Wireworms, seedcorn beetles, slender seedcorn beetles, and seedcorn maggots are minor soil pests that sometimes cause stand loss by feeding on the seed or germinating embryo. Insect damage that occurs below the soil surface is difficult to predict. It is also difficult to determine the need for a soil insecticide or seed treatment.
A decision to treat is made even more difficult because there are no rescue treatments for these insects. Therefore, insecticides must be applied at planting time if economic damage is anticipated based upon field history, previous crop or cover (such as CRP), heavy manuring during the spring, or if cool weather conditions are anticipated to delay germination. After the seed is planted, the chance of insect damage is greater the longer germination is delayed.
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 the seed before or during germination and hollow it out. Death of seedling plants also can occur when wireworms tunnel into the seedling stalk. Sometimes wireworms bore into the stalks of larger plants and tunnel several inches above the soil surface.
Seedcorn maggots are occasional pests of both germinating corn and soybean seed that cause reductions in yield because of stand loss. The greatest potential for injury to seeds is when animal manure is spread on the soil or when live, green plants are killed in the spring prior to planting. The adult seedcorn maggot (a fly) is attracted to these areas of rotting vegetation to lay her eggs. Damage is more likely to occur during a cool, wet spring when germination is delayed. Fields that only have last years crop residue on the soil surface or are no-till fields should not have problems from seedcorn maggots. Germinating seeds alone are not sufficient to attract large populations of seedcorn maggots.
Seedcorn beetles and slender seedcorn beetles mostly feed on other insects and rarely attack seeds. They are occasionally found under the same conditions and in the same areas that are infested with wireworms.
Recommendations for 1994 field corn are as follows:
If large populations of wireworms are present (based on problems during previous years or following grassy field conditions), or if corn rootworm or white grub damage is expected, a seed treatment will not provide protection. Apply a soil insecticide in furrow or t-banded.
If small populations of wireworms are expected, a seed treatment containing both diazinon or lindane (such as Agrox D-L Plus or Kernel Guard) should provide protection. This also will protect against seedcorn maggots, seedcorn beetles, and slender seedcorn beetles.
If a cover crop is killed in the spring or animal manure is spread on the field, and the seedcorn maggot is the only insect of concern, use a seed treatment on corn and soybeans. One packet of seed treatment (1.8 ounces of insecticide) will treat 50 pounds of seed. Read and follow all label directions before using any insecticide.