TWN Info Service on Biosafety
11 November 2025
Third World Network
www.twn.my
Dear Friends and Colleagues
Bt and New RNA Interference Technologies Ineffective Against Corn Rootworm
Corn rootworms have caused billions in yield losses in corn in the US. Genetically modified Bt corn targeting rootworms was first introduced in 2003. After several years, field-evolved resistance began to emerge. Seed companies responded with the “pyramid” strategy, combining two different Bt proteins to kill the same pest. A new line of defense came in 2022, with the commercialization of RNA interference (RNAi). It works by “silencing” specific genes in the pest, essentially shutting off functions the insect needs to survive. But the effectiveness of GM crops using Bt plus RNAi has been undermined because of the pre-existing resistance to Bt.
A study analysing two decades of published field data for rootworms across the Corn Belt in the US has found that their field-evolved resistance to Bt proteins is associated with increased root injury on corn producing Bt proteins and the recently introduced RNAi trait. This reduces yield and is especially concerning because of limited previous exposure to this new trait, showing that there is no silver bullet. The results consistently show that in fields where resistance to Bt has evolved, the combination of Bt and RNAi provides less protection from rootworm damage.
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 to delay resistance.
With best wishes,
Third World Network
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Item 1
‘NO SILVER BULLET’ — PEST RESISTANCE THREATENS CORN’S NEWEST BIOTECH DEFENSE, STUDY WARNS
University of Arizona; Farm News Media
https://www.michiganfarmnews.com/-no-silver-bullet-pest-resistance-threatens-corn-s-newest-biotech-defense-study-warns
25 September 2025
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 from the University of Arizona (U of A).
Drawing on decades of data across multiple states, entomologists at the university 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.
“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,” said 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 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 Yves Carrière, study co-author and professor in the U of A Department of Entomology.
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.”
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Item 2
ROOTWORM RESISTANCE TO BT ASSOCIATED WITH INCREASED INJURY TO CORN PYRAMIDS COMBINING BT PROTEINS AND RNA INTERFERENCE
Bruce E. Tabashnik and Yves Carrière
PNAS Vol. 122 | No. 38
https://doi.org/10.1073/pnas.2518683122
https://www.pnas.org/doi/10.1073/pnas.2518683122
18 September 2025
Significance
Crops genetically engineered to produce pest-killing proteins from Bacillus thuringiensis (Bt) control some major pests and reduce insecticide sprays. However, evolution of resistance to Bt crops has decreased these benefits. Analyses of published data for rootworms, the most damaging corn pests in the United States, show that their field-evolved resistance to Bt proteins is associated with increased root injury on corn producing Bt proteins and a recently introduced RNA interference trait targeting rootworms. This reduces yield and is especially concerning because of limited previous exposure to this new trait. Recommendations to sustain control include combinations of traits that are each highly effective, more host plants that do not target rootworms, and integrating transgenic corn with crop rotation and other tactics.
Abstract
Crops genetically engineered to produce insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) have revolutionized pest management, but their benefits have been reduced by evolution of practical resistance in at least 31 cases. To delay evolution of resistance, farmers have shifted from crops producing one Bt protein to crops called pyramids that produce two or more Bt proteins or other traits targeting each pest. Here, we focus on resistance to transgenic corn pyramids in the western corn rootworm (Diabrotica virgifera virgifera) and northern corn rootworm (Diabrotica barberi), which cost farmers in the United States $2 billion yearly in yield losses. We analyzed 998 relevant data values for 2005 to 2023 from 12 published field studies. The results support the hypothesis that rootworm resistance to Bt proteins Cry3Bb and Gpp34/Tpp35Ab reduces the efficacy of pyramids that produce one or both of these proteins and an RNA interference trait (DvSnf7) targeting rootworms, despite no apparent strong cross-resistance between the Bt proteins and DvSnf7. The reduced efficacy of pyramids producing Bt proteins and DvSnf7 entails increased root injury and emergence of adult beetles. Efficacy of DvSnf7 was substantially lower for reducing root injury than emergence. Because root injury decreases yield, the increased root injury has immediate practical consequences. More sustainable control may be achieved by deploying pyramids with traits that are each highly effective against rootworms, increasing the abundance of host plants that do not target rootworms, and combining transgenic corn with crop rotation and other control tactics in integrated pest management programs.