Caterpillar Damage in Your Corn Ears

Caterpillar Damage in Your Corn Ears
Sep 22, 2022

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By John Tooker

When silage harvest is underway, folks scout their fields to assess crop progress. This effort, of course, includes inspecting plants and opening husks, and some people find insects and damage that they would prefer not to see. This scouting can reveal caterpillars in ears, so I thought I would spend some time reviewing what is likely to be found and what it means for yield and seed choices.

In Pennsylvania, you can usually find evidence of four caterpillar species in corn ears: corn earworm, European corn borer, fall armyworm, and western bean cutworm. Feeding by any of these species causes direct damage to kernels, but they also facilitate entry to ears of various ear molds, which of course can be concerning in their own right, particularly if they produce mycotoxins. For more information on ear molds, see this recently updated article from Dr. Alyssa Collins. The most abundant of these four caterpillar species tends to be corn earworm. Most Bt corn hybrids offer some protection against earworm, fall armyworm, and corn borer species, but control is not 100%. And earworm is steadily evolving resistance to the main Bt toxins so damage from earworm appears to be getting a little worse each year. Nevertheless, earworm populations in Pennsylvania tend to be patchy and are rarely uniform across fields. My advice is usually to ignore typical, minor ear damage. Research that we conducted from 2010-2012 clearly indicated that caterpillar infestations in ears across PA were very low and the caterpillars were unlikely to be causing meaningful yield loss, even in non-Bt corn. I expect this situation to be the same even a decade later, but if you are seeing corn ears that are overwhelmed by feeding, it is probably time to consider switching to hybrids with two Bt toxins.

If the caterpillars in your corn ears are western bean cutworm, you may have a more severe problem. From 2009-2015, Penn State Extension educators trapped for western bean cutworm, but the moth populations never amounted to much and scouting revealed only a handful of caterpillars (Figure 1), so we discontinued our monitoring efforts. In other Great Lakes states, however, particularly on sandy soils, western bean cutworm has been causing economic damage to ears, even in Bt hybrids meant to control it (e.g., the Herculex trait, Cry1F protein). In fact, research has revealed that western bean cutworm has evolved resistance to the Cry1F protein. If you have heavy ear infestations, it will be important to know which caterpillar species is responsible. Particularly if you are in the northern part of the state, I encourage you to determine if your ear caterpillars are western bean cutworm or something else. The identity of the species matters a great deal and may explain the level of damage. Western bean cutworm lacks the strong longitudinal stripes of corn earworm and has an obvious brown collar (called its pronotum) behind it head that has three small stripes that parallel the body (Figure 1).

Lastly, for farmers who planted non-Bt hybrids, European corn borer can be a concern. In the last 20 years, this pest species has become a non-issue for most field corn growers, but older folks will remember that European corn borer is a pest of historic importance that regularly caused 5-20% yield losses for Pennsylvania corn farmers. Recall that this is the pest species against which Bt corn was first introduced. Widespread adoption of Bt corn has controlled populations of corn borer so well that its populations are near historic lows. As a result, some folks can plant non-Bt corn hybrids and experience greater profits than when using Bt seed, mostly because of lower cost of non-Bt seed, but pockets of corn borer remain and you need to understanding your local populations to know the risk they pose to your crops. Before moving to large acreages of non-Bt, I encourage growers to plant for a few years a small number of acres of high-yielding non-Bt hybrids and then scout these non-Bt acres to see how much damage occurs. If damage is low, cautiously increase acreage the next year and so on, but continue to scout to determine if corn borer populations are growing. If populations get too big, switching back to Bt hybrids should knock back the corn borer populations and perhaps non-Bt acres will again have an advantage. This might be an approach to consider if want to squeeze more profit out of each acre, but you have to understand your local pest populations. (If you want to discuss this approach in more detail, please get in touch.)

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