Monitoring Worm Pests in Sweet Corn

According to the 2018–2019 Agricultural Statistics Annual Bulletin, approximately 13,000 acres of sweet corn were planted in the Commonwealth in 2018, with a value of utilized production approaching $25,000,000. Sweet corn, like many vegetable crops in Pennsylvania, can be impacted by an array of insect pests that can reduce yield and/or crop quality. To manage these pests effectively, growers are encouraged to use IPM techniques on their farms to reduce the number of pesticide applications made while preserving both crop yield and quality.

Corn Earworm

Corn earworm, or CEW, is the most serious economic pest observed by sweet corn growers in Pennsylvania. CEW is not capable of overwintering in Pennsylvania and migrates into the state from the south each year. Corn earworm moths appear tan or off-white, with one dark spot in the middle of each forewing and a dark band at the end of each hind wing. Female corn earworms lay eggs singly on fresh corn silk. Corn earworm eggs typically hatch in 2–6 days (temperature dependent), and larvae move down the silk into the developing ear to begin feeding. The larvae of corn earworm can be quite variable in color and may appear light green, tan, brown, pink, maroon, or black in coloration. Light or dark stripes also can be observed running the length of the body, and small, microscopic spines can be detected covering the entire body. The head of the corn earworm larvae is always light brown in color, irrespective of the color of the larva’s body.

Figure 2. Corn earworm comes in many color morphs (left, photo Purdue Extension). And in the right photo, the corn earworm on the left has a light-colored head capsule, while the fall armyworm on the right has a prominent upside-down ‘Y’ on the head capsule due to the light band on the edges of chitinous plates. Image: G. Dively, U. of MD.

Corn earworms can significantly impact sweet corn ear quality if fields are left unprotected. Data from the Midwest indicates that "as little as 25 CEW adults can lay eggs on every ear in an acre of sweet corn." Corn earworm larvae are impossible to control once they have entered the corn ear, so control efforts must be focused on killing the larvae before they enter the ear.

Management of corn earworm is best accomplished by deploying either pheromone or blacklight traps on the farm. Pheromone traps are frequently the most practical traps that can be utilized on farms since they do not require electricity or a battery to power them during overnight hours. Pheromone traps can be of different designs, but the most frequently used traps include the nylon Scentry Heliothis trap, the Maryland Wire Cone trap, or the Hartstack trap. Trap placement for corn earworm is critical when monitoring for corn earworms. Adult corn earworm moths are attracted to silking plants, so to ensure successful trapping, all corn earworm traps should be placed in or adjacent to fields of silking sweet corn.

Corn earworm larva grow approximately 1 mm a day. When larvae are detected, measure the length of the larva to determine when it hatched. Use this data to evaluate your trapping data. If the average length of corn earworm larvae detected in the corn ears is 5 mm, look at your trapping records to see how many adult moths you caught in your traps 7 to 11 days ago. If your trap counts were low or no adult moths were detected in your traps during this period, you may have old or contaminated pheromones that may have impacted your trap counts. Consider replacing your pheromones and make sure that you take steps to prevent cross-contamination of the lures when you are handling pheromone lures for multiple moth species.

Corn Earworm Thresholds

Corn earworm thresholds were developed by Dr. Galen Dively at the University of Maryland on older large-stemmed full-season sweet corn varieties that are believed by entomologists to have a greater capacity to withstand feeding injury. Please use the data in the table below as a guide when making pest management decisions in your sweet corn plantings.

Fall Armyworm

Fall armyworm is another sweet corn pest that will not overwinter in Pennsylvania and must migrate from the south each season to infest the sweet corn crop. Fall armyworm moth hindwings are grayish-white in color, and the male moths will have white triangular spots on their dark gray forewings near the tips and center. The female moth forewings tend to be grayish brown in color with oval or rounded spots. Adult female moths will lay 100 eggs or more on the surfaces of corn leaves. These eggs often appear fuzzy or moldy in appearance because they are covered by hairs from the female moth’s body.

Fall armyworm larvae can vary in color and may range in color from tan to green to black. Yellow to dark stripes will run the length of the body, and four prominent dots can be seen on the top of the last body segment. Mature fall armyworm larvae have a distinctive upside-down white “Y” on the front of the head or face of the caterpillar, which makes them easy to identify.

