Early Season Insect Scouting in Corn

By Laura Flandermeyer and Dane Elmquist

Why scout for insect pests?

On average, we lose 20% of our yield to insect pests. The first step of a successful insect management program is scouting. Understanding if your pests have reached economically damaging levels is important prior to choosing a management strategy. At crop emergence, check for aboveground feeding damage or wilting, and search for plants cut at the base for larval insects.

If you find concerning damage, check around the plant for the culprit. The best way to identify a pest is to catch it in the act. Early season feeding damage by many insect pests may look similar. Correctly identifying the pest and its life stage is important prior to choosing a management strategy.

Economic thresholds refer to the pest density or level of damage at which a control measure is needed to prevent economic loss.

Consult economic thresholds when deciding whether to control an insect pest. Economic thresholds refer to the pest density or level of damage at which a control measure is needed to prevent economic loss. An economic loss would occur when the cost of the insect damage (e.g., yield or quality) exceeds the cost of control.

Beneficial pests and conservation practices

Remember, not all insects are pests! Numerous beneficial insects like pollinators, parasitoids, and generalist predators live in our croplands and provide ecosystem services that are valuable for agricultural production. Spraying a broad-spectrum insecticide may end up hurting your yield!

Conservation cropping practices, like reduced tillage and cover crops, can provide food and habitat for beneficial insects (e.g., ground beetles) in the spring and early in the growing season. Alternatively, cover crops and reduced soil disturbance can also support field crop pests in certain scenarios (e.g., moths, flies, and slugs). As a result, diligent scouting for early-season pests becomes especially important in conservation cropping systems.

Early season insect scouting in corn

Black cutworm scouting

Black cutworm larvae (abbreviated to ‘BCW’) are significant pests for early-stage corn. These larvae are known for their ability to cut off young corn plants at the base, often causing severe damage. BCW larvae are grayish black caterpillars up to 1 ¼” long (Figure 1). The adult moths (Figure 2) overwinter in southern states and migrate to Wisconsin in the spring, often on storm fronts. These flight times are hard to predict.

Use information from Wisconsin’s Department of Agriculture, Trade and Consumer Protection’s (DATCP) Pest Monitoring Network to track the timing of BCW arrival in Wisconsin to optimally time field scouting. Sign up for Insect Pest Text Alerts from the Bick Lab at UW-Madison to be alerted about BCW in your area. BCW has 4 or more generations per year, but the 1st and 2nd generation larvae cause problems in young corn plants.

Small BCW larvae (< 0.5” long) feed in the whorl and scar corn leaves. Larger larvae can cut V1-V2 corn plants at the ground or tunnel into V3-V4 plants below ground (Figure 3). Tunneling can lead to wilting, stunting, discoloration, or plant death. BCW larvae feed at night and will hide in the soil during the day.

Your field might be at higher risk for BCW damage if you:

• Plant corn into soybean, corn or wheat crop residue
• Plant corn after grass cover crops (e.g., cereal rye)
• Have weedy areas or low, wet spots
• Have late-planted corn fields
• Have cool or wet soil conditions

Female moths prefer to lay eggs in crop residue, weedy areas, or low, wet spots in fields. Green cover crops are also attractive egg-laying sites. Timely termination of cover crops can reduce the attractiveness of fields to egg-laying females. Late cover crop termination or “planting green” can potentially create a green bridge where BCW larvae move to corn when the cover crop is terminated. Scouting for BCW is critical when planting corn after a cereal rye cover crop. Cool and wet soil conditions slow seedling growth which increases risk of BCW damage.

Scout for BCW larvae by walking a W-shaped pattern in the field. Count 5 sets of 50 consecutive plants (250 total plants) and record the number of damaged plants. Ideally, you should collect two larvae to determine the instar (stage). Dig up damaged plants to check for larvae in the soil. They are usually within 2” of the plant. Next, determine the instar of the BCW by measuring the width of the head capsule. A key for BCW head capsule sizing can be found in Pest Management in Wisconsin Field Crops (A3646).

