One of the strengths of U.S. agriculture is that university researchers and Extension specialists are continually re-evaluating their recommendations for controlling pests in crop production systems.
Scientists in Mississippi and Tennessee, for example, revised their treatment thresholds in non-Bt corn after studies indicated a more conservative approach might be resulting in reduced yields, says Scott Stewart, Extension entomologist with the University of Tennessee.
“We’ve gotten quite a bit more aggressive based on data,” he says. “We’re also triggering more applications based on moth trapping. It used to be ‘treat if 20 percent of your plants are infested.’ Well, our data show that by the time 20 percent of your plants are infested, you’ve got yield loss.
“So, we’re saying to treat when first generation, whorl stage, 5 percent or more of your plants are infested with either eggs or small larvae. Or, if you run pheromone moth traps, treat when you’ve caught 50 or more moths in a week.”
After tassel and up until R-3, a new recommendation is to treat when you have 10 percent or more infested plants. “Really, we’re saying anytime you catch 100 or more moths per week during the crop’s sensitive stage, consider a treatment in the next four or five days.”
The reality, says Stewart, is that the vast majority of Tennessee corn growers manage corn borers with Bt corn. And they have new tools that can help them with Bt refuge requirements. “With the old Bt technologies — YieldGard, Herculex, Bt3 — if you’re in an officially-designated cotton-growing area, there’s supposed to be a 50 percent non-Bt refuge. In a non-cotton-growing area that refuge could drop to as little as 20 percent.”
As for the newer technologies, companies are putting in more traits. As a result, “They’re getting redundant control of corn borers — built-in resistance management is the hope. Because of that, for most of the varieties in cotton- and corn-growing areas, the refuge requirement is reduced.”
In cotton-growing areas, producers can reduce the non-Bt refuge requirement to 20 percent. In a corn-growing area, growers have more options.
“With some of them, you can go down to a 5 percent refuge,” notes Stewart. “You can plant a refuge in a bag where the seed is mixed in. Some require a 10 percent refuge, depending on the traits. Some stick to 20 percent. Do your best — make an honest effort.”
Another way producers can help with their insect control is to get a good stand.
“Corn is one of the most sensitive crops we have to stand loss and uneven stands,” Scott says. “You need the right plant population, you need an even plant population. We’ve come a long way. We now have vacuum planters. We’re doing so much better in our corn production.”
With a good Bt package, Stewart says, corn is rather simple to grow once it’s out of the ground with a good stand. It’s not an intensive insect-management crop, which is attractive. “Poll all the entomologists in the Mid-South and I’d guess every one of them would say you need an at-planting insecticide. Either use a seed treatment, an in-furrow treatment, or both.
Stewart hasn’t seen Southern corn rootworm in 10 years except in a few sweet corn fields. “That’s because we’re using seed treatments. But remember, some cover crops can be a ‘bridge’ to the main crop.”
If you’re buying corn seed, said Stewart, “You’re going to get an insecticide seed treatment. It’s standard. The one you get pretty much depends on which company you buy from — DEKALB and Poncho, Pioneer and Cruiser. Standard rates are 250 milligrams.
“Ninety percent of the time those are very good seed treatments. In most of my tests, that’s all that’s needed.”
However, that isn’t always the case. There are some risk factors to consider, says Stewart. “Do I choose one over the other? Should I special order a higher seed treatment rate? There is a trend in the industry to move up to a rate of 500 milligrams as a base treatment. My experience has been that the higher rate is more beneficial as you move farther south.”