In evaluating Bt corn hybrids in Alabama during 2011, there were perhaps more questions than answers, due primarily to low insect pressure.

“There are trends, and they show it might not pay off every year to plant a Bt corn hybrid here,” says Kathy Flanders, Auburn University Extension entomologist. “You have to pick the best hybrid for your field, the one that is best adapted to your area.”

Flanders discussed the use of Bt corn hybrids during the recent Central Alabama Corn Production Meeting, held in Autaugaville.

“In 2010, we had high corn borer populations in north Alabama and high caterpillar populations in south and central Alabama,” says Flanders. “Then, in 2011, we had low borer populations and lower caterpillar feeding, so that’s reflected in not seeing much difference in yields between the Bt corn and non-Bt corn this past year.”

All of the Bt corns being sold now are going to work on stalk borers, so if you farm in north Alabama or extreme southwest Alabama, where growers have problems with stalk borers, any of the currently available Bt corn varieties probably will help you, she says.

“In central Alabama, growers don’t have a problem with stalk borers, so the main problems will be with corn earworms and fall armyworms attacking the ear. So your yield increases probably will come from protecting the corn ears,” she says.

The newest kid on the block as far as Bt corn hybrids is the Agrisure Viptera trait, and it’s the only one that provides protection against corn earworms, says Flanders. In trials last year, hybrids with the Agrisure Viptera trait kept the corn ears very clean, she adds.

“Will these do you any good or help you with your yield?” asks Flanders. “Nothing was really significant in 2011. The ones with the more advanced Bt technology seem to give you some higher yields than if you don’t have Bt, but it wasn’t statistically different this past year.”

Trials are on-going to help determine if any of the Bt corn hybrids help with aflatoxin prevention, says Flanders. “This past year, under extreme drought stress, there were lower amounts of aflatoxin in some of the newer technologies. But aflatoxin is very variable from plot to plot, and we’ll have to look at this over several years. Caterpillar feeding and drought stress are only two things that affect aflatoxin. There are other things that increase the pressure.”

Select seed treatment based on risk

Flanders advises corn producers to select their seed treatment based on their risk. “If you’re conventionally tilled, and you’re rotating, you’re probably at low risk for any early season insects, so any of your low-rate seed treatments probably would work fine. If you start getting some risk factors, then you would pick a higher rate of seed treatments.”

Growers with generally rotated, no-till fields might be at higher risk for soil insects, she says.

“No-till is more of a risk for soil insect damage, so if you can, get a slightly higher rate of your seed treatment. The 500-rate is a good overall average if you’re planting into a lot of residue.”

Then, she says, select your Bt corn technology. “It won’t be the first consideration when you’re selecting your corn hybrid, and you might not always help yourself by planting Bt corn.”

In other Bt corn-related issues, there were reports this past year from Iowa of resistance to Bt corn by field populations of the Western corn rootworm (WCRW). WCRW is sometimes called the billion dollar pest because of the extent of damage it causes to corn and the cost of its control.

(For a look at how the resistance problem developed last summer, visit http://southeastfarmpress.com/grains/severe-root-damage-bt-corn-confirmed-illinois. Additional information can be found here).

Bt corn with activity against rootworms has been available to producers since 2003 and have been effective at preventing rootworm damage across the Corn Belt.

A common characteristic of all the fields with resistance was that Bt hybrids expressing the Cry3Bb1 protein had been used for three or more consecutive years in these fields. 

So should growers in the Southeast be worried about this occurrence?

According to Dave Buntin, University of Georgia grain crop entomologist, the Western corn rootworm has been spreading eastward and southward for decades and was first found damaging corn in northwest Georgia about 1990. 

“It is now present in the northern two-thirds of the state,” says Buntin. “WCRW has only one main host, corn.  It has one generation per year and over-winters in the egg stage in the soil. Female beetles are attracted to silking corn and lay eggs in the soil. If corn is planted in the same field the following year, larvae attack the corn roots.”

All but a small percentage of the corn grown in Georgia, he says, is rotated with other non-host crops which effectively prevent WCRW damage.  

“The handful of fields I have seen with damaging levels of WCRW in Georgia all have been on farms with cattle or dairy operations where corn is being grown continuously for grain or silage.

“In a few cases where crop rotation was not an option, hybrids with a Bt rootworm trait have been very effective in controlling these infestations,” he says.

Soil-applied insecticides or the use of Poncho 1250 seed treatment are other useful control measured, he adds. “Reduced susceptibility by WCRW to Bt corn should not be an issue that directly affects most corn producers in Georgia. Nevertheless, this is an example of the importance of not relying on the repeated use of a single method and insecticide for pest control. 

“Instead, an integrated approach with rotation of crops and insecticides, including Bt toxins, will help to prevent the development of resistance by target pests.”

(There are some who believe it is harder to break down the stalks of Bt corn in preparation for another crop. The answer to that is found here).

phollis@farmpress.com