‘Recalibrating’ Corn Hybrid Trait Decisions

An advantage of writing for a regional farm publication such as Farming is the opportunity to localize crop management recommendations. This is because the challenges affecting farmers in the eastern U.S. are somewhat different than for farmers on the West Coast. Although heavy late-winter rain and snow – primarily in northern California – has improved the drought situation, water shortages are a frequent issue for farmers in California and the arid Southwest but seldom for us. Conversely, California farmers don’t have to worry about the impact on alfalfa winter survival of subzero temperatures and the freeze-thaw cycle!

There are also regional differences in insect pest pressure, though for one particular insect species the difference may be narrowing. I’ve never been too concerned about the impact of European corn borer on corn grown in the northeastern U.S. Cornell University entomologists note that corn borer pressure in this region is much less common than in the Corn Belt, in most cases low enough that control isn’t recommended – neither insecticide applications nor the use of corn hybrids genetically resistant to this insect pest.

I got onboard after doing two years of research at Miner Institute in New York: We discovered that while the Bt trait completely prevented corn borer damage, this had little or no impact on corn yield. There was visible damage to the non-Bt hybrid, but apparently the damage wasn’t enough to impact yield. We sometimes call this type of damage “cosmetic injury.”

We saw something similar many years ago with the alfalfa blotch leafminer, before natural parasites caused a tremendous decrease in the population of this alfalfa pest. The leafminer does its feeding inside the alfalfa leaflet, resulting in white blotches that give the pest its name. The visual symptoms were worrisome to farmers, but research found that insecticide applications didn’t result in enough yield or quality increase to pay for the cost of the application. I told farmers not to apply insecticides to control leafminers; not only wasn’t it economical but it also would kill beneficial insects in the field – and there are many such insects, including alfalfa weevil parasites and ladybugs.

Corn borer populations decrease

In recent years even farmers in areas where European corn borers have been a problem in the past are finding little economic impact from the use of corn hybrids with the Bt resistance trait. Penn State University entomologists are now saying that corn borers may not be the problem they once were. There are still pockets in Pennsylvania where there’s a fair amount of corn borer pressure, but in other areas there may not be high enough populations to warrant the use of Bt corn.

A caution: Corn borers can occur in isolated pockets, with high populations in one area but not in fields less than 10 miles away. Therefore, field scouting is needed to determine the status of this insect, and August is a good month to do so. Corn borer eggs hatch in mid-summer, and by late August corn borer larvae should be boring holes into corn stalks. Look for broken tassels and the characteristic holes, which are about the size of a BB shot. What has apparently happened in the Corn Belt is that the corn borer trait has been so successful that it’s resulted in a dramatic decrease in the number of corn borers.

As previously noted, our research at Miner Institute found that the Bt trait was 100 percent effective in preventing corn borer damage. We harvested corn stalks in both Bt and control plots – both hybrids had identical genetics except for the Bt gene – then split the stalks, looking for corn borer damage. In the study’s two-year duration, we found that from one-quarter to almost half of the stalks in the non-Bt plots had visible corn borer damage. But the Bt corn plants had not a single borer hole or other sign of corn borer feeding.

So it might be time to “recalibrate” your seed corn selection, especially involving genetically modified traits. If European corn borers aren’t a problem, you don’t need hybrids with this trait. This is for two reasons: economics and ecology. Traited seed costs more than untraited seed with the same base genetics, so why pay extra for something you don’t need? And you shouldn’t be using genetically modified traits that you don’t need, because continued use of a trait increases the chances of the insect pest developing resistance. To date, resistance has been confirmed with Western corn rootworms but not with European corn borers. Even so, why risk it?

Should you plant untraited corn in 2017?

So much for corn borers; how about corn rootworms? There’s no sign that populations of this devastating insect pest are declining – in fact, the area of the U.S. affected by Bt-resistant Western corn rootworms has been increasing. However, in much of the eastern U.S. and particularly on dairy farms, corn is rotated with alfalfa and other hay crops, in many cases resulting in fewer years of continuous corn. A crop rotation consisting of corn and soybeans hasn’t proven to be nearly as effective in managing corn rootworms as one involving corn and a perennial forage crop.

Also worth noting: To date, Bt-resistance has been confirmed in Western corn rootworms but not Northern corn rootworms. Farmers who limit continuous corn production to three years may be able to plant corn hybrids with no genetically modified traits. In the eastern U.S., corn rootworms are almost never an economic problem in first-year corn and usually not in second-year corn either. So a farmer can plant non-Bt seed for those years. For the third year he could either plant hybrids with the corn rootworm trait, or use seed corn pretreated with the highest rate of insecticide. Seed treatments have reportedly had control problems where rootworm pressure was very high, but rootworm pressure may not be high in third-year corn. There are also herbicide tolerance traits – glyphosate and others – so you should decide before you order seed corn what herbicides you’ll use in each corn field. If you’re not going to use a postemergence application of glyphosate, why pay a technology fee for seed corn with this trait? It pays to be knowledgeable about what you’re buying, and what each of these traits cost – and the cost differs between seed companies. Because of several factors – not the least of which is the prospect of lower corn prices – more farmers are buying untraited seed corn, so the availability of untraited seed corn should be increasing.