Two groups of white grubs are found in Iowa croplands: the true white grub, which can cause significant stand loss in corn, and the annual white grub, which does not cause stand loss.
True white grubs, larvae of May or June beetles (the big brown ones that fly around yard lights during summer evenings), feed on the roots of seedling plants, causing them to wilt and die. They have a three-year life cycle and can cause stand loss during two of these three years.
Annual white grubs do not cause stand loss to corn or soybeans. They have a one-year life cycle and the grubs are nearly finished feeding and ready to transform to adults about the time corn planting occurs.
The occurrence of true white grubs in any field is difficult to predict. Examining the soil surface during spring tillage, especially near wooded areas with willow or cottonwood trees, may provide clues as to the occurrence of white grubs. If grubs are found, they should be collected and correctly identified to determine their potential for economic damage to corn.
White grubs are C-shaped, creamy-white in color, and covered with tiny bristles. True white grubs can be separated from annual white grubs by examining the pattern of bristles on the bellyside of the last tail segment (called the raster). The raster of true white grubs has a narrow, smooth space with two rows of parallel bristles (patterned like a zipper). In addition, there are many scattered bristles on both sides of the "zipper." Annual white grubs have scattered bristles with no "zipper" on the raster. Entomologists at North Dakota State University have estimated that one or more true white grubs per cubic foot of soil will cause significant stand reductions in a seedling crop. This would be a reasonable threshold to use in Iowa.