Corn rootworms, pests responsible for billions of dollars in yearly crop losses, are evolving resistance that weakens even the latest biotechnology controls, according to a new study published in the journal Proceedings of the National Academy of Sciences. 

Drawing on decades of data across multiple states, University of Arizona entomologists found that field-evolved resistance to Bacillus thuringiensis, or Bt, is undermining the effectiveness of corn that targets rootworms with the combination of Bt and RNA interference, or RNAi, a new biotech control that turns the rootworms' own genetic instructions against them.

The research team analyzed extensive field data collected over the past two decades in 12 previous studies, including millions of rootworms evaluated across the Corn Belt, which extends from western Ohio to eastern Nebraska and northeastern Kansas.

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"The results consistently show that in fields where resistance to Bt has evolved, the combination of Bt and RNA interference provides less protection from rootworm damage," Bruce Tabashnik, lead author on the study and head of the U of A Department of Entomology. 

"This isn't lab data," he added. "This is real-world, on-the-ground field data gathered from university and industry research across multiple states."

Billion-dollar beetles

Corn rootworms are called "billion-dollar bugs" for good reason, according to Tabashnik. 

"They're really nefarious, insidious pests," he said. "For each of the past two years, rootworms have caused about $2 billion in yield losses to corn in the U.S., and there's another billion dollars that's being invested yearly to control them."

Hidden underground, these beetle larvae gnaw away at the life blood of America's most valuable crop. In the arms race against these pests, genetically modified crops have been an important tool, increasing yields while reducing farmers' reliance on broad-spectrum insecticides that can be harmful to people and the environment.

Bt corn is engineered to produce proteins from the soil bacterium that kill pests but are not toxic to humans or wildlife, explained Tabashnik. Bt corn targeting rootworms was first introduced in 2003. 

"It was highly effective," he said, "but after several years, field-evolved resistance began to emerge."

Too much of a good thing

Resistance to Bt arises from Darwinian selection, the same process that produces resistance to antibiotics. Because Bt corn was so good at beating back corn rootworms, many growers began to plant it year after year, according to Tabashnik. 

While it wiped out most of the pests, some of them that were naturally, genetically less susceptible inevitably survived and passed on their resistance to the next generation.  

Seed companies responded with the "pyramid" strategy, combining two different Bt proteins to kill the same pest. 

"The idea is that if the insect is resistant to one Bt protein, the other one will kill it," Tabashnik said. "And that helped. But before long the rootworms evolved resistance to both types of Bt proteins."

A new line of defense came in 2022, with the commercialization of RNAi. It works by "silencing" specific genes in the pest, essentially shutting off functions the insect needs to survive. Unlike broad-spectrum insecticides, RNAi targets a gene that is specific to a pest but not present in humans, animals or beneficial insects.

Use of RNAi against corn rootworms is the first application of this technology to control a crop pest. It kills more slowly than Bt and is used in transgenic corn only together with Bt. 

"It was never meant to stand on its own. It was intended to be a one-two punch with Bt," Tabashnik said. "But by the time the RNAi was actually commercialized, the effectiveness of Bt plus RNAi was already undermined because of the pre-existing resistance to Bt."

Losing the biotech advantage

The results are cause for concern, according to the research team, as they underscore the risk of even this latest technology losing its bite against the corn pest. The researchers advise using integrated pest management strategies, including age-old practices such as crop rotation, as well as planting conventional corn "refuges" alongside transgenic crops to allow susceptible pest survival and delay resistance.

"Farmers have been relying on this new technology with the expectation that it would last for many years. Our study shows the lifespan of this tool could be much shorter unless resistance management is taken seriously," Tabashnik said. 

"It's a reminder that there's no silver bullet. Evolution doesn't stop. Rootworms adapt, and if we don't diversify our approaches, we'll keep chasing our tails."