Hybrid megapests evolving in Brazil are threat to crops worldwide

Two “megaplagues” that are already a major problem for farmers around the world, the cotton bollworm and the corn earworm, crossed paths in Brazil and exchanged genes conferring resistance to pesticides. Developing hybrid strains could devastate soybeans and other crops in Brazil and around the world if they cannot be controlled, threatening global food security.

“This could become a huge problem,” says Chris Jiggins of the University of Cambridge.

Many countries notably import soy from Brazil to feed humans and animals. “It kind of feeds the world,” Jiggins says.

More than 90 percent of soy grown in Brazil is genetically modified Bt soy containing an integrated pesticide. If yields fall due to pest resistance, this will cause the price of many foods to rise even further. It could also increase deforestation and greenhouse gas emissions, with farmers compensating by clearing more farmland.

Corn earworm (Helicoverpa zea) is a butterfly native to the Americas whose caterpillars eat most plant parts. They are particularly damaging to corn, but also feed on many other plants, including tomatoes, potatoes, cucumbers, and eggplants (aubergines).

In Brazil, H. zea was not a major problem for farmers growing soybeans, as it tends not to feed on the crop. But then, in 2013, the cotton bollworm (Helicoverpa armigera) was detected in Brazil. H. armigera is a relative of H. zea which is widespread throughout Eurasia. Both moths have been described as megapests because they are very damaging and difficult to control.

“They’re pretty exceptional pests, so I think it’s warranted,” Jiggins says. “Controlling the movement of butterflies is almost impossible. They travel very great distances.”

H. armigera also feeds on a wide range of plants and, unlike H. zeait thrives on soy, which caused huge problems for farmers when it arrived in Brazil. “It cost Brazilian agriculture billions of dollars,” says Jiggins.

This problem has been largely solved with the introduction of Bt soy, which is genetically modified to produce a protein made by soil bacteria. Bacillus thuringiensis which is toxic to most insects.

We thought that H. armigera And H. zea could not interbreed, but in 2018 genetic analysis revealed a few hybrids between the species. Jiggins and his colleagues analyzed the genomes of nearly 1,000 moths collected in Brazil over the past decade.

They found that a third of H. armigera now carry genes conferring resistance to the Bt toxin – and they got these genes H. zea. Bt corn was first introduced to North America in the 1990s, where some H. zea the strains have developed resistance. These resistance genes appear to have spread to South America and are now crossing species. For now, the hybrid H. armigera That hasn’t been a major problem, Jiggins says, but that could change as resistance spreads.

The transfer went both ways – almost all H. zea in Brazil now have a gene conferring resistance to a class of insecticides called pyrethroids, acquired from H. armigera. “We’re just blown away by how quickly this happened,” Jiggins says.

“With global connectivity and climate change reducing barriers to species range expansion, these megapests are likely to be a growing global problem, as is the increasing rate of biological invasions in general,” says Angela McGaughran of the University of Waikato in New Zealand.

Farmers are supposed to plant non-Bt crops next to Bt crops to create refuges that slow the spread of resistant pests. However, in many countries these guidelines are not followed.

Plant manufacturers are introducing new strains of Bt crops that produce two, three, or even five different Bt proteins to combat resistance. “But bringing these new products to market is expensive and slow, so it’s best to maintain the effectiveness of current Bt proteins with resistance management tactics, including refuges from exposure to Bt crops,” says Bruce Tabashnik of the University of Arizona.

Although hybridization can spread resistance, Tabashnik says the main problem is evolution within species. In China, strains of H. armigera have independently developed resistance to the original Bt toxin, he says.