Figure 3. Fall armyworm larva, on the left, note the “Y” shape on the front of the head. Photo: Russ Ottens, University of Georgia, Bugwood.org

Fall armyworm larvae will feed on the entire aboveground portion of the corn plant and feed in the whorls until the corn tassels emerge. Infested plants will often appear ragged in appearance, and fecal matter will often be observed on the leaves. Fall armyworms, unlike European corn borers, will not bore into the stalk of the corn plant and will focus on attacking the developing ears. Entomologists typically recommend that fall army worm be targeted in the late vegetative pre-tassel stage with insecticides.

There are no definitive thresholds for Fall Armyworm (FAW), but entomologists in the Mid-Atlantic recommend growers treat FAW during the early whorl stage when more than 15% of the plants are infested. Pyrethroid insecticides may not provide good control of FAW due to pesticide resistance.

European Corn Borer

Adult female European corn borer moths are yellowish brown in color with wavy or zigzag-like markings or bands running across the wings. Males are darker in coloration with olive-brown markings on the wings. Both female and male European corn borer moths have a wingspan approximately one inch across.

Figure 4. (Left) Female adult European corn borer. (Right) Male adult European corn borer. Photos: Frank Peairs, Colorado State University, Bugwood.org

 Females lay their eggs in groups of 5 to 50 on the undersides of corn leaves near the ear leaf. In their lifetime, a single female European corn borer moth can lay between 500 to 1000 eggs. The eggs are white in color and resemble fish scales because of the way that they overlap each other on the undersides of the corn leaf. European corn borer eggs hatch in approximately seven days under warm summer temperatures.

Upon hatching, European corn borer larvae will first feed on leaf tissue. European corn borer larvae do not feed completely through the corn leaf, so some membranous tissue will remain, resulting in damage described as "window paning." Later, the larvae will move to the whorl to feed before tunneling into stalks or sweet corn ears.

Pheromone traps can be used to monitor for the presence of European corn borer, but some entomologists report that pheromone traps are less reliable than black light traps for monitoring European corn borer flight activity. Owing to concerns about the reliability of pheromone traps, growers should scout 100 plants at the whorl stage for the presence of larvae and/or eggs. When tassels are visible, growers can begin to use black light traps and/or pheromone traps for monitoring purposes. Pheromone traps are best placed in grassy areas adjacent to sweet corn, with the mouth of the trap approximately 2–3 feet high over the grassy area.

Insecticide Recommendations for Worm Pests in Sweet Corn

Insecticide selection is critical when developing a strategy for controlling worm pests in sweet corn. Insecticide resistance coupled with a loss of efficacy in the pyrethroid class of insecticides can make managing worm pests in sweet corn much more difficult for growers. When developing your sweet corn worm management program, consider selecting and rotating classes of insecticides with different modes of action to delay insecticide resistance.

There are a limited number of insecticides outside of the pyrethroid classification that are labeled for corn ear protection. Synthetic pyrethroid insecticide can still be legally applied, but due to localized resistance concerns, some growers may not observe good control of resistant biotypes of corn earworm. Pyrethroid resistance concerns tend to increase as the growing season progresses. If pyrethroid insecticides are to be used for managing sweet corn worm pests, consider using these products earlier in the growing season when less pest pressure may be observed.

Labeled insecticides for sweet corn worm pests outside of the synthetic pyrethroid class of insecticides include:

• Lannate LV (Group 1A) PHI *See label.
• Blackhawk 36 WDG (Group 5) PHI: 1 day.
• Radiant SC (Group 5) PHI: 1 day.
• Intrepid Edge (Group 18 + 5) PHI: 3 days.
• Vantacor (Group 28) PHI: 1 day.

Synthetic pyrethroid insecticides labeled for managing worm pests in sweet corn include:

• Asana XL (Group 3A) PHI: 1 day.
• Baythroid XL (Group 3A) PHI: 1 day.
• Brigade 2 EC (Group 3A) PHI: 0 day.
• Hero EW (Group 3a) PHI: 3 days.
• Lambda-Cy 1EC (Group 3A) PHI: 1 day.
• Mustang Maxx (Group 3A) PHI: 1 day
• Permethrin 3.2 EC (Group 3A) PHI: 1 day.
• Tombstone (Group 3A) PHI: 0 day.
• Warrior II (Group 3A) PHI: 1 day.
• Combination insecticide products labeled for worm pests in sweet corn containing a pyrethroid insecticide include:
• Besiege (Group 3A + 28) PHI: 1 day.
• Ethos XB (Group 3A + Bacillus amyloliquefaciens) PHI: 1 day.
• Savoy EC (Group 3A + 4A) PHI: 7 days.