The insect’s instar tells you how long the black cutworm will continue to feed and damage corn in your field. For example, a 6th instar BCW will feed for approximately 14 more days and could cut four plants if they are in the 1-leaf stage.

Treatment is suggested for BCW if 2-5% of plants have been cut. It is not recommended to use insecticides as a “preventative approach” for BCW because it is difficult to predict outbreaks and could lead to pesticide overuse. Diligent field scouting and use of a rescue treatment if necessary is more economical and reliable. Spot treatment should be considered if damage is isolated in the field.

True armyworm scouting

True armyworm larvae (abbreviated to ‘TAW’) are above ground corn pests. TAW larvae are brownish-green, hairless caterpillars up to 2” long with dark and light stripes down their backs (Figure 4). The larvae have pale brown heads with dark markings. Like BCW, the adult moths overwinter in southern states and migrate to Wisconsin in the spring (Figure 5).

Use information from Wisconsin’s DATCP Pest Monitoring Network to track the timing of TAW arrival in Wisconsin to optimally time field scouting. Sign up for Insect Pest Text Alerts from the Bick Lab at UW-Madison to be alerted about TAW in your area.

In April or May, TAW moths arrive in Wisconsin and lay rows and clusters of greenish-white eggs in lush, green grasses at night. We have already seen the first 2025 flight of TAW in Wisconsin. The eggs hatch in 7 to 10 days. The small larvae feed on the grasses at night or on cloudy days and tend to hide in the soil or under foliage during the day. There are 2-3 generations per year in Wisconsin. The first generation is usually small but can still cause damage. The larger second generation of larvae occurs in July.

TAW larvae feed on foliage of corn plants and cause ragged tears. In severe cases, larvae will strip leaves (Figure 6).

Your field might be at higher risk for TAW damage if you:

• Plant corn after grass cover crops (e.g., cereal rye)
• Plant corn into a no-tilled alfalfa field
• Have grassy, weedy areas in the field
• Have cool or wet soil conditions

TAWs can migrate from pastures and small grains and invade corn fields. Damage is usually highest along the field edge or near grassy spots where female moths prefer to lay eggs. Green cover crops are also attractive egg-laying sites. Timely termination of cover crops can reduce the attractiveness of fields to egg-laying females. Late cover crop termination or planting green can potentially create a green bridge where TAW larvae move to corn when the cover crop is terminated. Scouting for TAW is critical when planting corn after a cereal rye cover crop.

Scout for TAW by checking 5 sets of 20 corn plants (100 plants total) when walking a W-shaped pattern across the field. Record the number of damaged plants and the number of true armyworms per plant. Infestations may only be in weedy zones or along field edges so you should scout in several areas of the field.

Treatment for TAW is suggested if you find two or more larvae (3/4” or smaller) per plant on 25% of the stand, or one larva (3/4” or smaller) per plant on 75% of the stand. Spot treat the field if possible.

Slug scouting

Slugs are above ground pests in corn. Slugs are molluscs (Figure 7), not insects. This means slugs cannot be controlled with insecticides.

Slugs typically overwinter as eggs in the soil, but adults can survive mild winters. Slugs feed on plant surfaces with “rasp-like” mouthparts. In corn, slugs scrape leaves and leave a window-pane damage then ragged or shredded holes (Figure 8). The damage causes defoliation, but young corn plants have a good chance to recover since the growing point is underground.

Your field might be at higher risk for slug damage if you:

• Have crop residue or weedy field conditions
• Have no-till or reduced till fields
• Have cool or wet soil conditions
• Have a wet spring following a mild winter
• Use insecticide seed treatments, which harm slug-predator insects

Slugs are nocturnal feeders. During the day, they seek shelter in soil crevices and under crop residue. Cool, wet conditions slow down the growth of young plants and make them more vulnerable to slug feeding.

