Sweet corn growers battle resistant corn earworm

Sweet corn growers across the Midwest have faced another year and a new round of battling their most recent foe: pyrethroid-resistant corn earworm (Helicoverpa zea).

Researchers in the upper Midwest have faced a good-news, bad-news situation in research plots the last couple of years.
Photo courtesy of Monsanto.

The problem seems more pronounced the farther north and west one goes, with Minnesota and Wisconsin reporting the greatest problems. The economic impact varies by year and by geography.

“It’s not a clear-cut situation; we’re dealing with a moving target,” says Eric Burkness, senior research fellow at the University of Minnesota. “The situation changes based on when the moths arrive here from the southern United States and what insecticides they encounter along the way. We may have a year with very high resistance or a year with no problem at all,” Burkness says.

“In Ohio, we are not seeing the loss of efficacy that they are seeing farther west of us,” says Jim Jasinski, Ohio extension IPM coordinator located in Champaign County. He ran a series of sweet corn trials at South Charleston, in Clark County, to see whether he could identify the problem seen in the upper Great Lakes.

As little as five years ago, a standard spray program all but eliminated corn earworm, allowing growers to produce plump ears filled all the way to the end. Suddenly, growers noticed that sometimes their sprays were not working as well. Effectiveness of the routine pyrethroid sprays dropped to 90 percent and then went lower than that in some cases.

Growers producing for the wholesale market know that buyers like to see damage well under 5 percent. In places like Indiana and Ohio, where much of the sweet corn is sold to the fresh market retail channels, this is a serious problem. In places where much of the corn is handled through roadside markets, consumers are a bit more forgiving, but still don’t like to see unsightly ear tips or worms in their corn. Ironically, the damage penalty is not too serious in the Minnesota-Wisconsin area where the problem first was detected; those growers serve process markets where ear ends typically get chopped off.

“All of a sudden we noticed this trend and we didn’t understand why the pyrethroids were not working well. We did not know why it was happening,” says Celeste Welty, extension entomologist and IPM specialist at the Ohio State University.

There was no question the market needed some help fast, and in the beginning there were not a lot of alternatives. As the problem became commonplace across the Midwest, teams of researchers went to work.

The best defenses

The growers are fighting back. The message today is to focus on proper application timing, scout fields well and, if there are high corn earworm counts, to alternate families of chemistries.

“Our primary focus is on timing,” says Burkness. “Growers have to be aware of the best time to spray in relation to the sweet corn’s growth stage.”

This puts a lot of pressure on growers to know what is happening in their fields, since broad research findings can vary by county, or even areas within a county. “We haven’t fully pinned down what is going on,” Burkness continues.

Moths migrate into Minnesota, for example, in early August. “Researchers have a network of traps from down in the Gulf States up through Iowa and Minnesota and into the East,” he continues. The results are posted to the PestWatch Web sites. Minnesota’s tracking page is at www.vegedge.umn.edu. In the East, Penn State coordinates efforts at www.pestwatch.psu.edu.

Wherever you track the moths’ flights, it is important to have some idea of the paths of the moths to your area. “It all depends on the weather where you are located whether the infestation will be greater or lower,” Burkness says.

Pyrethroid-resistant corn earworm continues to impact the Midwest’s sweet corn production.
Photo courtesy of Eric Burkness.

In the best of all worlds, a grower facing the problem could simply migrate away from the pyrethroids; the problem is cost. The new materials are expensive—quite expensive. There are some products based on new chemistry that are on the market right now, but they are much more expensive than traditional materials. The good news is that they seem to work. Welty says the two best bets for Ohio growers are DuPont’s Coragen or Dow’s Radiant. Burkness agrees that Coragen does the job and would add Belt from Bayer. Syngenta has a product pending registration on sweet corn called Voliam Xpress, which is a combination of the active ingredients in Coragen and Warrior. There is another product called Hero from FMC that combines two different materials, bifenthrin and zeta-cypermethrin (Mustang). Some growers have reported success with it.

Researchers in the upper Midwest have faced a good-news, bad-news situation in research plots the last couple of years. The good news for those producing sweet corn is that earworm infestations have been light; the bad news is that low populations make it tough to get reliable research data.

“We don’t see consistent results year to year as far as the pyrethroids go,” Burkness says.

