Caterpillar damage in your ears – AgFax

Corn ear feeding injury. Photo: Mississippi State University

When the silage harvest is underway, people survey their fields to assess the progress of the crop. These efforts, of course, include inspecting the plants and opening the husks, and some people find insects and damage they’d rather not see. This scouting can reveal caterpillars in the ears, so I thought I’d spend some time reviewing what can be found and what that means for yield and seed selection.

In Pennsylvania, you can usually find evidence of four types of caterpillars in corn ears: the corn earworm, the European corn borer, the fall armyworm, and the western bean armyworm. Feeding any of these species causes direct damage to the kernels, but they also make it easier for various ear molds to enter the ears, which of course can be a concern in itself, especially if they produce mycotoxins.

For more information on ear molds, see this recently updated article from Dr. Alice Collins. The most common of these four types of caterpillars is usually the corn earworm. Most Bt corn hybrids provide some protection against species of earworm, earworm, and corn borer, but control is not 100%. And the earworm is steadily developing resistance to the major Bt toxins, so the earworm damage gets a little worse every year.

However, earworm populations in Pennsylvania tend to be patchy and rarely homogeneous across fields. My advice is to generally ignore typical minor ear injuries. The research we conducted in 2010-2012 clearly showed that earworm infestations across PA were very low and that caterpillars were unlikely to cause significant yield loss, even in non-Bt corn. I expect this situation to be the same even ten years from now, but if you’re seeing corn ears that are overflowing with feeding, it’s probably time to consider switching to dual Bt hybrids.

If the caterpillars in your corn ears are legume worms, you may have a more serious problem. From 2009 to 2015, Pennsylvania educators trapped the bean moth, but the moth population was never large and scouting revealed only a few caterpillars, so we stopped our monitoring efforts. In other Great Lakes states, however, especially on sandy soils, bean jay causes economic damage to ears, even in Bt hybrids designed to control it (eg, Herculex trait, Cry1F protein).

In fact, studies have shown that the bean jay has developed resistance to the Cry1F protein. If you have severe ear infestations, it will be important to know which type of caterpillar is responsible. In particular, if you are in the upstate, I encourage you to determine if your earworms are bean worms or something else.

The identity of the species is of great importance and can explain the level of damage. The western bean weevil lacks the strong longitudinal stripes of the corn borer and has an obvious brown collar (called the pronotum) behind the head with three small stripes running parallel to the body.

Finally, for farmers planting non-Bt hybrids, the European corn borer can be a concern. Over the past 20 years, this pest species has become a non-problem for most field corn producers, but older folks will remember the European corn borer as a historically significant pest that regularly caused yield losses of 5-20% to Pennsylvania farmers. Recall that this is the type of pest against which Bt corn was first introduced.

Widespread use of Bt corn has controlled the corn beetle population so well that its population is near historic lows. As a result, some people can plant non-Bt corn hybrids and make more profit than using Bt seed, mainly because of the lower cost of non-Bt seed, but outbreaks of corn borer remain and you need to understand the local population , to know the risk they pose to your crops.

Before moving to large non-Bt areas, I encourage growers to plant a small number of acres of high-yielding non-Bt hybrids for a few years and then survey those non-Bt acres to see how badly the damage is occurring. If the damage is light, increase the acreage carefully the following year and so on, but continue scouting to determine if the corn beetle population is increasing. If populations become too large, switching back to Bt hybrids should suppress the corn beetle population and perhaps non-Bt acres will once again be favored.

This may be an approach worth considering if you want more profit per acre, but you need to understand local pest populations. (If you would like to discuss this approach in more detail, please get in touch.) Caterpillar damage in your ears – AgFax

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