Scout for slugs immediately after planting and emergence. Check wet, low-lying or weedy parts of fields that are likely to provide good slug habitat. You can use a refuge trap to scout

for slugs. Traps, commonly made from roofing shingles, rest on the soil surface and slugs seek shelter underneath during the day.

There are no economic thresholds for slug control in Wisconsin. Molluscicidal baits may be used in corn.

Hop vine borer scouting

The larvae stage of hop vine borers are pests of corn. Hop vine borer larvae have a tan colored head, cream colored body and “false” eyes behind the head (Figure 9).

Hop vine borer eggs overwinter on perennial grasses in Wisconsin. The eggs hatch in the spring and larvae migrate to nearby corn fields.

Hop vine borers tunnel into the belowground portion of corn stems (Figure 10). The whorl may appear dead in damaged plants while other leaves are green.

Your field might be at higher risk for hop vine borer damage if you:

• Have field margins with perennial grasses
• Had within field late season weed patches the previous year

Scouting for hop vine borers should start when corn emerges. Scout along fence rows, waterways or field margins. Check for damage in areas of the field that have hop vine borer history.

Treatment may be warranted at the time of plant emergence if yearly infestations occur. Chemical applications should be made while hop vine borer is migrating to the field before larvae tunnel into the corn plants below ground. Once the larvae are in the plant, they cannot be controlled with an insecticide. Target applications to the field perimeter to minimize costs and non-target effects.

Stalk borer scouting

Stalk borers are above-ground corn pests that feed on foliage and tunnel into young corn stems. Stalk borer larvae have a tan to orange colored head with a black stripe on each side, and a fleshy yellowish-brown body with a black saddle-like marking in the center (Figure 11). They can be up to 1 ½” long.

Stalk borers and hop vine borers have similar life cycles. Stalk borers overwinter in Wisconsin as eggs on perennial grasses like quackgrass and wirestem muhley. Giant ragweed is one of the few broadleaf weeds that is a preferred host for stalk borer. In the spring, the eggs hatch and small larvae feed on the perennial grasses. When the larvae get too big for the grass, they migrate to nearby corn. The easiest time to control stalk borers is during the migration from the perennial grasses to the corn field.

Small larvae cause pinhole feeding damage on corn. Larger larvae feed on the foliage and tunnel into the aboveground portion of corn stems (Figure 12). If boring has occurred, the central leaves of the plant will wilt. Small plants are more susceptible to injury. Plants are not likely to be killed by stalk borer damage once they reach V7 stage.

Damage is often found near locations with perennial grasses, like fence rows, grassed waterways, or terraces. Stalk borer damage will often be along field edges where eggs were laid the previous fall.

Your field might be at higher risk for stalk borer damage if you:

• Have field margins with perennial grasses
• Had within field late season weed patches the previous year

Scout for stalk borer when 1,300-to-1,400-degree days (base temperature 41˚F) have accumulated. You should look for stalk borers moving from grass to nearby corn. Check for damage in the four to eight rows near field edges. Check corn whorls for larvae and count the number of damaged plants.

Economic thresholds for stalk borer in corn are based on leaf stage, market value and expected yield (Table 2-8 in A3646). If treatment is needed, control for stalk borers at 1,400-to-1,700-degree days. Timing is important. Once larvae have tunneled into the stalk, you won’t be able to control them with an insecticide.

White grub scouting

White grubs are below ground pests in corn. The grubs are larvae with tan heads, cream-colored bodies curved in a C-shape, and six legs (Figure 13). As adults, these insects are known as scarab beetles. There are several species of white grubs in the Midwest, including May/June beetles (Figure 14), Japanese beetles (Figure 15) and masked chafer (Figure 16). Japanese beetles and masked chafer have a 1-year life cycle and May/June beetles have a 2-3-year life cycle.

White grubs feed on corn seedlings. They consume roots, feed on the mesocotyl and sometimes tunnel into the stalk below ground. Damage appears as wilted, stunted, or discolored seedlings (Figure 17). Plants may fail to emerge, leaving gaps in rows.

