed to achieve control.
We have also started our soybean aphid survey and no aphids have been detected so far. In states where aphids overwinter (remember they are still considered a migratory pest for us) either no or very low levels of aphids have been detected. A combination of low aphid overwintering plus numerous beneficial insects are combining to keep aphids at low numbers in areas to our north and west. As always, the situation can change quickly so we will keep you updated throughout the season.