David Owens, Extension Entomologist, owensd@udel.edu
Alfalfa
Any alfalfa that was recently cut should be swept for potato leafhopper to ensure that it does not damage regrowth. Recently cut alfalfa less than 6 inches in height has low thresholds, near 1-2 per 10 sweeps.
Soybean
Dectes stem borer has been emerging from the soil. In years that I have monitored for them, their populations have peaked between July 7 and 14. We have passed 1500 degree days and will be at 1800 by mid-week. Kelly Hamby, Alan Leslie, and I put together a Dectes fact sheet that significantly expanded on a Joanne Whalen fact sheet: https://extension.umd.edu/resource/dectes-stem-borer-management-soybeans-fs-1196/. Please note that there are suggestions for insecticide use and decision making in the fact sheet as a matter of literature and label review but, based on a lot of work that Joanne and Bill Cissel did, I do NOT generally recommend an application for this pest. However, read the Decision Making and Management sections of the fact sheet.
Scout for defoliation, particularly on recently planted double crop fields. Virginia reported some spots this week with heavy grasshopper feeding. We may have a bit more grasshopper this year on account of last year’s dry late summer/fall. Asiatic garden beetle is active now, and sporadically causes damage to soybean.
Full season beans should be scouted for stink bug. Stink bugs begin moving into soybean between R2 and R4. Thresholds for seed production are 2 per 15 sweeps, for regular beans 4-5 per 15 sweeps and for Plenish soybean, I suggest (on account of price difference, longer dry period, and not research based) using a threshold of 3.5- 4 bugs per 15 sweeps.
Field Corn
Much of our corn crop is approaching tassel push or is at early silking, V14-R1. Stink bug thresholds are lowest in V-14-VT corn. Look for stink bugs between the leaf above the primary ear and the leaf below the primary ear. Thresholds are based on number of bugs per 100 plants. At late whorl through tassel push, the threshold is 10 bugs per 100 plants, but at R1-R2 (silk to blister), it is 28 bugs per 100 plants. Keep in mind stink bugs tend to be edge species, meaning that if you go in even just a little bit, their numbers tend to drop off quickly. Any corn field adjacent to small grain or headed cover crop should be scouted. Tax ditches and other weedy non-crop habitat area can also harbor stink bugs. It is my firm belief that the vast majority of insecticides tank mixed with fungicides are applied to fields in which stink bugs are not present in numbers great enough to justify treatment and this application is made too late to effect meaningful control. Furthermore, lambda cyhalothrin is an extremely common pyrethroids used, but we have seen in many spray trials it not work as well on brown stink bugs, our most common corn stink bug. IF you have a field that is near threshold and IF that field is going to be treated before silking, consider bifenthrin.