Corn earworm moths typically blow up from the South into the Midwest, entomologists note. For this reason, a year-to-year rotation is unlikely to solve the problem for Midwesterners. Those moths that migrate North are the ones that have survived the aggressive spray programs of Southern growers.

Early season sweet corn usually escapes the corn earworm scourge. It is the middle-season and late-season corn crops that come under attack.

Just as the corn comes into silk, air currents blow the moths up from as far south as Louisiana. “They get dropped out depending on the weather fronts,” Jasinski explains. “Our biggest flights are from August to September, the peak of the sweet corn season.”

Jasinski says Ohio growers are not complaining about the loss of efficacy that other growers are noticing. “I’d say, based on what we have seen in this area so far, it is not a problem,” he says.

Where the problem exists, entomology researchers are still looking at the most effective rotation of sprays. Rotation should be done within a year’s spray schedule. Most growers do about six sprays each year. Some growers who have noticed a slippage in pyrethroid efficacy are running a regular mix of new chemistry and pyrethroid like this: NC, P, NC, P, NC, P. Others are doing their first two sprays with a new material so the rotation looks like this: NC, NC, P, P, P, P.

The second program has the benefit of requiring only two of the more expensive new sprays, rather than three.

An Area to Worry About, An Area to Relax

If you are an Eastern Corn Belt area sweet corn grower who likes to worry, worry about western bean cutworm. Odds are if you are a grower from Nebraska, you are familiar with WBC, an established pest on dry beans. It mainly stuck to the Western Corn Belt and was an understood pest. For some reason, it began moving east through Iowa and has been found into Michigan.

In Ohio, Celeste Welty has seen moths, but not many WBC larvae. It could be a year or two until the pest becomes a factor in the Buckeye state, but it is something growers need to keep on their checklists. Don’t start worrying until later in the season; this pest is not expected to show up until after the European corn borer or the fall armyworm arrive.

“WBC is on our radar. We know about it and we are monitoring it,” says Minnesota’s Eric Burkness. He says it is found all the way into Pennsylvania. “It is widespread, but in low numbers,” he adds. In most regions it is not yet an economic pest. “It can happen, but it is rare,” he adds.

However, the news is brighter on the corn flea beetle front. Corn flea beetle transmits Stewart’s wilt. The good news, according to Welty, is that Ohio State is predicting corn flea beetle numbers to be negligible through the 2010 season. Growers can thank the cold weather this past winter for that.

What About TG Sweet Corn?

Whatever the pesticide program a grower uses, most have at least been exposed to a pitch on planting transgenic sweet corn varieties. “Transgenic crops work well,” says Ohio State IPM Expert Jim Jasinski. Most entomologists across the Midwest would agree with that assessment.

The problem with TG crops is the consumer perception of transgenic materials. Let’s face it, one of the real attractions of buying corn directly from a farmer at a roadside stand is knowing that the corn is fresh and locally produced. People who are attracted to good, local produce are likely to be much more concerned about the other qualities of the corn they eat than the person who grabs a box of frozen ears out of a grocery freezer.

On the other hand, the extensive use of Bt field corn actually is helping sweet corn growers. Minnesota’s Eric Burkness says they find Bt field corn reducing the populations of corn borer and that cuts pressure on sweet corn fields.

“Typically, sweet corn has more pesticides than field corn,” he says. So even though it might seem borers would seek refuge in sweet corn fields, the reality is that Bt field corn has lessened the borer’s impact. The University of Minnesota plans more work in the near future to confirm that.

Modified sweet corn does work, too, Jasinski says. Consumers worried about chemical sprays might find comfort in knowing the amount of material used is reduced, and the corn still looks good. “They work quite well against corn borer and earworm,” he says. “There is a new generation coming that is even better for earworm,” he adds.

The disclosure issue is one the grower must face. “If the customer asks, you have to tell them yes or no,” Jasinski says. That can be difficult for growers who have one block of TG corn and another of traditional.

Growers using TG materials should also be aware that some secondary pests that a normal spray schedule gets, like aphids, may show up in the fields, since the amount of chemical is reduced.

Curt Harler is a freelance writer and a frequent contributor to Moose River Media. He resides in Strongsville, Ohio.