Researchers utilized genomic analysis to explore the genetic modifications leading to resistance against transgenic crops in wild populations of the corn earworm, also referred to as the cotton bollworm, or Helicoverpa zea. Their findings indicated that the resistance developed in the field did not correlate with any of the 20 genes previously thought to be linked to resistance to the insecticidal proteins found in these genetically engineered crops.
Unchecked insect pests can wreak havoc on crops. To tackle this issue and lessen the reliance on insecticide applications, scientists have genetically modified crops to produce bacterial proteins that effectively eliminate major pests without harming humans or wildlife. However, the extensive cultivation of these transgenic crops has triggered swift adaptations in some pests. A recent study published in the Proceedings of the National Academy of Sciences uncovers an unexpected genetic foundation for resistance to transgenic crops in a key agricultural pest in the United States.
Researchers from the University of Arizona’s Department of Entomology within the College of Agriculture, Life and Environmental Sciences employed genomic tools to identify the genetic factors behind resistance to transgenic crops in field populations of the corn earworm, or cotton bollworm, scientifically named Helicoverpa zea. They found that the resistance observed in this destructive pest was not linked to any of the previously identified 20 genes that were thought to convey resistance against the pest-targeting proteins in these genetically engineered crops.
According to Bruce Tabashnik, the senior author and head of the U of A Department of Entomology, “The corn earworm is one of the most formidable pests globally, capable of rapidly developing resistance to genetically modified crops in the field. We previously identified 20 genes with mutations that provide resistance to pest-targeting proteins based on earlier studies done with lab-selected strains of corn earworm and resistant field populations, as well as other related pests. We refer to these 20 genes as ‘the usual suspects.’ However, to our surprise, none of them were implicated in the field-evolved resistance we studied.”
A new ally in the ongoing fight against pests
To defend against the corn earworm and various major caterpillar and beetle pests, some crops have been genetically altered to produce proteins from the common bacterium Bacillus thuringiensis, or Bt. Unlike traditional broad-spectrum insecticides, Bt proteins specifically target a limited number of insect species. Broad-spectrum insecticides function as nerve poisons, while Bt proteins are only toxic when consumed, binding to specific gut receptors that most non-pest species, including humans, lack. Due to their effectiveness and safety profile, Bt crops are cultivated in many countries, covering over a quarter billion acres annually. In the United States, by 2024, the area planted with Bt varieties comprised 86% of corn and 90% of cotton. Nonetheless, the emergence of resistance among pests like the corn earworm has reduced the advantages of Bt crops.
The corn earworm is one of the most economically significant pests in the U.S., causing damages and expenses amounting to hundreds of millions of dollars each year. It targets a diverse array of crops, such as corn, cotton, soybean, and tomato.
A twist in the genetic code
To investigate the genetic basis of field-evolved resistance in the corn earworm, the U of A scientists partnered with researchers from Texas A&M University, who conducted bioassays to evaluate resistance by testing field-derived insects.
“Bioassays are typically conducted to assess whether insects exhibit resistance by exposing them to Bt proteins in controlled lab conditions,” explained Luciano Matzkin, co-author and entomology professor at U of A.
Normally, the insects tested in bioassays are discarded afterward. In this innovative collaboration, frozen specimens from the bioassays at Texas A&M were sent to U of A for DNA extraction and sequencing, enabling the researchers to scan the entire genome for genetic variations between resistant and susceptible corn earworm caterpillars. The genomic analysis ultimately encompassed 937 corn earworms collected from 17 locations in seven states across the southern U.S. from 2002 to 2020.
“We meticulously investigated the 20 genes that had previously been implicated in pest responses to Bt proteins. Our findings suggest that changes in these particular genes are not responsible for resistance in wild corn earworm populations,” stated Andrew Legan, a postdoctoral fellow at U of A and the study’s lead author. “Instead, we linked the resistance to a group of genes that were duplicated in certain resistant field populations. However, it remains unclear how many of these genes actually contribute to resistance or the mechanism through which they do so.”
Even though researchers could not pinpoint the cause of resistance to a specific gene, they emphasize that this study highlights a critical insight: the genetic underpinnings of resistance can vary significantly between field and laboratory settings. This understanding is essential for developing effective monitoring methods for resistance in the field. The results further demonstrate how bioassays, when paired with genomic analyses, can provide valuable information. As this study illustrates, the genetic examination of insects preserved from routine bioassay monitoring can clarify the genetic basis of resistance evolved in the field, while bioassay results deliver immediate insights into the resistance status in the field, aiding in management decisions.
