This week, a couple Iowa crop consultants reported grubs are easy to find this spring. I assume they are digging in crop soils to evaluate seed germination. Last year, there was some grub feeding injury in seedling corn and so it is a good time to look for seed and seedling injury around the state. Learning how to distinguish annual grubs (e.g., masked chafers and Japanese beetle) from true white grubs (May and June beetles) with a longer life cycle is important.
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 (aka, the grub butt) and look for the anal opening shape and also the arrangement of raster hairs. 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.
Grubs are more common to see feeding during the seed and seedling stages. Scouting is recommended to determine stand loss. 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. Other great identification resources include the MSU grub ID guide and OSU grub ID guide.