Thanks to ISUEO Field Agronomist Meaghan Anderson and Extension Entomologist Ashley Dean, I had a great field scouting adventure in central Iowa. First visiting the field for poor germination and other issues, Meaghan was digging in the soil in search of more information and found grubs! So of course, she invited Ashley and me for a closer look. I have been hearing about grubs in cornfields, particularly north of I-80; for about a week. With people assessing germination and checking stand counts, it is also a good idea to look for early-season pest activity --- like grubs!
It was easy to find the grubs, as we were digging in the furrow of rows, about 2-3 inches below the soil surface. Most of the grubs were small, less than ½-inch in length. My best guess is that there were 1-2 grubs per square feet. We did not note any root pruning or obvious signs of feeding injury from the grubs, but also unclear what they might have been eating. It was difficult for me to make a species identification in the field, since it is based on raster hairs on the tip of the abdomen. If you really want to know what species you have, refer to this online guide to scarab rasters.
Most of the scarab beetles are not field crop pests. Japanese beetle would be the most prominent scarab pest in Iowa. I would expect Japanese beetle grubs to be larger or even pupated by now. The size of grubs we found yesterday makes me wonder if it was the true white grub with a multi-year life cycle, which eventually turns into May/June beetles. Read more about true white grub management here.