Dominic Reisig, an entomologist at North Carolina State University, received a phone call in 2013 from a farmer who was having trouble with fall armyworms (Spodoptera frugiperda), an insect pest that costs farmers in the southeastern United States tens of millions of dollars each year.
This seemed odd because the farmer had planted Bt corn, a genetically-modified variety that produces a protein that should be fatal to armyworms. In fact, after 18 years of intensive Bt-maize planting, there has never been any documentation of field resistance to Bt corn in armyworms or any other moth pests in the mainland U.S. north of Florida or Louisiana.
However, Dr. Reisig knew that a strain of fall armyworm from Puerto Rico had been documented as being resistant to a certain Bt protein called Cry1F — which is one of many Bt proteins used in corn and other crops. So, he and other entomologists from corn-growing states designed experiments to determine whether the North Carolina armyworms were also resistant to Cry1F, and if so, how much of a threat they might be to corn growers. The results of their study were published in the journal PLOS ONE.
In addition to the NC sample, the researchers gathered armyworms from Louisiana, Florida, Georgia, and Texas, and they fed them different varieties of insect-resistant corn. While some of the corn varieties expressed only one Bt protein, like Cry1F, others — known as “stacked” or “pyramided” varieties — expressed more than one protein.
They’re probably from Puerto Rico
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.
“It’s a pretty solid assumption,” Dr. Reisig said. “Fall armyworms can’t overwinter in North Carolina because it’s too cold, and a previous study by Rodney Nagoshi and others showed that the North Carolina armyworms are genetically similar to those from Puerto Rico.”
However, just to be sure, samples of the insects have been sent to two different labs for DNA testing.
Unlikely to be a problem for corn farmers
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.
“Switching to a different Bt toxin, such as a hybrid that expresses Vip, or pyramiding will solve the problem for corn farmers,” Dr. Reisig said, while also warning that the resistance should not be taken lightly.
“This is a huge wake-up call for farmers north of Louisiana and Florida — this is definitely something to keep an eye on,” he said. “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.”
One of his co-authors, University of Georgia Professor David Buntin, agreed: “This indicates that we have to manage these traits through good stewardship to minimize resistance. But from a practical standpoint, the fall armyworm is a migratory pest that tends to be very late in the season, so we haven’t seen much in the way of field failures. But it does suggest that we need to move to pyramided products and phase out these single-gene products because the resistance to pyramided products is much less likely to occur in the field.”
In addition to corn growers, cotton farmers should take also heed because the Cry1F-resistant armyworms could impact WideStrike cotton, a variety that also contains the Cry1F protein.
“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,” Dr. Reisig said. “Corn is less threatened, since it is at its most vulnerable in the spring, when fall armyworm populations are still low in North Carolina.”
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