Corn earworm, Helicoverpa zea
Corn earworm moths have a wingspan of about 2.5-3.8 cm (1-1.5 inches) and are white or tan with irregular wing markings. These moths are nocturnal and can often be found resting beneath foliage during the day. Females lay white or pale green, spherical eggs singly on younger leaves and buds of hemp. As the eggs mature, they become yellow and gray and develop ridges. The eggs are very small, about 0.5 mm (0.02 inches), and each female moth can lay as many as 25 eggs in one day. Young larvae are small and dark with black bristles and, after hatching, are typically concentrated near plant reproductive structures. Later instars are about 25.5 mm (1 inch) long and are highly variable in color. Larvae can be green, brown, pink, yellow, or a combination of these colors. They also have alternating dark and light stripes running lengthwise along the top of the thorax and abdomen, and brown or black heads with rows of dark bristles or bumps along the body. Pupae are brown or dark red.
- The corn earworm, also referred to as the bollworm or tomato fruitworm, is a major pest of sweet corn in Colorado.
- Caterpillars prefer to feed on ears of corn, which results in the destruction of kernels and the presence of emergence holes in the shuck. In some areas, corn earworm populations do not survive the winter and are reestablished annually when moths migrate north from suitable overwintering sites.
- Monitoring and scouting are critical to corn earworm management. Visual plant inspections for eggs and pheromone traps can be used to help determine when control measures are necessary.
- If chemical control programs are used to suppress corn earworm, proper rotation of chemistries is recommended to address increasing concerns over pesticide resistance.
- Other commonly used control tactics include use of resistant corn varieties and planting date modifications.
Caterpillar of corn earworm feeding on a corn ear. Corn earworm is a major pest of sweet corn in Colorado, especially in southern and western regions. Adults are moths that can migrate up to 300 miles in a single night, which plays an important role in the dispersal of this pest. The larvae bore into corn ears and consume kernels. Image credit: John C. French Sr., Retired, Universities: Auburn, GA, Clemson and U of MO, Bugwood.org
Corn earworm moth visiting a flower. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Corn injured by feeding of corn earworm. Image credit: Clemson University – USDA Cooperative Extension Slide Series, Bugwood.org
Ear of corn with husks pulled back to expose feeding injuries due to corn earworm. Image credit: Ward Upham, Kansas State University, Bugwood.org
A Heliothis trap deployed in a corn field for monitoring corn earworm. These traps can be baited with pheromone lures to attract male moths, which can be seen at the top of the trap. Image credit: Eugene E. Nelson, Bugwood.org
Life history and habits
Corn earworm overwinters as a pupa in soil, though the weather in some areas may reduce survival of overwintering pupae. Published data suggest that corn earworm cannot overwinter in regions north of 40° latitude, such as Nebraska, northern Utah, and northern parts of Colorado. Regions where it cannot overwinter are repopulated each season after moths emerge and migrate north from overwintering sites. It is worth noting that milder winters increase corn earworm’s ability to overwinter. The number of generations each year depends on temperature, with four or more generations observed in southern Utah, two or three generations in northern Utah, but only two generations in Nebraska. Therefore, the number of generations in Colorado likely varies across the state, possibly with two or three generations produced each year in the northern regions and four or more in southern parts of the state.
Moths lay eggs on the silks of host plants in the summer, and each female can produce over 1000 eggs during her lifetime. Females can also lay eggs on weeds and other vegetables when corn silks are unavailable. After hatching, young larvae migrate to the tip of a developing corn ear where they consume kernels and contaminate the corn ear with frass (excrement). The larvae of corn earworm are cannibalistic, which usually limits their density to one or two per ear.
Feeding of corn earworm caterpillars can begin on whorls early in the season. In corn, there will be visible injury to the ear tip since caterpillars feed on kernels, damaged corn silks, mold growth within the ear, and there may be an increase in sap beetle and earwig infestations since corn earworm feeding produces an entry hole near the tip of the husk. There may also be extensive excrement present at the ear tip.
Monitoring for corn earworm activity is key to effective management of this pest. It can be done through pheromone traps and scouting.
In western Colorado, chemical treatments are based on the abundance of moths captured in pheromone traps. The use of Heliothis traps baited with pheromone lures can aid in monitoring the nocturnal moths; however, the pheromone lures are only attractive to male moths and should be replaced every few weeks. For a list of U.S.-based companies that sell these lures, please visit the Oregon State University webpage. Traps can be deployed at the border of fields and maintained throughout harvest, with regular inspections twice each week and lures replaced every two weeks. During each trap inspection, captured moths should be emptied from traps and killed by crushing, freezing, or placing the moths in a jar containing alcohol or nail polish remover. When using traps, it is important that growers can distinguish between corn earworm moths and other lepidopteran species that fly during the daytime. Using degree days in conjunction with these monitoring techniques will help inform decisions related to management. With good record keeping practices and consistent monitoring protocols, these data can be used to predict infestation levels.
Sweet corn has a low tolerance for feeding of corn earworm, and the timing of insecticide treatments should coincide with egg hatch. Sampling for eggs can begin as early as following corn emergence and should intensify when silks form. Corn planted late that is silking during the late summer and early fall is especially susceptible to corn earworm injury. Large numbers of eggs on fresh corn silks indicate that the population can reach destructive levels. It is recommended to begin insecticide applications during the silking stage when eggs begin to hatch.
Planting resistant varieties of sweet corn with long silk channels, tight husks, and fast growth provides an effective cultural control against corn earworm. Late-planted corn is more susceptible to yield loss due to corn earworm infestation, and early plantings of sweet corn may escape significant injury and require fewer insecticide treatments. To protect late-planted corn, insecticides can be applied every other day during the silking period until harvest. In areas where corn earworm can survive the winter, fall tilling can help reduce the survival of overwintering pupae.
There are several natural enemies of corn earworm. Insect predators of corn earworm larvae include lacewings, minute pirate bugs, and damsel bugs. However, their natural abundance is typically insufficient to provide acceptable control of corn earworm.
Trichogramma wasps are naturally occurring parasitoids that reproduce by parasitizing caterpillars, killing them as a result. These wasps can be purchased and released in fields, a management tactic that requires specific timing and maintenance of wasp populations. This has shown limited success in neighboring states like Utah.
Transgenic Bt sweet corn is now approved for commercial production. It is recommended that Bt sweet corn be incorporated into later plantings since moth counts in pheromone traps tend to be high late in the season. However, feeding injury can still be significant in plantings of Bt sweet corn, and adequate protection may require regular insecticide applications.
Insecticide treatments are most effective when they coincide with egg hatch, and least effective after caterpillars bore into corn ears. For effective control, insecticide treatments should begin when the first silks appear within a field. Beginning insecticide applications too late greatly reduces their efficacy. In sweet corn, it is recommended that insecticides be applied with a high-clearance sprayer, with four hollow-cone nozzles per row targeted at the ear zone. For a visual representation of this sprayer coverage, as well as alternative options for applying insecticides in commercial sweet corn, visit the University of Tennessee factsheet. Proper coverage of corn ears is essential for effective control of corn earworm, and it is worth noting that ground applications tend to provide superior coverage of corn ears when compared with aerial applications.
Pyrethroid insecticides have traditionally been the primary chemical control against corn earworm. However, some earworm populations in the Midwest have developed resistance to pyrethroids, and they tend to be more effective at lower temperatures. Therefore, it is now recommended that pyrethroids be rotated with other classes of insecticides. Reapplications will often be necessary to maintain an active insecticide residue on susceptible plant structures as young larvae continue to emerge through the growing season. More information on chemical control of corn earworm in sweet corn is available here.
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