May 14, 2010

As corn begins to emerge, be alert to the potential damage that may be caused by early season insects.

Most early season insect damage to corn is caused by wireworms, white grubs, or cutworms. These insects often are associated with fields that have been in pasture or CRP where the grasses were allowed to grow for more than one year. It is rare to see these problems in continuous corn, but exceptions happen.

These insects feed underground (wireworms and white grubs), or on or below the soil surface (cutworms), so detection requires observing plant damage and digging in soil around the plant.

Wireworms

Wireworms (Figure 1) are the larvae of click beetles. The adult beetles prefer to lay eggs in grasses and the larvae can remain in that stage for up to six years, depending on the species. Wireworms are our earliest corn pests, as they may feed on the seed before germination, causing reduced plant emergence. Later feeding may kill or stunt small emerged plants.

All wireworm feeding is done underground. Wireworms are white, yellow, orange, or brown with hard shells. They tend to be more numerous in fields that have been in grass or pasture or fields that have had grassy weed problems. Wireworms prefer cooler soil temperatures, under 70°F, so fields that were planted early or have heavy surface residue may be at higher risk than tilled fields.

Treatment. There is no rescue treatment for wireworms, so farmers with a high probability of problems need to use a planting time treatment to prevent stand reduction. The development of seed treatments like Cruiser and Poncho has really reduced the incidence of wireworm damage. They are excellent early season stand protectors. One potential problem with seed treatments is that they are so commonly used that resistance is a possibility. Granular soil insecticides and liquids also work well.

White Grubs

White grubs (Figure 2) are the larvae of May (or June) beetles. They also prefer to feed on grasses and very rarely affect crops other than corn. There are two basic types of grubs. Annual grubs complete their development in one year and are not considered serious pests. Three-year grubs, however, can damage corn severely in the last two years of their larval stage. The larvae overwinter deep in the soil, and as the soil warms they begin feeding on plant roots. Damage to corn may not occur until the corn is in the 2- to 6-leaf stage. This is difficult because up to the time of feeding, the stand may look fine. Often grub damage is near shelter belts where the adults may congregate to feed and mate.

Treatment. Like wireworms, there is no rescue treatment for white grubs. Again, high risk areas need to be treated at planting. Products for white grub control are similar to wireworm control.

Replanting. If wireworm or white grub damage is serious enough to warrant a replant, use planting time treatments, although the odds for damage diminish with the warming of the soil.

Cutworms

Cutworms and other insects may hinder emerging corn plants this spring even if seed was treated with insecticides or Bt corn hybrids were used. High populations of insects may overwhelm the protection method, regardless of whether it was an insecticide applied at planting (liquid, granular or seed treatment) or a Bt corn hybrid.

Also, in some cases products are not labeled for the full spectrum of Nebraska insects. For example, Herculex I Bt corn hybrids list black cutworm on the label, but not other soil cutworm species.

Cutworms can cause serious damage to corn in the first couple weeks after emergence so it is important to scout fields for damage. Several species of cutworms attack corn. The severity and the area affected will vary greatly, depending on species involved, previous crop history, and weather conditions. The black cutworm does not overwinter in Nebraska, and infestations depend on moth movement in southerly winds in the spring.

Fields with winter annual weeds, or abundant crop residue are more attractive to the egg-laying moths in the spring. Other cutworm species (dingy, army, Sandhills) overwinter as partly grown caterpillars. Remember that early detection of a problem is essential because most of the cutting occurs within seven days of plant emergence.

Treatment. Generally, a postemergence "rescue" treatment should be considered if cutting is observed on 5% or more of plants and the worms are one inch or less in length. Rescue treatments are effective in controlling soil cutworms. Ambush 2E, Asana XL, Baythroid, Cobalt, Lorsban 4E, Mustang Max EC, Warrior, Proaxis, Pounce 3.2EC, or other insecticides with similar active ingredients (generics) will give satisfactory control as post-emergence sprays. If soil is dry or crusted, rotary hoeing immediately before or after Lorsban (chlorpyrifos) application may enhance control. The other insecticides are pyrethroids and should not be incorporated.

More Information

• On insecticide products and rates at the UNL Department of Entomology Web site.
• On managing cutworms in UNL Extension NebGuide G1153, Corn Cutworms, available online or at local extension offices.

Keith Jarvi
Extension Educator in Dakota, Dixon, and Thurston Counties
Northeast REC, Norfolk
Bob Wright
Extension Entomologist, Lincoln