The research team, led by Yucheng Wang and Fei Yang of the University of Minnesota together with David L. Kerns of Texas A&M University, Graham P. Head of Bayer Crop Science, Dawson Kerns of Louisiana State University, and Bruce E. Tabashnik of the University of Arizona, set out to answer two questions with direct consequences for resistance management. First, does selection for resistance to Vip3Aa cause cross-resistance to Cry proteins, which would undermine the pyramided crops that produce both toxin classes? Second, do Cry proteins and Vip3Aa interact synergistically against insects that are already resistant, a property that could extend the useful life of multi-toxin Bt crops?

To address these questions, the researchers generated five strains of H. zea carrying more than 100-fold resistance to Vip3Aa in a susceptible genetic background. The strains originated from F2 screens of field populations collected in Louisiana, Mississippi, and Texas between 2019 and 2020, a technique that exposes hidden recessive resistance alleles carried by mating pairs of field-collected insects. Each strain was then crossed repeatedly with a laboratory-susceptible strain, BZ-SS, obtained from a commercial supplier, and reselected with Vip3Aa in the laboratory. After five to six rounds of backcrossing and selection, the resulting strains, designated LT70-Vip, AC4-Vip, M1-Vip, R2-Vip, and R15-Vip, showed such profound resistance that even the highest concentration of Vip3Aa tested, 100 micrograms per square centimeter of diet, killed no more than 15 percent of larvae, implying resistance ratios exceeding 500-fold.

The team then tested these Vip3Aa-resistant strains against five Cry proteins used in commercial Bt crops: Cry1Ab, Cry1Ac, Cry1A.105, Cry1Fa, and Cry2Ab. The verdict was strikingly inconsistent. Across 23 bioassay comparisons, seven LC50 values were significantly higher than those of the susceptible strain, seven were significantly lower, and nine did not differ significantly, a seven-to-seven split that matches exactly what would be expected if no consistent cross-resistance existed. The overall mean cross-resistance ratio was 1.2, statistically indistinguishable from the value of 1.0 expected if selection with Vip3Aa had no effect on susceptibility to Cry proteins. Only Cry1Ac showed a statistically significant mean positive cross-resistance, and only one strain, AC4-Vip, displayed significant overall positive cross-resistance across the Cry proteins tested.

What astonished the researchers was not the absence of a trend but the sheer magnitude of the variation around it. The 23 cross-resistance ratios spanned an extraordinary 1100-fold range, from 0.06, indicating 17-fold negative cross-resistance to Cry1Fa in strain LT70-Vip, to 65-fold positive cross-resistance to Cry1A.105 in strain AC4-Vip. That range is roughly 49 to 70 times wider than those reported in previous reviews of cross-resistance between Vip3 and Cry proteins in other lepidopteran species. The authors suggest that the genetic diversity of the field-derived resistance alleles, which complementation tests place at three distinct loci among the five strains, likely contributes to this variability, as does the unknown mechanism of resistance in most of the strains. Only LT70-Vip has a biochemically characterized mechanism, reduced binding of Vip3Aa to midgut brush border membrane vesicles, and as expected under the receptor model of Bt toxicity, that strain showed no significant cross-resistance to any Cry protein.

The second half of the study tackled synergy, an interaction in which two toxins combined kill more insects than would be predicted from the independent action of each toxin alone. Previous surveys suggested synergy between Cry and Vip proteins is relatively rare, appearing in only about 19 percent of bioassay evaluations reviewed in 2020, though more recent studies on fall armyworm and peach fruit moth reported far higher frequencies. Critically, nearly all prior synergy work used susceptible insects. To test synergy against resistant insects, the team created a triple-resistant strain, TRE-RR, by crossing the Vip3Aa-resistant LT70-Vip strain with a Cry-resistant strain derived from Texas field corn, then selecting successive generations with all three toxins. The resulting strain exhibited resistance ratios of 265 for Cry1Ac, 495 for Cry2Ab, and greater than 96 for Vip3Aa.

