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 three-year life cycle and can cause stand loss during two years of their three-year cycle. The adult stage is the called the May or June beetle.
Annual white grubs have not caused stand loss in Iowa to either corn or soybeans. They have a one-year life cycle. The grubs are nearly finished feeding in the spring about the time that corn is planted. The adult stage is called the masked chafer.
Like many other soil insects, it is sometimes difficult to predict when and where true white grubs are going to be found. Problems can be expected in corn fields following pasture or grassy CRP ground. But stand loss also occurs in continuous corn. Here the problem is usually found adjacent to areas bordered by cottonwood or willow trees. The beetles feed on the leaves of these trees at night, then fly down to the ground to lay their eggs. 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.
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 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). There are many scattered bristles on either side of the zipper. Annual white grubs have the 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 will cause stand loss in seedling corn. This would be a reasonable threshold to use in Iowa.
Recommendations for 1994 corn are as follows:
In corn following corn, a soil insecticide is not recommended for true white grubs if they were not found prior to planting or if stand loss did not occur last year. In corn following soybeans, the same recommendation applies. In corn following pasture or grassy CRP ground (3 or more years with grass), true white grub problems are more likely to occur, and a soil insecticide is recommended at planting. Soil insecticides are not recommended if only annual white grubs are found.
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 during the replanting.
Granular insecticides labeled for control of true white grubs in corn and rates per 1,000 row feet are: Counter 15G (8 ounces in furrow or t-banded), Counter CR (6 ounces in furrow or t-banded), Force 1.5G (8-10 ounces in furrow or t-banded), Lorsban 15G (8-16 ounces in furrow or 16 ounces t-banded), Thimet 15G (8 ounces t-banded), and Thimet 20G (6 ounces t-banded). Dyfonate II 15G and Dyfonate II 20G are labeled for suppression of white grubs.