Hybrid megapests evolving in Brazil are a threat to crops worldwide

Two “mega-sixes” that are already a major problem for farmers around the world, the cotton bollworm and the corn bollworm, have interbred in Brazil and exchanged genes conferring resistance to pesticides. The hybrid strains that are evolving could devastate soybeans and other crops in Brazil and around the world if left unchecked, threatening global food security.

“It has the potential to be a huge problem,” he says Chris Jiggins at the University of Cambridge.

In particular, many countries import soybeans from Brazil for both human and animal feed. “They kind of feed the world,” Jiggins says.

More than 90 percent of soybeans grown in Brazil are genetically modified Bt soybeans containing an embedded pesticide. If yields were to fall due to pests becoming resistant, this would lead to further increases in the price of many foods. It could also increase deforestation and greenhouse gas emissions as farmers compensate by clearing more farmland.

Corn earworm (Helicoverpa zea) is a moth native to America whose caterpillars eat most parts of plants. It mainly damages maize, but also feeds on many other plants including tomatoes, potatoes, cucumbers and eggplant(s).

in Brazil H. zea this was not a big problem for soybean farmers because it tends not to feed on the crop. But then, in 2013, the cotton worm (Helicoverpa armiger) was detected in Brazil. H. armigera is related H. zea which is widespread throughout Eurasia. These two moths have been described as mega-pests because they are so damaging and difficult to control.

“They’re pretty exceptional pests, so I think it’s justified,” Jiggins says. “Controlling the movement of moths is almost impossible. They move great distances.”

H. armigera also feed on a wide variety of plants and unlike H. zeait thrives on soybeans, so it caused huge problems for farmers when it got to Brazil. “It cost Brazilian agriculture billions of dollars,” says Jiggins.

This was largely solved by the introduction of Bt soybeans, which are genetically modified to produce a protein made by a soil bacterium Bacillus thuringiensis which is toxic to most insects.

It was thought H. armigera and H. zea could not cross but in 2018 genetic analysis revealed several hybrids between species. Jiggins and his colleagues have now analyzed the genomes of nearly 1,000 moths collected in Brazil over the past decade.

They found that a third H. armigera they now carry genes for resistance to Bt toxin – and they have acquired those genes H. zea. Bt corn was first introduced in North America in the 1990s, where some H. zea strains have developed resistance. These resistance genes appear to have spread to South America and are now interbreeding. Hybrid for now H. armigera they haven’t been a major problem, Jiggins says, but that could change as resistance spreads.

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

“With global connectivity and climate change together reducing barriers to species range expansion, such mega-pests are likely to be a growing global problem, as is the generally escalating rate of biological invasions,” he says. Angela McGaughran at the University of Waikato in New Zealand.

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

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

While hybridization can spread resistance, Tabashnik says the main problem is evolution within a species. In China tribes H. armigera they have independently evolved resistance to the original Bt toxin, he says.

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