Against this formidable laboratory adversary, synergy emerged clearly. In diet bioassays conducted at two concentrations, combinations of Vip3Aa with Cry1Ac produced significantly lower observed survival than expected under independent action at both concentrations tested, and Cry2Ab combined with Vip3Aa was synergistic at the higher concentration. Most remarkably, the three-toxin combination of Cry1Ac, Cry2Ab, and Vip3Aa was synergistic at both concentrations, meaning that even larvae equipped with resistance to every toxin individually died more often when the toxins were delivered together than their separate toxicities would predict. The finding is consistent with the authors’ earlier analysis of field data from Bt corn showing synergy between Cry1Ab and Vip3Aa against susceptible H. zea, but it is, to the authors’ knowledge, the first demonstration of Cry-Vip synergy against a strain resistant to both toxin classes.

The practical implications are substantial. If synergy between Cry proteins and Vip3Aa operates in the field against resistant H. zea, as it appears to against susceptible populations, it would kill some insects that survive either toxin alone, reducing the intensity of selection for Vip3Aa resistance and potentially slowing its evolution. That would be welcome news for an industry and a grower community watching early warning signs of Vip3Aa resistance accumulate in the United States, where practical resistance to Vip3Aa has already been documented in Brazil. The synergy finding also carries a cautionary note for resistance monitoring. Current surveillance relies partly on sentinel plots of corn producing Cry1Ab plus Vip3Aa, from which a phenotypic frequency of resistance is calculated by comparing larval abundance in Bt ears with that in non-Bt ears. If synergy kills some larvae that are resistant to either toxin alone, this approach could underestimate the true frequency of Vip3Aa resistance, a discrepancy the authors recommend resolving by comparing estimates from F2 screens with those from sentinel plots.

The study also refines the scientific understanding of how Bt resistance evolves and spreads. Because Vip3Aa and Cry proteins bind to different midgut receptors, strong cross-resistance between them was not expected, and the new results largely vindicate that expectation while revealing how messy the underlying biology can be. Mechanisms other than altered receptor binding, such as changes in toxin processing within the midgut, can confer broad cross-resistance, and the authors note that determining the resistance mechanisms in the remaining strains could explain the puzzling differences between R2-Vip and R15-Vip, which share a resistance locus yet differed 13-fold in their mean cross-resistance to Cry proteins. For now, the message for resistance management is cautiously optimistic: pyramiding Vip3Aa with Cry proteins remains a sound strategy, the lack of consistent cross-resistance means Cry resistance alleles should not compromise Vip3Aa efficacy, and synergy may quietly buy precious time for one of agriculture’s most valuable pest control technologies as its chief adversary continues to adapt.

Subject of Research: Cross-resistance and synergy between the Bt toxins Vip3Aa and Cry proteins in Vip3Aa-resistant strains of the corn earworm Helicoverpa zea

Article Title: Evaluating cross-resistance and synergy between Vip3Aa and Cry proteins from Bt in six strains of Helicoverpa zea derived via F2 screens

Article References: Evaluating cross-resistance and synergy between Vip3Aa and Cry proteins from Bt in six strains of Helicoverpa zea derived via F2 screens. (n.d.). https://doi.org/10.1007/s44297-026-00073-8

Image Credits: AI Generated

DOI: 10.1007/s44297-026-00073-8

Keywords: Helicoverpa zea, corn earworm, Bt crops, Vip3Aa, Cry proteins, cross-resistance, synergy, insect resistance, transgenic crops, resistance management, Bacillus thuringiensis, pest control

