Novel Mechanism of Resistance to Bt Crops in a Major Agricultural Pest

Insect pests can destroy crops if they are not controlled. To limit damage and lessen the need for insecticide sprays, crops have been genetically modified to produce bacterial proteins that kill important pests without endangering humans or wildlife.

However, some pests have quickly adapted to the extensive use of these transgenic crops. According to a recent study published in the journal Proceedings of the National Academy of Sciences, one of the most significant crop pests in the US has a novel genetic basis for resistance to transgenic crops.

Using genomics, researchers from the University of Arizona Department of Entomology in the College of Agriculture, Life and Environmental Sciences examined the genetic alterations in field populations of the corn earworm, also called the cotton bollworm or Helicoverpa zea, that result in resistance to transgenic crops.

They found that none of the 20 genes previously linked to resistance to the pest-killing proteins in transgenic crops were linked to field-evolved resistance in this voracious pest.

The corn earworm is one the world's most challenging pests in terms of its ability to quickly evolve resistance in the field to genetically engineered crops. We identified 20 genes that harbor mutations conferring resistance to pest-killing proteins based on previous work with lab-selected strains of corn earworm as well as resistant field populations and lab strains of other lepidopteran pests. We call these 20 genes 'the usual suspects.' Contrary to our expectations, in seeking the culprit for field-evolved resistance of corn earworm, none of the usual suspects were guilty.”

Bruce Tabashnik, Study Senior Author and Head, Department of Entomology, University of Arizona

A New Tool in the Never-Ending Battle with Pests

Crop plants have been genetically modified to generate proteins from the common bacterium Bacillus thuringiensis, or Bt, to defend them from the maize earworm and a few other significant caterpillar and beetle pests. In contrast to broad-spectrum pesticides, Bt proteins are effective against a small number of insect species.

Bt proteins can only be poisonous if consumed and then attach to particular gut receptors that are not present in the majority of non-pest species, including humans. In contrast, broad-spectrum insecticides are nerve poisons. Bt crops are cultivated on more than a quarter billion acres annually in dozens of countries due to their safety and effectiveness.

In 2024, 90% of the cotton and 86% of the maize cultivated in the US were Bt cultivars. However, the advantages of Bt crops have diminished as a result of pests like corn earworms developing resistance.

With annual damages and expenses totaling hundreds of millions of dollars, the maize earworm is one of the most economically significant pests in the US. It targets a variety of crops, such as tomatoes, corn, cotton, and soybeans.

A Twist in the DNA

The U of A researchers worked with colleagues at Texas A&M University who had tested insects collected from the field using bioassays to assess resistance and examine the genetic basis of maize earworm resistance that has evolved in the field.

Bioassays are used routinely to determine if insects are resistant by exposing them to Bt proteins in the lab.”

Luciano Matzkin, Entomology Professor and Study Co-Author, University of Arizona

Once bioassays are finished, the tested insects are often thrown away. To scan the full genome for genetic changes between the resistant and susceptible maize earworm caterpillars, the insects from bioassays carried out at Texas A&M were frozen and transferred to the University of Arizona for DNA extraction and sequencing.

The genome investigation comprised 937 corn earworms from 17 sites in seven southern US states, sampled between 2002 and 2020, including some previously sequenced specimens.

We carefully examined 20 genes that affected how pests responded to Bt proteins in previous studies. Our evidence indicates changes in these genes are not causing resistance to Bt crops in wild populations of the corn earworm. Instead, we found resistance was associated with a cluster of genes that were duplicated in some resistant field populations. But it remains a mystery as to how many of these genes contribute to resistance and how they confer resistance.”

Andrew Legan, Postdoctoral Fellow and Study First Author, University of Arizona

Researchers said the study serves as a crucial reminder that the genetic foundation of resistance might vary between the field and the lab, even while it does not identify a particular gene as the origin of resistance. This is a crucial disclaimer for creating instruments that track resistance in the field.

The outcomes also show how genetic analysis can be used with bioassays. The study demonstrates that while the results of routine monitoring bioassays can be used to quickly ascertain the level of resistance in the field and aid in management decisions, genomic analyses of resistant and susceptible insects preserved from these tests can help clarify the genetic basis of field-evolved resistance.