Corn insects in the soil: wireworms and white grubs

During the next several weeks, farmers will check the soil before and during planting for a variety of reasons. When they find an un-familiar insect, they will wonder if it is a pest. Correct identification of an insect is the first step in determining if there is a potential problem. Descriptions of several pests and of other insects that are often mistaken for pests follow.

Crane fly larvae do not injure corn seeds or seedlings, but they do feed on decaying organic matter in the soil. They could be considered beneficial insects. They are often thought to be cutworms, but they do not have legs. They do have four, soft horns on their tail end, which cutworms dont have, and are light gray or brown without stripes.

Ground beetle larvae are often mistaken for wireworms because they are about the same size and shape. They may be completely black or light-colored with dark, square-shaped spots on their backs. They can be identified by their two large jaws and two soft, slender projections on the tail. Ground beetle larvae can reach a length of 114 inch. Ground beetle larva are beneficial and feed on other insects, including cutworms.

Wireworms. Two groups of wireworms (hard bodied and soft bodied) attack corn seed and occasionally soybean throughout Iowa. They are not serious pests in terms of annual acreage infested, but they occur often enough to cause severe stand loss in some fields. The probability of crop damage from wireworms is low, except where corn follows a grassy situation such as pasture or CRP ground, or where problems have appeared during the past several years. Problems can persist in a corn field because wireworms live for two to six years.

Wireworms damage corn in several ways. Early-season damage occurs when larvae bore into and hollow out the seed before or during germination. Death of seedling plants also can occur when wireworms tunnel into the seedling stalk. Some-times wireworms bore into the stalks of larger plants and tunnel several inches above the soil surface.

White grubs. 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 the two years of their three year cycle.

Annual white grubs have not caused stand loss in Iowa to either corn or soybeans. They have a one-year life cycle, and the grubs are nearly finished feeding in the spring about the time that corn is planted.

As is the case with many other soil insects, it is difficult to predict when and where true white grubs will be found. Problems can be expected in corn fields following pasture or grassy CRP ground. But stand loss also occurs in continuous corn, and in Iowa the problem is usually found adjacent to areas bordered by cottonwood or willow trees. 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). Also, 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 is a reasonable threshold to use in Iowa.

Recommendations for 1995 corn are as follows:

  • In corn following corn, a soil insecticide is not recommended for true white grubs or wireworms if they were not found prior to planting or if stand loss did not occur last year.
  • In corn following soybeans, follow the recommendation above.
  • In corn following pasture, sod, or grassy CRP ground, true white grubs and wireworms 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 or wireworm damage occurs. If a corn stand is severely damaged and must be replanted, use a soil insecticide during the replanting. Labeled granular insecticide are listed. Read and follow all label directions.

Updated 04/13/1996 - 1:00pm