Agronomic Crop Scouting Report

David Owens, Extension Entomologist,

Western corn rootworm adults are active and worth scouting for if planting corn in the same field as this year’s, and especially in western Kent or New Castle County where the ground is heavier. Threshold for next year management (Bt variety with rootworm traits or soil insecticide application) is 1 beetle per plant. I was asked recently what I thought of insecticides tank mixed with a fungicide application. Threshold for stink bugs are 10 bugs per 100 plants for tassel push or tasseling plants but go up to 28 bugs per 100 plants during the early silk stages. Most stink bugs in corn tend to be brown stink bugs which are difficult to kill with pyrethroids. Of them, bifenthrin does the best job, but is also a bit more expensive. In the absence of scouting, a prophylactic tank mix may be of some benefit around edges only, but I don’t see it as paying for itself in field interiors. Of course, this is difficult to manage if a field is being treated by plan.

Dectes stem borer is active. We have hit 1500-degree days which some folks have suggested as a time to apply an insecticide. I DO NOT recommend insecticide for Dectes based on a lot of work that Joanne Whalen performed. There’s a great DSB fact sheet that explains management considerations here:

Reports of spider mite activity and fields treated came in this week. Spider mite thresholds are half of the plants showing some stippling over 1/3rd of leaves. In 2020, we had a mite trial on station in which they caused heavy defoliation in untreated check plots at R2. Yield was affected, but not by nearly as much as one might think. Dimethoate will slow mites down and reduce them temporarily. Abamectin and Etoxazole are the best products and my ‘go-to’ miticides. If considering a mite treatment, these two will take them out and keep them from coming back. There are generics available.