Corn Insect Guide

Insects rob Tennessee corn producers of about five percent of their potential yields on an annual basis. However, severe pest infestations can cause complete crop loss. While pesticides play an important role in crop protection, they should be used only when there is the potential for damage severe enough to cause economic loss. There are several cultural practices that can be used to reduce insect problems and minimize pesticide use. Scouting fields for insect infestations and monitoring pest populations with pheromone traps can provide an estimate of insect pressure in a field, and thus, help to guide any treatment decisions.

Prevention

Early Planting: Planting field corn early, during the recommended planting window, will reduce the chances of crop damage from several insect species. For example, corn borers and fall armyworm are frequent pests of late-planted corn in Tennessee.

Weed Control: Certain insects carry (or transmit) virus diseases in corn. By controlling weeds such as Johnsongrass early in the season, the chances of leafhoppers and aphids transmitting viruses to corn are reduced. When planting corn in fields known to be heavily infested with Johnsongrass, choose a hybrid with good tolerance to the Maize Dwarf Mosaic Virus (MDMV) complex.

Tillage: No-tillage production can increase soil insect pest problems in many cases. Cutworms, wireworms, white grubs, seedcorn maggots and lesser cornstalk borers may build up in grass sod or where previous crop residue has been left on the soil surface at planting. Burndown with herbicides well in advance of planting (3-4 weeks) can reduce the risks of infestation. Look for white grubs, wireworms and any other insects that may be exposed during land preparation.

Seed and At-Planting Insecticide Treatments: Almost all seed corn comes treated with insecticide. These insecticides will control or suppress a number of seed and seedling insect pests. Insecticide seed treatments, specifically clothianidin (e.g., Poncho) and thiamethoxam (e.g., Cruiser), have largely replaced the use of in-furrow insecticides which were applied at planting. However, at-planting insecticides or higher labeled rates of insecticide seed treatments can be used for supplemental control of seed and seedling pests (see Below Ground Pests ).

Bt Corn Traits: Bt corn for the control of corn borers is typically recommended on at least part of a grower’s acreage and particularly in late planted fields. The table below is intended to provide growers with the information needed to help them select among the various Bt trait packages offered by seed distributors. Some trait packages may also express Bt proteins that control western and northern corn rootworm. These are uncommon pests in Tennessee, and Bt traits for corn rootworm control are seldom needed. However, continuous corn production in the same field increases the likelihood of western corn rootworm infestations.

Resistance management guidelines for Bt corn require a producer to plant a refuge of non-Bt corn. Some newer trait packages require a smaller refuge of non-Bt corn. Please refer to the grower licensing agreement and refuge guidelines provided by the company for complete details.

Scouting Corn

Seedling Corn: Check twice weekly for cutworms, seedcorn maggots, armyworms, white grubs and other pests of seedling corn. Walk in a zigzag pattern through the field, checking at least 10 places in the field. Count the number of damaged plants in 10 feet of row. Check at least 100 plants. Look for silken tubes at the bases of plants for lesser corn stalk borers. Plants less than 12 inches tall are most susceptible to injury.

Whorl-Feeding Insects: Corn fields should be checked at least weekly until the crop is mature to determine the presence of insect pests or their damage. Walk in a U-shaped pattern over the field. Sample 10 plants in 10 locations on a weekly basis, but fewer plants can often be checked depending upon pest density. To check for live larvae, cut open at least two (or more) plants in each sample and record the number of larvae.

Look on the undersides of leaves for fall armyworm or corn borer egg masses. Southwestern and European corn borers lay their eggs in an overlapping pattern that appears like small fish scales. However, southwestern corn borer egg masses are usually smaller (2-8 eggs) than those of European corn borer (10 or more eggs). Fall armyworms lay their eggs in clusters of 50 to several hundred on corn leaves and other vegetation.

Silking/Tasseling Stages: Examine plants for European and southwestern corn borers. Look for egg masses or small larvae feeding on the leaves. Corn borers lay their egg masses on the middle third of the plant near the ear zone. Check on the undersides of leaves for these egg masses. Small larvae may be found between ear husks or behind leaf collars. It is important to correctly identify larvae which are found because corn borers, corn earworm and fall armyworm may all be present. Treatment for insect pests during this stage will be more difficult. Insecticidal control for corn borers in tasseling corn is generally not as efficient as for plants in the whorl stage. Small larvae are more easily controlled than larger worms.

Black Light and Pheromone Traps: Black light traps can be used to monitor movement of adult insects. Pheromone (sex-attractant) traps are also used to monitor various insect flights, such as southwestern corn borers. Light or pheromone traps can be used to complement a scouting program. Traps can be used in each county or on individual farms to provide producers with advance warnings of insect infestations.

Relative Efficacy and Refuge Requirements of Selected, Commercially Available Bt Corn Products

* 50% and 20% non-Bt corn refuge requirement in cotton and corn areas, respectively.
** 20% non-Bt corn refuge is required in cotton and corn areas.
*** 20% and 5% refuge requirement in cotton and corn areas, respectively.
**** 5% refuge in bag system in non-cotton areas; a separate 20% non-Bt refuge is required in cotton growing areas.
***** 10% refuge in bag system in non-cotton areas; a separate 20% non-Bt refuge is required in cotton growing areas.

Premixed Insecticide Products

* Caution: When using Cobalt Advanced with ALS herbicides such as Accent, Capreno, Halex GT, Steadfast, Lightning, Option or Resolve, the possibility for plant injury exists. See herbicide label for restrictions.