White grubs in early corn

Some early-season stand loss from white grubs in corn has been reported from scattered locations across Iowa. 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 kill seedling plants by feeding on the roots. They have a 3-year life cycle and can cause stand loss during 2 years of their 3-year cycle.

Annual white grubs have a 1-year life cycle, and the grubs are nearly finished feeding in the spring about the time that corn is planted. I have never been able to confirm that annual white grubs cause stand loss in either Iowa corn or soybean, but University of Illinois entomologists consider them a problem in central and southern parts of Illinois.

True white grub with zipper-like parallel row of hairs on underside of tail.

True white grub raster on left and annual white grub raster on right.

As is the case with many soil insects, it is difficult to predict when and where true white grubs will be found. Problems can be expected in cornfields following pasture or grassy Conservation Reserve Program ground. But stand loss also occurs in continuous corn, and in Iowa the problem is usually, but not always, found adjacent to areas bordered by cottonwoods or willows. Sometimes the grubs are found far from any trees and the reason for their occurrence in a field is often a mystery.

Examining the soil during spring tillage, especially near these wooded areas, may reveal white grubs. If grubs are found, collect and correctly identify them to determine their potential for economic damage to corn. Both kinds of white grubs are C-shaped, creamy white, and covered with tiny bristles. True white grubs can be separated from annual white grubs by examining the pattern of hairs on the raster (the bellyside of the last tail segment). The raster of the true white grub has a narrow, smooth space with two rows of parallel bristles (patterned like a zipper). Also, there are many scattered bristles on either side of the zipper. Annual white grubs have scattered bristles on the raster, but no distinct pattern like the zipper.

Entomologists at North Dakota State University have estimated that one or more true white grubs per cubic foot of soil can cause stand loss in seedling corn. This threshold is also reasonable for Iowa. There are no rescue treatments after true white grub damage occurs. If a corn stand is severely damaged and must be replanted, use a soil insecticide that is labeled for white grubs during the replanting.

This article originally appeared on page 84 of the IC-486(10) -- May 21, 2001 issue.

Updated 05/20/2001 - 1:00pm