Diamondback moths love broccoli. They’re also fond of cabbage, cauliflower, and related crops. And they quickly evolve resistance to insecticides and crops genetically modified to kill them. But frustrated farmers might get a new weapon against them: genetically engineered versions of the moths that mate with wild pests and cause half their offspring to die. Yesterday, researchers reported that these moths, developed by the biotechnology company Oxitec, survive well on actual farms.
“This is important,” says Max Scott, an entomologist at North Carolina State University who was not involved in the study. The technology, he says, “has excellent potential.” But it’s unclear what happens next with the moths, as Oxitec says it is only “evaluating potential opportunities.”
The approach is a version of sterile insect technology, which has been used for decades to control and eradicate a fly known as screwworm, a livestock parasite, and a few other pests. The insects are blasted with x-rays, making them sterile. Then, the neutered males are released to find females and when the insects mate, there are no offspring.
One challenge is that large numbers are required because the radiation leaves the males less vigorous than their wild rivals. “Radiating insects is like a using sledgehammer,” says Tony Shelton, an entomologist at Cornell University who studies the diamondback moth and led the current study. “You can get the same result by just tweaking genes, and they will behave normally.”
That tweaking was done by Oxitec, which also funded the new field trial. The company is more famous for its mosquitoes, which it has tested in Brazil and other tropical countries to combat dengue fever and other diseases. (In October 2019, the city of Indaiatuba, Brazil, began to use the Oxitec insects to help reduce its mosquito populations.) The moths were engineered in the same way: Researchers at the company assembled a “lethality gene” called tetracycline transcriptional activator variant (tTAV), by combining DNA from the bacterium Escherichia coli and the herpes simplex virus; then they added it to the insects.
The idea is that when modified males mate with females in the wild, they pass on their tTAV gene. The gene prevents the female offspring from developing, and they die as larvae. But male offspring survive and half inherit tTAV. After these males grow up and mate with other wild insects, the next generation of female offspring also dies, further shrinking the population. The Oxitec insects carry a gene for a fluorescent marker as well, allowing them to be identified in the wild.
The modified males have another attractive trait. They could help maintain the effectiveness of insecticides and genetically modified crops the diamondback moth has evolved resistance to. That’s because the modified males added to a field don’t have the resistance genes, as they were bred in the lab from a susceptible strain.
In 2015, Shelton and his colleagues showed in a greenhouse study that the modified moths could knock down a population in three generations. The new trial—started in 2017—was to evaluate the behavior of the insect in a real field, where weather and predators can make life more challenging.
The researchers released several thousands of Oxitec moths in a cabbage field in New York. They then placed traps with scented lures throughout the field to see how far the insects might travel. The modified moths behaved the same as normal moths; 95% ventured less than 35 meters from where they were released, meaning that they stayed in the field. They also lived as long as normal moths, the team reported on 29 January in Frontiers in Bioengineering and Biotechnology.
The scientists did not measure the ability of the moths to reduce the population in the field. But in lab experiments, they determined that the modified moths were just as good at finding mates, and these females laid as many eggs. A mathematical model based on these data suggested the modified moths will be effective at controlling pests, Shelton says.
A drawback of this approach is that releasing insects is more complicated to use than simply spraying insecticides, and probably more expensive as well, Scott notes. In addition, organic farmers, who are a large market for biological control because they aren’t allowed to spray synthetic insecticides, can’t use genetically modified insects.
The next step would be to test the moths in warmer locations that have larger infestations of diamondback moths. Shelton won’t be around to see that, as he plans to retire. “I’ve taken this as far as I can,” he says. “I hope some other scientists continue the work with the diamondback moth and other insects.”
Neil Morrison, who directs agricultural research at Oxitec, said in a statement that the company is “evaluating potential opportunities in regions where diamondback moth management is challenging for farmers.” But, he added, the research has shown “significant promise.” Meanwhile, the company is continuing to research the technology to control two other major pests. One is a moth called the soybean looper that has evolved resistance to insecticides in Brazil, where it also damages cotton and corn. The other is the fall armyworm, which is even more omnivorous and a rapidly growing problem in Africa.