The technical logic of transgenesis is straightforward but powerful. Where a breeder seeking a target trait must find it within the primary gene pool of a crop, a transgenic approach can source the gene from any organism, including microbes, unrelated plants, or even the crop’s own genome rearranged in new ways. The major traits incorporated into commercial transgenic crops to date fall into four broad categories: herbicide tolerance, insect resistance, tolerance of abiotic stresses such as drought and salinity, and nutritional enhancement. Insect-resistant crops typically deploy genes derived from the soil bacterium Bacillus thuringiensis, whose crystalline toxin proteins, modified for enhanced expression in plant tissues, disrupt the gut of specific larval pests while leaving beneficial insects largely unharmed. Herbicide-tolerant crops carry enzymes insensitive to broad-spectrum herbicides such as glyphosate, allowing farmers to control weeds with simpler spray regimes. The review emphasizes that these are not exotic interventions but extensions of a long tradition of genetic exchange that plant breeders have practised for a century, from the transfer of leaf-rust resistance from Aegilops umbellulata into wheat in the 1950s to the wide hybridization programmes that underpin modern triticale.

The global adoption record is striking. Since the first commercial plantings in 1996, biotech crops have spread to tens of millions of hectares across more than two dozen countries, with soybean, maize, cotton and canola dominating the area. Meta-analyses of agronomic and economic outcomes, including the widely cited work of Klümper and Qaim, indicate that transgenic varieties have generally delivered reduced pesticide use, higher yields and improved farmer profits, with the largest gains accruing to smallholders in developing countries. Economic assessments covering 1996 to 2020 estimate cumulative farm income gains in the tens of billions of dollars and document reductions in pesticide spraying and associated carbon emissions. The review notes that these benefits have not been uniform, and that outcomes depend heavily on the trait, the crop, the pest complex and the stewardship practices that accompany deployment, but the overall pattern of evidence supports the view that the technology, when well managed, has been a net positive for productivity and environmental load.

India’s own experience with Bt cotton is the centerpiece of the domestic evidence base. Approved for cultivation in 2002, Bt cotton expressing the Cry1Ac toxin, and later stacked events, spread rapidly to cover more than ninety percent of the cotton area, transforming India from a cotton importer into one of the world’s largest producers and exporters. Yields rose, insecticide sprays against bollworms fell, and farm incomes improved in the early years of adoption. Yet the review is candid about the cautionary side of the story. Field-evolved resistance in the pink bollworm, Pectinophora gossypiella, was documented in central India within a decade of introduction, and widespread infestations on Bt cotton have since eroded some of the early gains. The authors point to inadequate compliance with refuge requirements, the planting of non-Bt cotton strips designed to slow the evolution of resistant pests, and the persistence of unapproved or diluted seed markets as contributing factors. The lesson, they argue, is not that the technology failed but that stewardship, seed quality control and resistance management are inseparable from any transgenic deployment.

Safety concerns have shadowed transgenic crops from the outset, and the review confronts them directly. Novel gene combinations have raised questions about potential risks to human and animal health, biodiversity and the environment, and these questions have fuelled some of the most contentious episodes in modern science communication. The authors revisit the Séralini affair, in which a 2012 rodent feeding study linking glyphosate-tolerant maize to tumours was widely criticized on statistical and design grounds, retracted, and later republished elsewhere, with regulatory agencies including the European Food Safety Authority concluding that it did not demonstrate harm. They also note the monarch butterfly controversy, in which laboratory findings of Bt pollen toxicity to larvae were subsequently weighed against field evidence and habitat loss as the dominant driver of population declines. More broadly, comprehensive reviews of livestock feeding studies have found no consistent adverse health effects from approved GM feed, and the National Academies of Sciences, Engineering and Medicine concluded in 2016 that genetically engineered crops are as safe as their conventional counterparts when properly regulated. The review stresses that every commercial transgenic crop undergoes rigorous evaluation for agronomic performance, food and feed safety, and compositional equivalence before release.

One of the more provocative threads in the review is the argument that transgenes are not inherently hazardous and that nature itself has been moving genes across species boundaries for millennia. The cultivated sweet potato, a staple food for millions, carries Agrobacterium T-DNA sequences with expressed genes in its genome, the result of ancient horizontal gene transfer, making it in a literal sense a naturally transgenic crop. Widespread horizontal gene transfer between plants and bacteria continues to be documented in modern genomes. The authors argue that increasing public awareness of such facts is critical for building societal confidence, because the perception of transgenesis as an alien and uniquely dangerous intervention rests on a misunderstanding of both the technology and natural biology. Transparent communication, they contend, must become a core component of any deployment strategy rather than an afterthought.

