Corn Earworm - Snap Beans

K. Van Wychen Bennett, E. C. Burkness and W. D. Hutchison
Department of Entomology, University of Minnesota

Introduction

photo of corn earwrm

The corn earworm, Helicoverpa zea, has a wide host range, attacking many cultivated crops and weeds. Corn earworm (CEW) can cause serious damage to corn, tomatoes, cotton, vetch, and snap beans. In the upper Midwest, CEW may damage snap beans if infestations occur during the period between flowering to harvest.

Biology and Life cycle

CEW moths are green-eyed, buff-colored, stout moths with a wingspan of about 1½ inches. Their wings have irregular wavy lines and a conspicuous comma shaped mark on the front wings. Their hind wings are white with dark areas along the edges. Fully grown larvae are about 2 inches long. Alternating dark and light lines run lengthwise down their backs. The larvae can vary in color from olive-brown to light green to pink to brown to nearly black with a lighter colored underside. Their head capsule is a light brown. The eggs are about half the size of a pinhead and are yellowish. Females may prefer to lay their eggs on fresh corn silk but they also lay their eggs singly on the leaves of other plants such as snap beans.

CEW do not overwinter in the upper Midwest. There are generally two flights from the south. The first generation, which arrives around mid-June, usually is a small population. CEW usually do not damage snap beans because the bean plants are not at a vulnerable stage (i.e., bud stage to harvest). The second generation, however, is usually much larger and can cause more damage. The second flight of CEW starts to arrive in the upper Midwest during August; the arrival date will vary depending on the wind patterns coming from the Gulf Coast region. CEW usually fly at dusk or on warm, cloudy days.

Damage

Female CEW can lay about 1000 eggs during their lifetime. Beans are vulnerable to CEW larval feeding from bud stage to harvest. When the eggs hatch, the larvae begin feeding immediately. CEW can feed on the bean foliage which results in holes. They also feed on the outside of the pod, which causes surface damage, and burrow into the pods to feed on seeds. Other insects may also cause damage to the leaves and pods. Bean leaf beetle adults chew small round holes in the leaves. The bean leaf beetle adults also chew on the bean pods. European corn borer larvae also feed on the leaves and pods and may bore into the stems. The feeding holes in the pods may provide entry points for fungal pathogens and increase disease incidence. CEW also eat other larvae that they encounter, including their own species. Besides causing holes in the pods and leaves, the larvae can be a contaminant when the beans are processed.

photo of corn earworm damage

Management

You may easily monitor adult population levels with either a pheromone or a blacklight trap although a pheromone trap provides more accurate results. The female sex pheromone is a mixture of (Z)-9-hexadecenal and (Z)-11-hexadecenal. Check pheromone traps daily starting in July. Implement control measures when CEW exceed the following thresholds:

Number of Moths
Treatment
Less than 5 moths/blacklight trap, or less than 10 moths/pheromone trap No treatment needed
5 moths/blacklight trap, or 10 moths/pheromone trap Treat vulnerable beans (bud stage to 7 days before harvest)
25 moths/blacklight trap, or 100 moths/pheromone trap Treat with an effective CEW insecticide if beans are at a vulnerable stage. Beans should be treated on a 5-7 day schedule
100 moths/blacklight trap, or 500 moths/pheromone trap Treat beans with an effective CEW insecticide. May need to shorten the intervals and increase rates. Beans should be treated on a 5-7 day schedule

Chemical control

Commercial Growers

If thresholds have been reached, several insecticides can be used to control CEW. To ensure proper use of insecticides, refer to the most recent edition of the Midwest Vegetable Production Guide. A avoid spraying when snap beans or crops in neighboring fields are flowering. Although bees do not pollinate snap beans, they collect nectar and pollen from the bean flowers. If sprays are necessary during flowering, spray very early in the morning or in the evening when the bees are less likely to be gathering nectar or pollen in the field or garden.

Home Gardener

The home gardener may also pick the larvae off the plant and destroy them.

Cultural Control

Avoid planting snap beans next to corn. Corn, especially tasseling corn, is very attractive to CEW and CEW often lay their eggs on corn silks.

Biological Control

Several Trichogramma species parasitize CEW eggs. Other common wasp parasites include Brachymeria ovata, Microplitis croceipes, and two Tachinid species. Generalist predators such as lacewing larvae, minute pirate bugs, and damsel bugs feed on CEW eggs and small larvae.

Selected References

  • Cornell University. 2003. Vegetable Disease ID and Management http://vegetablemdonline.ppath.cornell.edu/
  • Foster, R. & B. Flood. 1995. Vegetable Insect Management. Meister Publishing Co. Willoughby, Ohio.
  • Metcalf, R.L. & R.A. Metcalf. 1993. Destructive and Useful Insects. 5th Edition. McGraw-Hill, Inc. New York.
  • University of Minnesota Extension Service BU-7094-S. Midwest Vegetable Production Guide for Commercial Growers; http://www.btny.purdue.edu/Pubs/ID/ID-56/
  • Vegetable Crop Scouting Manual. Integrated Pest Management Program-University of Wisconsin Extension, Cooperative Extension Service. Madison, WI, 1998.