David Owens, Extension Entomologist; owensd@udel.edu
Soybeans
The usual defoliator suspects are active. Full season beans are beginning to enter the reproductive stages. As a reminder, defoliation thresholds are lower for R-stage soybean. Dectes stem borer continue to emerge from the soil. With the hot, dry weather, especially in the southern part of the state, be on the lookout for spider mites. Action thresholds are a combination of leaf defoliation and mite presence: 10% of plants with 1/3 or more leaf stippling and 20-30 mites per leaflet. Agri-Mek SC is the only Abamectin containing product labeled for use on soybeans. Zeal is another equally effective miticide labeled for spider mites on soybean. Of the pyrethroids, only bifenthrin is labeled for spider mites. If bifenthrin is used, go with its high label rate and in enough water to ensure thorough coverage. Two older insecticides that can be used include dimethoate and Lorsban. Lorsban is not systemic and will remove natural enemies. Scout the field soon after application and be prepared to re-treat. Be sure to follow label restrictions on reapplication intervals, in most cases you will need to switch products. Dimethoate will break down quickly in UV light, and needs to be absorbed by the leaf. Leaf absorption slows down when plants are drought stressed. Dimethoate is also sensitive to water conditions. Joanne wrote a great review of dimethoate and Lorsban in a WCU post on July 2, 2010. It can be found here: https://sites.udel.edu/weeklycropupdate/?p=2206
Alfalfa
Potato leafhopper populations have been heavier this year and across the region. On small alfalfa less than 3 inches, thresholds are 20 per 100 sweeps, increasing to 50 in 4-6 inch alfalfa and 100 per 100 sweeps in 7-11 inch alfalfa.