Gene flow and containment receive detailed technical treatment. Pollen-mediated transgene escape from cultivated fields to wild or weedy relatives has been demonstrated in crops such as rice, rapeseed and cotton, and the review surveys the strategies available for containment, including chloroplast transformation, male sterility systems, genetic use restriction technologies and spatial isolation. The risk profile, however, is highly crop-specific. A transgenic trait in a self-pollinating crop with no compatible wild relatives in the region of cultivation poses a very different ecological question than the same trait in an outcrossing species surrounded by interfertile wild populations. This crop-specific logic underpins the review’s central recommendation about where India should focus its transgenic efforts next.

Applying criteria that include the availability of target traits within the primary gene pool, consumption patterns, ecological risks and lessons from past experience, the authors identify transgrafted fruits, sugarcane, forage crops and maize as promising candidates for India. Transgrafting is technically elegant: a transgenic rootstock conferring disease resistance or other traits can be grafted with a non-transgenic scion, and in some systems mobile molecules such as transgene-derived small interfering RNAs travel from rootstock to scion, conferring virus resistance without genetically modifying the harvested fruit. Studies in grapevine, apple, sweet cherry, plum and blueberry have demonstrated the feasibility of the approach, and because the edible product may not carry the transgene, regulatory and consumer barriers could be lower. Sugarcane, propagated vegetatively and not grown from seed in the field, presents minimal gene-flow risk, while forage crops and maize offer clear targets in pest resistance and stress tolerance, with maize already the subject of global transgenic experience that India can draw upon directly.

The regulatory dimension is treated as decisive. India’s case-by-case framework, overseen by the Genetic Engineering Appraisal Committee, has approved only Bt cotton for commercial cultivation, and the long-pending case of genetically modified mustard illustrates how regulatory decisions can stall under public and political pressure. Meanwhile, the government has moved to clarify the status of genome-edited plants, issuing standard operating procedures for regulatory review of SDN-1 and SDN-2 categories of targeted mutagenesis, which do not introduce foreign DNA and are treated more leniently in many jurisdictions. The review argues that strengthening regulatory capacity, ensuring timely and science-based decisions, and addressing public concerns through transparent communication are essential for effective and socially acceptable deployment. It also points to the experience of Bangladesh, where Bt eggplant was commercialized for smallholders, as evidence that South Asian farming systems can benefit from well-managed transgenic introductions beyond cotton.

The overall message of the review is one of disciplined optimism. Transgenic technology is neither a panacea nor a menace; it is a tool whose value depends on choosing the right crops, the right traits and the right stewardship systems, and on building the regulatory and communicative infrastructure that allows evidence rather than fear to guide adoption. For India, with its mounting pressures of population, climate variability, shrinking arable land and degrading water resources, the authors argue that the strategic question is not whether to use transgenic innovation but where and how to deploy it first. Their answer, grounded in global evidence and Indian experience alike, points toward fruit crops protected through transgrafting, sugarcane, forages and maize as the sensible frontier, with Bt cotton’s mixed legacy serving as both inspiration and warning for the road ahead.

Subject of Research: Global transgenic crop innovations and their strategic application to Indian agriculture

Article Title: Advancing Indian Agriculture Through Global Transgenic Innovations

Article References: Dhillon, B., Singh, A., Sohu, V., & Rakshit, S. (2026). Advancing Indian Agriculture Through Global Transgenic Innovations. Indian Journal of Genetics and Plant Breeding, 86(3), 261-278. https://doi.org/10.1007/s44489-026-00030-3

Image Credits: AI Generated

DOI: 10.1007/s44489-026-00030-3

Keywords: transgenic crops, Bt cotton, India, transgrafting, gene flow, biosafety, herbicide tolerance, insect resistance, pink bollworm resistance, maize, sugarcane, genome editing