Insect damage that occurs below the soil surface is difficult to predict, and it is equally difficult evaluating the need for an insecticide. Wireworms, seedcorn beetles, slender seedcorn beetles, and seedcorn maggots are minor soil pests that occasionally cause stand loss by feeding on the seed or germinating embryo.
Determining the economic feasibility of applying a soil insecticide specifically for a minor soil pest is made even more difficult because no rescue treatments are available for these insects. Therefore, control measures must be applied at planting time if economic damage is anticipated based upon past field history, farming practices, or cool weather conditions that result in delayed germination. After the seed is planted, the chance of insect damage is greater the longer germination is delayed.
Wireworms infest corn and occasionally soybean fields and are found throughout Iowa. They are not considered to be serious pests in terms of annual acreage infested, but they occur often enough to cause severe stand losses in localized areas. Overall, the probability of any field suffering crop damage from wireworms is very low, except where the crop follows pasture or where problems have appeared during the past several years.
Wireworms damage corn in several ways. Early-season injury is caused by larvae boring into the seed before or during germination and hollowing it out. Death and injury of seedling plants also can occur when larvae tunnel into the base of the plant below the soil line. Sometimes wireworms bore into the stalks of large plants and tunnel several inches above the soil surface.
Seedcorn maggots are occasional pests of both germinating corn and soybean seeds, and cause reductions in yield because of stand loss. They are typically more of a problem in soybeans than corn and damage is more likely to occur during a cool, wet spring. With the adoption of conservation tillage practices, it was believed that seedcorn maggot injury would increase, but this has been shown not to be the trend. The greatest potential for injury to corn and soybean is when live, green organic matter or animal manure is incorporated into the soil in the spring. During the spring, egg-laying flies are attracted to soil containing green organic matter. The damage potential is minimal in no-tillage fields, and germinating seeds alone are not sufficient to attract large populations of egg-laying flies.
Seedcorn beetles and slender seedcorn beetles primarily feed on other insects and rarely attack seeds. They are occasionally found during the same conditions and in the same areas that are infested with wireworms or seedcorn maggots.
Recommendations for 1993 are as follows: A seed treatment containing both diazinon and lindane should provide good protection against the seedcorn maggot, seedcorn beetle, slender seedcorn beetle, and small populations of wireworms. However, if large populations of wireworms are present (based on problems during previous years or corn following pasture), or if corn rootworm or white grub damage is expected, then a seed treatment will not provide adequate protection against all of these pests. A soil insecticide will be necessary under these conditions and should be applied in-furrow or t-banded.
If cover crops or green organic matter are plowed under during the spring, apply a seed treatment to corn or soybeans if the seedcorn maggot is the only insect of concern. Fields that have crop residue remaining on the surface or are no-tilled should not have problems from seedcorn maggots. This same recommendation would apply to the seedcorn beetle and slender seedcorn beetle.