Western bean cutworm caterpillars have distinctive longitudinal lines in its collar, or pronotum, behind its head.

Now is the time to evaluate caterpillar damage in your corn.

As the season begins to wind down, folks are scouting their fields to assess crop progress. This effort, of course, includes inspecting plants and opening husks, and some people are finding insects and damage, which is concerning.

This scouting can reveal caterpillars in ears and leaf damage, particularly for those who planted non-Bt hybrids.

Extension entomologist John Tooker reviews what is likely to be found and what it means for yield and seed choices.

In ears, you can find corn earworm, European corn borer, fall armyworm, and western bean cutworm.

In Pennsylvania, the most abundant species tends to be corn earworm.

Most Bt corn hybrids offer some protection against these species, but control is not 100%, and farmers can be disappointed when they find worms in ears of their field corn. Keep in mind, however, that infestations tend to be patchy and are not usually uniform across entire fields.

Tooker’s advice is usually to ignore typical, minor ear damage. Research conducted from 2010-2012 clearly indicates that caterpillar infestations in ears across Pennsylvania were low, and the caterpillars were unlikely to cause meaningful yield loss, even in non-Bt corn. Tooker expects this situation to be the same in 2021.

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, so we discontinued our monitoring efforts. In other Great Lakes states, however, western bean cutworm has been causing economic damage to ears, even in Bt hybrids meant to control it (the Herculex trait, Cry1F protein). In fact, research from 2017 revealed that western bean cutworm has evolved resistance to the Cry1F protein.

If you have heavy ear infestations, it is 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.

Also, we have heard from a few farmers who planted non-Bt hybrids and have found extensive leaf and stalk damage from European corn borer.

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. This is a clever approach, but it depends on understanding local corn borer populations and is most viable when corn borer populations are low.

Before moving to large acreages of non-Bt, Tooker encourages 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 you are adventurous and want to squeeze more profit out of each acre. If you want to discuss this approach in more detail, contact Tooker at tooker@psu.edu

Leon Ressler is a Penn State Extension educator based in Lancaster County, Pennsylvania.


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