David Owens, Extension Entomologist, owensd@udel.edu
In general, rates of wireworm and white grub injury are lower now than they have been historically due to good weed control. However, last year there were more weedy fields than usual due to weather interference with herbicide programs. These weeds are attractive to egg-laying adult wireworms and white grubs. The eggs hatch in the late summer and larvae feed on plant roots and organic matter until late fall when they move deep into the soil to avoid freezing. As the soil warms up between the end of March and mid-April, wireworms and white grubs will start moving to the surface. You can sample for them using a couple of different methods. The first method is useful primarily for wireworm sampling and involves baiting a field for 2-3 weeks. Place a half cup of wheat and corn seed in a shallow hole about 4 inches deep and 9 inches wide, cover the seed back up, and secure a piece of black plastic like a garbage bag over the soil. The plastic helps warm the soil, the seed will germinate, and wireworms will come to it. If 1 or more wireworms are found per bait station, a soil insecticide should be used. The second method is the compact soil sample, useful for both grubs and wireworms. This involves digging a hole 8 inches x 8 inches x 6 inches deep. Consider a treatment if you find more than 1 grub per field. Ideally, 1-2 bait stations per acre or 5 – 10 soil samples per field will provide good sampling confidence. Efficacy ratings for various seed treatments can be found here: https://blogs.ext.vt.edu/ag-pest-advisory/files/2014/10/Field-corn-insecticide-seed-treatment-chart.pdf