Caterpillars in Corn: Earworm, Western Bean Cutworm or Corn Borer?

The end of the summer is a good time to scout for caterpillar damage in corn. What is present will tell you if you have pests that need to be managed next year.
Caterpillars in Corn: Earworm, Western Bean Cutworm or Corn Borer? - Articles

Updated: September 19, 2018

Caterpillars in Corn: Earworm, Western Bean Cutworm or Corn Borer?

Figure 1: Western bean cutworm caterpillar, showing the distinctive longitudinal lines in its collar (or pronotum) behind its head. (Image Credit: Eric Bohnenblust, Penn State)

As the season begins to wind down, folks are scouting their fields to assess crop progress, which includes inspecting plants and opening husks. Some people are finding caterpillars in ears and leaf damage, particularly for those who planted non-Bt hybrids. Here is 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 with the others being much less common. 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 the ears of their field corn. Keep in mind, however, that infestations tend to be patchy and are not usually uniform across entire 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 cause meaningful yield loss, even in non-Bt corn. I expect this situation to be the same in 2018.

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, 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), and in fact, 2017 research revealed that the 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. In particular, 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 its head that has three small stripes that parallel the body (Figure 1).

Lastly, we’ve heard from a few farmers who planted non-Bt hybrids and 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. Recall that this is the pest species against which Bt corn was first introduced.

Widespread adoption of Bt corn has controlled corn borer populations so well that they 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 due to the 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, I encourage growers to plant for a few years a few acres of high-yielding non-Bt hybrids and then scout these non-Bt acres to see how much damage has occurred. 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.

Authors

Insect ecology Plant-insect interactions Conservation biological control Chemical ecology Gall insects

More by John Tooker, Ph.D.