• Corn earworms do not overwinter north of I-70 so infestations in these areas migrate from the south every year.
• Once the larvae enter the corn ears, control with insecticides is difficult and not feasible.
• A number of Bt commercially available corn hybrids provide effective control of corn earworm.
• Decreased susceptibility of corn earworm to Cry toxins in Bt sweet corn was confirmed a couple of years ago and these are the same Cry toxins expressed in field corn.
• In 2019, reports of higher numbers of corn earworms observed feeding on Bt field corn, are raising concerns for potential decreased susceptibility to Bt traits.

Corn earworm identification and life cycle

Larvae of corn earworm can be green, yellow, dark brown and even light purple in color (Image 1). Three or four sets of alternating dark and light stripes run the length of the larval body. The head capsule is orange in color. Dark tubercles are found along their backs and sides. The skin is coarse in texture with black spines covering the surface of their body. This latter is one of the key characteristics that distinguish them from other striped larvae. When disturbed they curl into a C-shape. Adults are grayish-brown moths with irregular dark gray lines on the front wings and a dark area near the wing tip.

Image 1 below. Color variability of corn earworm larvae (Source: www.maine.gov)

Corn earworms overwinter as pupae in the southern states or in the Midwest states south of I-70 and migrate north to the Midwest/ Corn belt region in early May.Historically there have been two generations per year in the Midwest and multiple generations in areas with warmer climates (south). Adult females of the first generation lay eggs singly in the whorl while the second generation females lay eggs on corn silks. One single female can lay up to 1000 eggs. However because the larvae are cannibalistic only one larva is typically found per ear.Once larvae find the ear they continue feeding and move very little. After a few weeks, full grown larvae drop to the soil and burrows belowground to pupate. Ten to fourteen days later a new corn earworm adult emerges.

Damage

Although the first generation larvae that feed in the whorl cause some defoliation of the unfurled leaves (Image 2 above), the second generation is likely the most damaging one. Corn at VT to R1 is most attractive to egg laying by females. Second generation larvae may cause damage to the tassel, silks and kernels. Corn earworms enter the ear through the silk channel. As silks dry, corn earworms begin to feed on tip kernels (Fig. 3). This initial feeding can provide an entry point for secondary pests such as sap beetles and pathogenic fungi that can cause ear rots and produce mycotoxins. Corn earworms are an economic pest of sweet and seed corn mainly, but occasionally their numbers can raise concerns in field corn. However, University studies have demonstrated that even moderate kernel feeding by corn earworm, does not result in yield losses.

Image 3 below. Corn earworm feeding on ear tip (Source: GROWMARK, Inc.)

Scouting

It is recommended to scout at R1 or when significant moths are captured in the pheromone traps. The presence of large numbers of eggs on fresh corn silks indicates the potential for damaging populations. The least destructive way to determine corn earworm presence in the ear is by sampling the silk. If no corn earworm damage is found, the silks will remain attached when pulled back lightly. However if silks can be pulled off easily with light pressure or if they are not attached, it can indicate that corn earworm larvae are present or that they have caused damage.

Management

The adoption of Bt corn hybrids has made corn earworm management a lot easier than it was in the past. A number of Bt commercially available corn hybrids provide acceptable control of corn earworm as well as other Lepidoptera pests and western corn rootworm. Generally speaking, trait packages containing Cry1’s, Cry2’s and Vip proteins target Lepidoptera pests while Cry3’s proteins target corn rootworms. More specifically Cry1Ab’s target is European corn borer, while Cry2Ab2 was initially targeted against tobacco budworm, corn earworm, fall armyworm and other important pests of cotton. Cry1F provides good control of corn borers, fall armyworm and suppression of corn earworm. Michigan State University regularly publishes a very useful “Handy Bt trait table” with the list of pests controlled by various trait packages, resistance findings, new product information, refuge requirement etc. The latest version available was updated in May 2019. This table can be found at: https://agrilife.org/lubbock/files/2019/05/BtTraitTable-May-2019.pdf.

As you can see in this Bt trait table, corn earworm resistance has been confirmed for Bt toxins Cry1A.105 and Cry2Ab2 (in VT Double PRO, Triple PRO and Smartstax). Keep in mind that these resistance reports come from locations that may not be your particular one so it is possible that these traits may still provide acceptable control in your area. Bt products containing Viptera remain effective against corn earworm with no resistance reported to date. Corn earworm is also a pest of cotton in the south, where it takes the name of cotton bollworm. Bt cotton contains some of the same traits available in corn and so there is a lot of exposure to the same Bt toxins by this pest, which increases the risk of resistance. Now let’s remember that a good portion of the corn earworm moths migrating to the northern areas come from southern states. So, it is not surprise that if resistance is present there, some of those moths flying north may carry that resistance along and pass it on to the next generation.

Understanding your Bt trait package is extremely important in tracking insect resistance development. If decreased efficacy is noted for a Bt trait that should be effective at managing a certain pest it needs to be reported to your local seed dealer or extension entomologist. Keep in mind that sometimes seeing more corn earworms than normal feeding on corn ears does not necessarily mean that the Bt toxin is no longer effective. In some years we may have very large populations of corn earworm moths migrating north, at much earlier times than normal and so, local populations and the amount of damage observed will be higher due to that.

If the corn does not contain Bt traits that offer control against corn earworm, insecticide applications may be considered in the whorl stage if economic thresholds are met. Once the larvae enter the ears, control with insecticides is difficult and not economical as earworms are protected by the ear husk. Keep in mind that there have been reports of corn earworm resistance to pyrethroids as well but there are other insecticides that provide acceptable levels of control. For more information on insecticides labeled for corn earworm in corn: http://entomology.unl.edu/insecticide-treatment-options-corn-earworm-field-corn