Corn earworm, Helicoverpa zea
Corn earworm moths are about 25.5 mm (1 inch) long 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 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 pest of hemp, cotton, corn, peanuts, tomato, and many ornamental plants.
- Hemp grown for CBD is more susceptible to corn earworm attack than hemp grown for fiber.
- Monitoring for corn earworm moths is recommended to help growers determine when control measures are necessary based on changes abundance through the growing season.
Larva of corn earworm feeding on hemp. Corn earworm is a highly polyphagous pest that can feed on hemp, cotton, corn, peanuts, tomato, and many ornamental plants. Hemp grown for CBD is more susceptible than varieties grown for fiber or seed. While there are currently no treatment thresholds established for this pest, monitoring is still recommended to help inform decisions related to pest management. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Adult of corn earworm resting on hemp plant. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Egg of corn earworm on hemp leaf. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Hemp bud damaged by feeding of corn earworm. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Hemp with heavily damaged buds due to feeding of corn earworm. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
A Heliothis trap deployed in hemp field for monitoring corn earworm. These traps can be baited with pheromone lures which are attractive to male moths. Image credit: Whitney Cranshaw, Colorado State University, 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. Certain locations are not suitable for winter survival and can be repopulated after moths emerge and migrate north from overwintering sites. The moths actively lay eggs on host plants in the summer. There are two generations each year, possibly more depending on temperature.
Caterpillars feed by boring into buds and blossoms which destroys developing buds and seeds, reducing yield. Corn earworm will feed on leaves when reproductive tissues are not available. Furthermore, caterpillar feeding causes wounds in plant tissue that provide an entry site for microbial pathogens, and fungal infections with certain Fusarium spp. are particularly problematic in hemp. Corn earworm moths prefer flowering plants over plants that have not flowered. Hemp grown for CBD is more susceptible to attack by corn earworm than hemp grown for fiber or seed.
Begin visual inspections for corn earworm caterpillars and signs of feeding injury when plants begin to flower. The presence of excrement (frass), wilting leaves, and tunnelling in buds are signs of a corn earworm infestation.
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. 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 hemp fields and maintained throughout harvest, with regular inspections twice each week and lures replaced every two weeks. Captured moths should be emptied from traps during each check. When using traps, it is important that hemp growers can distinguish between corn earworm moths and other lepidopteran species that fly during the daytime.
There are currently no treatment thresholds for this pest in hemp. Therefore, it is recommended that growers record the quantity of corn earworm moths caught in traps. Chemical treatments should only be applied when these moths are abundant in traps.
Removing larvae by hand is a time consuming but effective management strategy. Before planting, preparing soil by destroying weeds with herbicides or mowing can help destroy overwintering pupae, though moths can still migrate from nearby areas. It is recommended that weeds are continually removed throughout the season to reduce food availability for caterpillars and help decrease their density.
There are limited insecticides registered for use in hemp. The combination of biological-based insecticides with commercially available biocontrol agents, such as Beauveria bassiana can provide effective control of corn earworm in hemp. Multiple applications may be necessary for effective protection during outbreaks when moths continuously lay eggs. For more information on treatment recommendations for biological-based pesticides, consult the Oregon State University webpage.
Britt, K., Taylor, S., and T. Kuhnar. 2019. Corn Earworm, Helicoverpa zea, a pest of hemp, Cannabis sativa, in Virginia. Virginia Cooperative Extension. Available https://www.pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/ENTO/ento-328/ENTO-328.pdf
Colorado State University. (n.d.). Corn Earworm. Colorado State University. Available https://webdoc.agsci.colostate.edu/hempinsects/PDFs/Corn%20earworm%20revised.pdf
Hirsch, K., and K. Kesheimer. 2021. Corn Earworm in Hemp. Alabama A&M and Auburn Universities: Extension. Available https://www.aces.edu/blog/topics/crop-production/corn-earworm-in-hemp/
Murray, M. (n.d.). Pests of Hemp in Utah: Disease and Arthropod Identification Guide. Utah State University: Extension. Available https://extension.usu.edu/pests/files/pubs/hemp-guide-webtest.pdf
Pulkoski, M., and H. Burrack. 2020. Corn Earworm and Tobacco Budworm in Industrial Hemp. NC State: Extension. Available https://content.ces.ncsu.edu/corn-earworm-and-tobacco-budworm-in-industrial-hemp
Shrestha, G. 2022. Corn Earworm Management in Hemp. Oregon State University: Extension. Available https://extension.oregonstate.edu/crop-production/hemp/corn-earworm-management-hemp