Your field might be at higher risk for white grub damage if you:

• Plant corn after pasture, alfalfa, sod or a weedy corn or soybean field
• Plant corn into a set-aside field
• Have cool or wet soil conditions

Cool and wet soil conditions slow seedling growth which increases the risk of white grub damage.

There is no rescue treatment. Only preventative control methods are effective. If you scout fields after planting, you should identify damage caused by white grubs and develop a field history to plan for management in future years.

Seedcorn maggot scouting

Seedcorn maggot is a below ground pest in corn. The name ‘seedcorn maggot’ is misleading, as the fly larvae feed on a wide range of food, including seeds, but also manure. Larvae called maggots are 3/16” long, white and cigar shaped (Figure 18). The adults look like small houseflies (Figure 19). Seedcorn maggots feed on the seed, hypocotyl and cotyledon (Figure 20). Damaged seeds may fail to germinate and emerge, leaving gaps in rows. A common indicator of seedcorn maggot infestation is stand loss. If you see stand loss, you can dig up the seed to identify the problem.

Seedcorn maggot has 4-5 generations per year. The first three generations (360, 1080, and 1800 degree-days, base temperature 39˚F) are most problematic in corn.

Your field might be at higher risk for seedcorn maggot damage if you:

• Plow down green manure
• Apply livestock manure
• Have recently tilled
• Have cool or wet soil conditions

Adult flies are attracted to decaying organic matter and newly tilled fields where they prefer to lay eggs. No-till fields are less attractive to egg-laying females. Cool and wet soil conditions slow seed germination and emergence which increases the risk of seedcorn maggot damage.

There are no economic thresholds for seedcorn maggot, nor are there reliable rescue treatment options. You must scout fields to identify damage caused by seedcorn maggot and develop a field history to plan for management in future years. Management options include in-furrow insecticides or seed treatments.

Wireworm scouting

Wireworms are below ground pests in corn. The hard-shelled, copper colored larva have three sets of jointed legs and can damage corn seed (Figure 21). The adults are click beetles (Figure 22). There are many species with different life cycles which can last 2-5 years, meaning if you have wireworm issues one year, you will likely have issues in following years.

Wireworms move within the soil profile. The larvae attack the seed below ground. Damaged seeds may fail to germinate and emerge, leaving gaps in rows. If wireworms enter the plant below the growing point, you will see holes in emerging leaves. If the plant is entered above the growing point, the emerged plants will show wilting or stunting injury.

Your field might be at higher risk for wireworm damage if you:

• Plant corn after pasture, alfalfa or sod
• Plant corn after a weedy corn or soybean field
• Have cool or wet soil conditions

The adults are attracted to grassy weeds where they prefer to lay eggs. Due to the extended 2–5-year life cycle of the larvae, the second or third year after alfalfa or sod may have more wireworm damage. Cool and wet soil conditions slow seed germination and emergence which increases the risk of wireworm damage.

There is no rescue treatment. You must scout fields to identify damage caused by wireworms and develop a field history to plan for management in future years. Management options include in-furrow insecticides or seed treatment.

Billbug scouting

Billbugs are occasional above ground pests in corn. These 3/8” dark-colored weevils feed at the base of corn seedlings, creating holes in the plant stem (Figure 23). As the corn plant grows, earlier feeding holes can be seen on the leaves more prominently (Figure 24). The damage is usually non-economic and is frequently confused with that of other early season corn insect pests.

Your field might be at higher risk for billbug damage if you:

• Have grassy weeds or yellow nutsedge

Billbugs overwinter as adults in the soil and emerge in May. They feed on grasses and prefer to lay eggs in nutsedge where the larvae grow and develop.

There are no economic thresholds for billbug in Wisconsin. Treatment may be warranted if adult feeding is killing plants and reducing the stand. There is no rescue treatment for larval feeding.