Corn earworm

Larvae are brown-headed caterpillars that range in color from pale green to pinkish to brown, with alternating dark and light stripes running lengthwise down the body (Figure 1). When first hatched, larvae are very small, about 1/10 inch (1.5 mm), but grow up to 1 1/2 (3.8 cm).

Corn earworm adults are moths about 3/4 inch (2 cm) long, with a wingspan of 1-1 1/2 inch (2.5-3.8 cm). They range in color from olive green to tan to dark, reddish brown, with the hind wings usually lighter than the front wings (Figure 2). The front wings have a distinctive, darker spot near their center, and both front and hind wings have darker bands near the outer margins. Eyes of adult male moths reflect a distinctive green color when light is shone into them.

Pupae are dark brown, oblong, about 3/4 inch (2 cm) long, and can be found buried 2-4 inches (5-10 cm) deep in the soil (Figure 3).

Eggs are tiny, about half the size of a pin head, and pale green when first laid, but gradually turn grey as they near hatching (Figure 4).

Biology

There are typically three generations of corn earworm per year in Idaho, particularly in the Treasure Valley.

Corn earworm overwinters in the pupal form buried 2-4 inches deep in the soil in most regions, though in some cold, northern areas they cannot survive the winter and must migrate into the region each summer. Adults emerge beginning in late May and extending into June and are most easily observed at night, when they are active. After emergence, adults mate and females begin laying eggs on host plants. Females prefer to lay eggs (oviposit) on corn silk, but will lay eggs on other parts of the corn plant or other host plants if fresh corn silk is not available. A single female can lay up to 1,000 eggs, which take between two and 10 days to hatch, depending upon temperatures. After hatching, larvae immediately begin feeding. When ready to pupate, larvae move into the soil, where they build their protective pupal casing. For the first and second generation, adults emerge after 10 to 25 days. The third generation overwinters as pupae in regions where temperatures allow.

Damage

Corn earworms are present throughout the growing season but are most abundant during late summer.

In corn: Larvae can feed on many plant parts such as leaves, tassels and whorls, but prefer to feed within corn ears (Figure 5). Larvae feed within the tip of the ear, leaving damage, mold and large amounts of frass (excrement). This reduces quality and storage life. Larval feeding can also cause indirect damage, decreased pollination which leads to poor earfill. Larval feeding also creates holes in the ear that can lead to earwig and sap beetle infestation.

In tomato, pepper and other fruits and vegetables: Larvae tunnel into fruits, reducing quality and leading to secondary infection. Larvae also chew leaves, stems and flowers, reducing yield quality and overall plant health.

Monitoring

Pheromone-baited cone or net traps can be used to monitor corn earworm. For corn, traps should be deployed by early June or just before corn is in the silk-forming stage. Traps should be placed along field edges, as close to the corn as possible. Traps are most effective when the bottom of the trap is at the same height as the corn silk, and lures are replaced every two-three weeks (or as recommended by the manufacturer). Check traps twice weekly until the first moth is caught and daily thereafter. When checking traps to determine if economic thresholds have been reached, make sure to only count corn earworm moths, as other moths are likely to be found in traps as well. Calculate average moths captured over three-four nights to account for variability. The economic threshold for corn earworm in sweet corn is 0.2 moths per night, and reapplication may be necessary, depending on subsequent moth captures (view under Management/Chemical section for reapplication thresholds).

Monitoring efforts can also be guided by tracking degree days. Degree days, also often called growing degree days, are a measure of the heat units over time. As insect development is dependent upon temperatures, counting up these degree days can help predict the timing of different developmental events throughout the season, and helps account for differences in weather each year. Different insects undergo development in different temperature ranges, so for each insect there are unique lower and upper development thresholds between which development will occur. For corn earworm, the lower development threshold is 55 degrees Fahrenheit and the upper development threshold is 95 degrees Fahrenheit. To calculate degree day accumulation in your region and determine what stage of development moths are likely in based on this accumulation, use the calculators.

Management

Primary management tactics

Trapping using pheromone lures is a cornerstone of corn earworm management in commercial settings. If economic thresholds are passed and treatment with insecticides is deemed necessary, applications should be timed to coincide with egg hatch. For home gardens, early planting can help ensure harvest occurs before damage is most likely.

Cultural

• Early planting often helps avoid significant damage
• Plant resistant varieties, often those with tight husks
• For home gardens, a clothespin can be used to pinch the point where the silk enters the ear, preventing larvae from accessing the ear
• Fall tillage or removal of dead plants can cause mortality in overwintering populations, leading to fewer earworms emerging in the spring

Biological

Avoiding the use of broad-spectrum insecticides can help preserve natural enemies of corn earworm such as:

• Generalist predators (e.g., lacewings, minute pirate bugs and damsel bugs)
• Parasitoids (e.g., Triochogramma wasps)

Chemical

• Apply insecticides before larvae enter the ear, usually beginning within two days of the beginning of silking
• Repeat as allowed by label or suggested below until silks turn brown (at which point moths no longer lay eggs on silks)
• Average moths trapped per night — reapplication interval by days
  • Less than 0.2 — None
  • 0.2-0.6 — five days
  • 0.7-6.5 — three days
  • More than 6.5 — two days
• Recommendations for pesticides to use in the management of corn earworm be found on the PNW Pest Management Handbooks website

Pesticide warning

Always read and follow the instructions printed on the pesticide label. The pesticide recommendations in this University of Idaho webpage do not substitute for instructions on the label. Pesticide laws and labels change frequently and may have changed since this publication was written. Some pesticides may have been withdrawn or had certain uses prohibited. Use pesticides with care. Do not use a pesticide unless the specific plant, animal or other application site is specifically listed on the label. Store pesticides in their original containers and keep them out of the reach of children, pets and livestock.

Trade names — To simplify information, trade names have been used. No endorsement of named products is intended nor is criticism implied of similar products not mentioned.

Groundwater — To protect groundwater, when there is a choice of pesticides, the applicator should use the product least likely to leach.