Caterpillar Damage in Your Corn Ears

As silage harvest is underway, folks have been scouting their fields to assess crop progress. This effort, of course, includes opening husks, and some people are finding 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 for next year.

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 molds, which of course can be concerning in their own right, particularly if they produce mycotoxins. For more information on ear molds, see Ear Rots in Your Corn Crop? by Dr. Alyssa Collins.

The most abundant of these four caterpillar species tends to be corn earworm. These caterpillars can have variable coloration and have prominent stripes down their bodies (Figure 2). The caterpillars enter the ear via the silk; therefore, their damage tends to be concentrated around the tip (Figure 2). They are also cannibalistic and readily eat their neighbors, so you rarely find more than one earworm per ear. Fall armyworm caterpillars can have prominent stripes down their sides but they tend to have variable coloration, including some pale versions that have more prominent spots along their bodies (Figure 3). Fall armyworm tends to enter the ear by boring through the husk mid-ear; so, if you find a hole in the side of the husk, you likely have fall armyworm in the ear. European corn borer caterpillars can also enter the ear through the husk, but these caterpillars tend to lack stripes and are easily identified by their pale color and regular distribution of small black dots, particularly in older larvae (Figure 4). Fall armyworm and European corn borer tend to occur singly in corn ears, though there are exceptions.

Most Bt corn hybrids offer some protection against corn earworm, fall armyworm, and corn borer species, but control is not 100%. Corn 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 a different Bt toxin or those that have two Bt toxins.

If the caterpillars in your corn ears are western bean cutworm, you may have a more severe problem. Western bean cutworm lacks the strong longitudinal stripes of corn earworm or fall armyworm and has an obvious brown collar (called its pronotum) behind its head that has three small stripes that parallel the body (Figure 1). This species is not cannibalistic, so if you find more than one caterpillar per ear, you may be dealing with western bean cutworm. During the summer of 2023, Penn State Extension educators trapped for moths of western bean cutworm around Pennsylvania and found variable populations. In some areas of Centre County, for example, some traps captured hundreds of moths per week. In other parts of the state, traps captured very few moths. In previous years, when we trapped for western bean cutworm moths, we found a poor correspondence between moth populations and caterpillars in nearby fields, but it makes sense to expect more caterpillars in proximity to traps with high numbers of moths.  Regardless, I encourage you to scout your fields to assess whether your corn ears hold populations of western bean cutworm.

In other Great Lakes states (e.g., New York, Michigan), 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, so it is not expected to control this pest species. If you have heavy ear infestations, it will be important to know which Bt traits you purchased and which caterpillar species is responsible. 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. If you find high levels of damage from western bean cutworm caterpillars next year, you may want to consider using a Bt hybrid that includes the Viptera trait (protein Vip3A). To understand available Bt traits and what they control, see "The Handy Bt Trait Table" (PDF) from Michigan State and Texas A&M.

Lastly, for farmers who planted non-Bt hybrids, European corn borer can be a concern. In the last 20 years, due to the widespread use of Bt hybrids, 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 the lower cost of non-Bt seed, but pockets of corn borer remain, and you need to understand 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.)