Corn Earworm - Sweet Corn

A.C. Morey, W. D. Hutchison, and E.C. Burkness
Department of Entomology, University of Minnesota

Introduction

The corn earworm, Helicoverpa zea, has a wide host range, attacking many cultivated crops and weeds throughout the U.S. Corn earworm (CEW) can cause significant economic damage to sweet corn, field corn, tomatoes, cotton, and snap beans. In the upper Midwest, CEW is most damaging to sweet corn when infestations occur during the period between row tassel and 100% silk.

photo of corn earworm larvae
CEW larvae
(Eric Burkness, University of Minnesota)

Biology and Life cycle

Adult CEW are a buff to greenish-colored moths with green eyes. Wing markings are irregular, with the most conspicuous mark being a dark comma-shaped spot on the forewings. Adults have a wingspan of about 1½ inches and are most active at night.  Females prefer to lay their eggs singly on fresh silk, but will also lay on the tassels if silks are unavailable. Eggs are about half the size of a pinhead and are yellowish.  Larvae go through about 6 molts, or instars. Early instars tend to be pale, darkening with time. Fully grown larvae are about 2 inches long and develop a wide range of colors from pale green to pink to almost black. Features distinguishing CEW from other common caterpillar pests include a light-colored head capsule and many black micro-spines along the body. Alternating dark and light lines run lengthwise down their backs. Mature larvae drop from the corn plant after about 18 days and pupate 1-2 inches down in the soil.

photo of corn earworm moth
CEW Moth
(C. Kulhanek, University of Minnesota)

CEW do not overwinter in the upper Midwest, with the northern limit generally estimated to be the 40th parallel. There are typically two flight periods each summer, for moths migrating from the southern states. The first period, which arrives around mid-June, is usually small and of little concern for sweet corn growers. However, the second period, beginning in late July to early August (Minnesota, Wisconsin), usually includes several flights. The duration and size of these late-season flights will vary considerably, depending on the wind patterns coming from the Gulf Coast region, and the size of southern source populations in a given year. As with most moth species, CEW moths are primarily nocturnal, but begin flights at dusk.

Damage

Female CEW can lay about 1000 eggs during their lifetime. Corn is vulnerable to CEW larval feeding from row-tassel to 100% silk.  The eggs hatch in about 2-6 days, and then larvae move down the silk into the developing ear where begin feeding immediately. CEW feed directly on the kernels and a mature larvae can easily consume 8-15 kernels, or the top 1-2 inches of the ear tip. Feeding holes in the ear may provide entry points for fungal pathogens and increase disease incidence. CEW are also cannibalistic, with slight older larvae eating younger instars; they may also agitate or bite other larval species attempting to feed in the ear. Besides causing loss of kernels, the larvae and frass can be a contaminant when the corn is processed.

photo of corn earworm damage
CEW larva feeding on ear
(E.C. Burkness, University of Minnesota)

photo of corn earworm eggs on silk
CEW eggs on silk
(Bill Hutchison, University of Minnesota)

Management

Once CEW larvae enter the ear tip, they are protected by the husk so the treatment window for effective control is very limited. Because incoming flights to a given production area are highly variable, it is therefore essential to monitor adult population levels with either a pheromone or a blacklight trap. We have found that the pheromone trap provides consistent results in the Upper Midwest region. 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:

photo of Hartstack trap
CEW Hartstack Trap
(Bill Hutchison, University of Minnesota)

Number of Moths per night*

Treatment

Less than 5 moths/blacklight trap, or less than 10 moths/pheromone trap
(per night)

No treatment needed

>5 moths/blacklight trap, or >10 moths/pheromone trap

Treat vulnerable corn (row tassel to 100% silk) with an effective CEW insecticide, reapplying every 2-5 days until brown silks appear.

25 moths/blacklight trap, or 100 moths/pheromone trap

Treat silking corn until brown silks. Corn should be treated on a 2-3 day schedule.

100 moths/blacklight trap, or 500 moths/pheromone trap

Treat silking corn with most effective CEW insecticide until brown silk.  Corn should be treated on a 1-2 day schedule.

*Pheromone traps and lures for H. zea can be ordered at Great Lakes IPM.

Chemical Control

If trap-catch 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.  The most commonly used insecticides for CEW are pyrethroids (e.g. Warrior, Brigade).  Organic options are also available (e.g. PyGanic, based on a natural pyrethrum).  Genetically modified sweet corn varieties containing Bacillus thuringiensis (e.g., Bt-11) toxins also provide a high level of control.  Avoid spraying insecticides when neighboring fields are flowering.  Bees collect nectar and pollen from many vegetable 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.

Cultural Control

Where possible, avoid late-planted corn.  As shown in Table 1, in central Minnesota, varieties harvested after the first week in August will tend to be at much greater risk of infestation. This is because these plants are silking and therefore vulnerable to egg-lay by CEW moths, during most of the late-season migration period.

Table 1. Risk of Corn Earworm (CEW) infestations in sweet corn in relationship to harvest dates (untreated plots); Rosemount, MN

2005

2006

2007

2008

Harvest date

%Ears-CEW

Harvest date

%Ears-CEW

Harvest date

%Ears-CEW

Harvest date

%Ears-CEW

8/5

0

7/31

0.9

7/30

0

8/11

40

8/8

0

8/3

0

8/6

4

8/14

17

8/15

2

8/14

6

8/16

20

8/20

17

8/30

18

8/28

34

8/28

16

8/28

37

9/15

60

9/15

90

9/12

96

9/12

23

*All data for GG63, Supersweet Jubilee Plus, and Providence for 2005, 2006, and 2007-2008, respectively; Rosemount, MN is approx. 44.7 lat., 93.1 long, an area where CEW is not able to successfully overwinter; annual late-season infestations result from long-distance migratory flights from the Southern U.S.

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 lady beetles, lacewing larvae, minute pirate bugs, and damsel bugs also feed on CEW eggs and small larvae.  In the Upper Midwest region, the generalist predators are likely having a more significant impact on CEW than parasitoids. Biological control will only be possible when alternative products such as Dipel™ (Bt), oils, or Bt sweet corn is used. 

Selected References

  • Cornell University. 2003. Vegetable Disease ID and Management http://vegetablemdonline.ppath.cornell.edu/
  • Cranshaw, W. 2004.  Garden Insects of North America.  Princeton University Press. Princeton, New Jersey. p. 260.
  • Flood, B.R., Foster, R.F., Hutchison, W.D., and Pataky, S. 2005.  Sweet Corn  In: Vegetable Insect Management [eds.] Foster, R. and Flood, B.R.  Meister Media Worldwide, Willoughby, OH.  pp. 39-64.
  • Hardwick, D.F. 1965. The Corn Earworm Complex.  Entomological Society of Canada Memoirs. 40:3-246.
  • Hutchison, W.D. 2006. IPM: A Risk Management Framework to Improve Decision-Making. Univ. of Minnesota Extension Service. St. Paul, MN. On-line. http://www.extension.umn.edu/distribution/horticulture/DG8229.html
  • University of Minnesota Extension Service BU-7094-S. Midwest Vegetable Production Guide for Commercial Growers.  Online.  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.