As Merle Haggard said in his song, Under the Bridge, “you’ll find some great grub feedin’ here below.” I got a couple calls about grub injury in seedling corn last week and how to identify annual grubs (e.g., masked chafers and Japanese beetle) from true white grubs (May and June beetles) with a longer life cycle.
First, we need to know important body characters to distinguish grubs from other immature insects in the soil. Grubs are immature stage of scarab beetles. They have creamy white or translucent bodies, with a brown head capsule. The tip of the abdomen, known as the raster, is usually dark in color, too. They have three pairs of legs near the head and lack fleshy prolegs. They are always in a c-shape and that behavior usually is unique compared to other soil-dwelling larvae.
To identify different kinds of grubs is a little more difficult. You have to look at the tip of the raster (grub butt) and look for the anal opening shape and also the arrangement of raster hairs. No, I'm not kidding. The anal opening can be shaped like a crescent or like a “Y,” and the raster hairs can be arranged into a crescent, triangle or like a zipper. The zipper can be closed (see photo below) or look like it is opening up. To learn more about grub identification, use these resources developed by Michigan State University and the Ohio State University.
Usually when grubs causing significant injury to corn roots, they are the true white grubs. Injury is more likely in early-planted corn and fields with no-till or reduced tillage practices. I helped write an ICM News article last year that reviewed management options for true white grubs. I encourage you to look for grub feedin' below while assessing plant stand this year.