David Owens, Extension Entomologist, owensd@udel.edu
Corn
Small grain harvest has begun, and with it, the potential to flush brown stink bugs out and into any adjacent corn. Also check near woodlines in fields that were soybeans late last year, because those Fall stink bugs overwinter in woods. Be sure to scout any corn younger than V6 and then again between V14-VT. In between these stages, stink bugs do not cause damage. North Carolina and Virginia revised threshold guidance last year given research efforts from 2016 onward. You do not need to treat if you have less than 4 bugs per 100 plants, and a treatment is advisable when there are greater than 10 stink bugs per 100 plants. You can find the full threshold table here: https://corn.ces.ncsu.edu/stink-bug-management-in-corn/. Grasshoppers are hatching out. A suggested threshold from Purdue is 3 grasshoppers per square yard.
Soybean
Active defoliators include bean leaf beetle, bean leafroller, grasshoppers and thrips. Thrips injury shows up as darkened or silvered leaf veins. Thrips pressure this year has been a bit higher, in part due to dry May weather. Often in soybean, thrips populations are transient. There are no thresholds for soybean in the mid-Atlantic. Unless seedling beans are drought stressed, not growing, and young growth is crinkled, do not worry about them. A report came in this week of spider mites beginning to make an appearance around field edges. Spider mites typically show up around the first week of June and build up to treatable levels in July. Their populations typically crash around August 10-15. In soybean, we have 5 options if a treatment is advisable (20 per leaflet, yellowing and defoliation beginning): Lorsban, Dimethoate, Bifenthrin, Agri-Mek SC, and Zeal. With dimethoate, we need soil moisture to help move that product into the leaf tissue. It is also sensitive to water quality. With dimethoate, Lorsban, and bifenthrin, I think of these products as 2-spray materials. They stunt spider mite populations briefly, but they quickly rebound because the products have short lived residual and do not kill eggs. Agri-Mek and Zeal in last year’s spray trials performed excellently at mid-rates. We have a spider mite fact sheet that hopefully is a useful resource for both field and vegetable crops: https://www.udel.edu/academics/colleges/canr/cooperative-extension/fact-sheets/two-spotted-spider-mite/.