The link between Lepidoptera, including corn earworms, and aflatoxin-producing fungi has been known for more than 40 years. Proving the effectiveness of fungicides and insecticides at controlling the pests was easy. Proving the economic value of doing so has been near impossible, until now.
With new generation traits, such as Monsanto's Genuity family of traits and Syngenta's Agrisure Viptera trait, the odds are getting better that control is worth the cost.
Corn earworms are nasty critters, even though they start out at only 2/100 of an inch long. Eating their way through the silk of a corn ear, larvae turn cannibalistic as they grow larger, munching away on each other as well as the immature corn. When they do enough damage, yield falls. In addition, they literally leave the door open for pathogens that produce mycotoxins such as aflatoxin. Tunneling also interferes with a plant's fluid movement to kernels, mimicking drought stress conditions that also encourage mycotoxin production. Real damage can be done at numbers that are not extremely high. If the industry is lucky, tainted corn is caught before it is fed to livestock or people.
Like the damage from corn earworm, damage from aflatoxin is also a numbers game, and therein lies the rub. Until recently, it was hard to justify foliar treatments to control the insect or fungicides to control mycotoxin-producing fungi, much less controlling the insect to reduce the risk of aflatoxin production. Even without the earworm, other pests can be a factor, or the fungi spores can enter through the silks on their own; no other pest is needed. There are just too many variables — high temperature and water stresses, crop density, growth stage, fertility and weed competition — to provide definitive data.
Traits May Be A Game Changer
Field trial's data sets are now suggesting the situation has changed with the new traits. Part of the improvement in economics is generic; that is, the result of a healthier plant. Although minor infestations can weaken a plant well before economic control thresholds are reached, control of minor injury also can ensure strong growth and strong immune response to attacks.
We've been looking at multiple traits from several companies offering 50 percent to 90 percent control of corn earworm in the ear compared to older technologies," said Angus Catchot, Extension entomologist, Mississippi State University. "The multiple traits are a significant improvement over single gene technology when it comes to corn earworm control."
The problem for Mississippi growers is that while the traits control corn earworm, measuring earworm impact on corn yield is difficult. It may be necessary to assess the cumulative impact of the pest on the broader crop system. "Most of the earworms, 95 percent or greater, whether found in soybeans or cotton, funnel through corn," said Catchot. "If we can stop them in corn, we may be able to reduce their impact in other crops also."
Scott Stewart, Extension entomologist, University of Tennessee, admitted that the new traits are doing a great job controlling earworms with the possibility of reducing mycotoxin levels, but the correlation remains unpredictable, given the sporadic nature of the fungus. He pointed out that although there was more aflatoxin-affected corn in western Tennessee than usual this year, he expected the level to be even higher, given the very hot and dry summer.
"The primary driver for the pathogens remains environmental," stressed Stewart. "You can have aflatoxin occur even in the absence of insects. Ear-feeding insects can passively spread the fungus around, and this can result in more kernels getting infected with the fungus. Thus, Bt traits can have an indirect effect by reducing the spread of the pathogen that causes aflatoxin, however, this does not always correlate with aflatoxin levels.
"The new traits definitely have a potential benefit of reducing mycotoxins, but it's hard to put a value on it," he said. "Often, mycotoxins are not a problem. There may be a case where you reduce aflatoxin, but it's not enough. You can knock levels down from 500 parts per billion to 200, but that's not enough. Of course, there may be a case where it's a difference maker, and the traits get the mycotoxin level down to an acceptable level."
Real Benefits or Just Hype?
Bruce Battles, agronomy marketing manager, Syngenta Seed, is excited about the potential he is seeing with his company's trait package. However, he admitted, it isn't the first time that researchers have gotten excited over the potential to control earworm vectoring of aflatoxin. "There was a lot of work in the 1990s when Bt traits were introduced for corn borer control," he recalled. "They demonstrated a reduction in mycotoxins, but not elimination. In the past, we just didn't have the tools."
Having the right tools makes all the difference, he suggested, pointing to trials showing the same genetics in three different hybrids. They compared a no-added traits hybrid in refuge acres, triple stack Agrisure 3000GT without the Agrisure Viptera trait and the Agrisure Viptera 3111 trait stack.
"We had nine locations over 2008 and 2009 with fumonosin reductions of up to 80 percent in parts per million," he said. "While we don't always correlate fumonosin reductions with corn earworm pressure data, we have correlated insect damage with aflatoxin levels. When we look at the amount of insect damage per trait of hybrid, it matched exactly with the amount of aflatoxin. The refuge hybrid averaged 268 ppb. That dropped to 128 ppb with Agrisure 3000GT without the Agrisure Viptera trait. With the Agrisure Viptera trait, aflatoxin levels dropped to 21 ppb. The Agrisure Viptera trait practically eliminated the aflatoxin."
Is Cost Justified?
Whether or not growers are able to justify the cost of the new traits for aflatoxin control may remain questionable. However, it may be the big picture, as Catchot noted, that will provide the cost justification and aflatoxin as simply one more benefit. He points to the benefit of reduced refuge acres and reduced losses due to Southwestern corn borer on those acres. He also points to improved genetics with all their features that should accompany the new traits.
"I don't think we fully grasp the impact of improved grain quality with the new traits," said Catchot. "There is data out of Georgia that shows a significant improvement in test weight. Although our corn acres have increased significantly, we still aren't getting the elite corn lines. Our growers are willing to plant and manage for high yield. As we see the new elite lines introduced in the South and Mid-South, we may see even greater benefit."