Armyworms develop resistance to Bt corn

November 18, 2014 by Matt Shipman, North Carolina State University
Fall armyworm on a corn leaf. Credit: Dominic Reisig

In fall 2013, Dominic Reisig got a phone call from a farmer in rural Hyde County, N.C. The farmer was growing corn, and it was literally falling apart in the field. What was going on?

Reisig is an entomologist at North Carolina State University; a sort of science detective who specializes in insects that pose a threat to crops. And the farmer had presented him with a mystery.

When Reisig arrived, he quickly determined that the culprits were fall armyworms (Spodoptera frugiperda), a pest species that costs farmers in the southeastern United States tens of millions of dollars each year.

What made this case so perplexing was that the farmer had planted a variety of genetically engineered that produces a Bacillus thuringiensis (Bt) protein that should be fatal to armyworms. But it wasn't.

As adults, S. frugiperda are unprepossessing moths with drab wings that fade from a mottled brown forewing to a creamy pale hindwing. But in their larval stage, S. frugiperda are fall armyworms – caterpillars the size of a pen cap that devour whole crops, from soybeans to cotton to corn.

In short, armyworms are cause for concern. Especially if they have developed the ability to devour the toxins farmers use to protect their crops.

Armyworms develop resistance to Bt corn
Corn plants displaying damage from fall armyworms. Credit: Dominic Reisig

So Reisig collected some of the armyworms from the farm in Hyde County and took them back to his lab, where he created a colony of the insects for study. And while he bred armyworms, he also grew two varieties of corn in his greenhouse – some Bt corn, like that grown by the farmer, and some non-Bt corn. Then Reisig fed the corn to both "normal" armyworms and to the armyworms he'd brought back from Hyde County.

Both armyworms could eat the non-Bt corn. But while the normal armyworms died when fed Bt corn, the Hyde County armyworms ate their way right through it – oblivious to the poisons that should have killed them.

Meanwhile, Reisig also reached out to a colleague at Louisiana State University named Fangneng Huang, an acknowledged expert on armyworms and resistance to Bt crops.

Huang had discovered armyworms that could cope with Bt proteins in the past, but none had been found further north than Louisiana or Florida. Could these NC worms be a new outbreak of the resistant pest?

Reisig sent samples of the Bt-resistant North Carolina armyworms to Huang for analysis. Huang ran tests on the NC armyworms and found that they were among the most Bt-resistant strains of armyworm that Huang had tested – equivalent to a strain found in Florida.

"This is a huge wake-up call for farmers north of Louisiana and Florida – this is definitely something to keep an eye on," Reisig says. "Resistance happens, and it's a stark reminder that we need to take steps – such as planting non-Bt 'refuge' crops near the Bt crops – to limit the development of resistant insect strains."

To date, fall armyworms have not been a major crop pest in North Carolina, since they can't survive a winter freeze. Studies have shown that these insects often migrate north from Florida, and this year Reisig found them in North Carolina corn fields as early as May.

The real fear is that these Bt-resistant armyworms will move from corn to cotton.

"That would be problematic, since cotton usually begins producing fruit (the marketable cotton bolls) in late July or early August – when fall armyworm populations have grown," Reisig says. "Corn is less threatened, since it is at its most vulnerable in the spring, when fall armyworm populations are still low in N.C.

"We've done greenhouse studies and found that this Bt-resistant armyworm can certainly eat genetically modified cotton; we just haven't seen it happen in the field yet," Reisig says.

Explore further: Insect-resistant maize could increase yields and decrease pesticide use in Mexico

More information: Huang F, Qureshi JA, Meagher RL Jr, Reisig DD, Head GP, et al. (2014) Cry1F "Resistance in Fall Armyworm Spodoptera frugiperda: Single Gene versus Pyramided Bt Maize." PLoS ONE 9(11): e112958. DOI: 10.1371/journal.pone.0112958

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not rated yet Nov 18, 2014
Now what?
3 / 5 (2) Nov 18, 2014
Shoddy "journalism"......they need to tell the whole story....not just cut out the parts that explain what is really going on.....
The North Carolina armyworms were indeed resistant to the Cry1F protein, which explained why they caused that North Carolina farmer so much trouble, since the corn variety that he was growing expressed the Cry1F protein only. This also led the researchers to believe that the armyworms probably originated from Puerto Rico.

5 / 5 (2) Nov 18, 2014
While the researchers found that this particular strain of fall armyworm is resistant to the Cry1F protein and a few others, they also found that they were not resistant to other Bt proteins, such as Cry2Ab2 or Vip3A — nor were they resistant to pyramided Bt corn varieties that contain more than one insecticidal protein. That's good news for corn farmers like the one from North Carolina who originally found the Cry1F-resistant armyworms, because it means they can still grow other Bt corn varieties in the future, reducing their reliance on chemical insecticide sprays.
4.5 / 5 (2) Nov 18, 2014
Well, farmers! Just keep planting the same thing year after year and relying on the failed culture of bugkillers to rescue you arses! See what you get. Now plant some other stuff and rotate your crops like your fathers were taught in the 1950's. Setting up a monoculture is always bad as the pests that dine on one crop tend to multiply tremendously, magnifying crop losses. Rotating crops keeps the pests off balance and diversifies the ecology of your fields, just like you were probably still taught at ag school. I know that banks make farmers stupid by hanging the lure of money and security in exchange for doing stupid things like corning your fields to death. Bankers are rape, ruin, and run boys. As soon as your soil is dead, they will drop you like a dead mouse.
not rated yet Nov 19, 2014
Yet another pesticide loses its efficacy. This should be no surprise. It would be more honest if, as new pesticides are marketed, they are accompanied with information estimating how many seasons they can be used before resistance develops. Thus farmers will better be able to predict when they need to consider new strategies.
Nov 19, 2014
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1 / 5 (1) Nov 19, 2014
I wonder how many people here just what Bt-corn actually is & how there is a correlation with the adoption of Bt-corn & increase in gastro-intestinal afflictions in USA...
Suggest you watch this documentary at your earliest convenience, it is reasonably well balanced & well structured, worthy of consideration:-
"Genetic Roulette - The Gamble of Our Lives"

For the least biased possible information on Bt-corn, here with references:-

It should be understood insect's gut is not that dissimilar to human gut in terms of chemical processes after all we have the same core amino acids, same DNA structures, same reliance on energy production etc

Could somebody from Monsanto or that favor's arbitrarily modifying natural genes that have stood the test of millions of years of robustness explain why having bacterial toxins included in our food which we have not adapted to should NOT have any deleterious effects ?

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