In 2008, a fruit fly known as the spotted-wing drosophila made its way from Southeast Asia to the continental US, likely hitching a ride on fruit shipments. First detected in California raspberry fields, the insect rapidly spread to other states.
Unlike the common fruit fly, which is attracted to rotting food, spotted-wing drosophila prefers ripening, healthy fruit. Using a serrated, tubelike organ, the females slice through fruit skin and deposit their eggs inside. When the eggs hatch, the emerging larvae destroy the crop. The invasive pests cause hundreds of millions of dollars in damage each year. To control them, growers rely on pesticides that kill insects indiscriminately, including both pests and helpful bugs. But scientists are working on new solutions that could one day replace—or at least limit—the need for spraying chemicals.
In greenhouses in Oregon last month, researchers with the US Department of Agriculture began testing one such approach: sterilized male flies. The gene-edited bugs, made by St. Louis–based biotech company Agragene, are meant to suppress wild fly populations. The idea is that if they were to be released into the environment, the sterilized males would mate with wild females, resulting in a fertility dead end. “We see this technology as being able to provide healthier fruit and vegetables without doing a lot of harm to the environment,” says Agragene CEO Bryan Witherbee.
Scientists at the company used the DNA editing tool Crispr to knock out two essential genes in fly embryos—one involved in male reproduction and another with female development. As a result, only sterile males hatch while the females die. “You don’t want to release females into the population, because those are the ones that are doing the damage,” says Stephanie Gamez, director of research and development at Agragene.
But before the company can release any gene-edited insects into the open, it first has to test them in contained greenhouses. Working with government researchers, and with permission from the USDA, the company is testing how well the edited males can bring down the populations of unedited flies in greenhouse conditions and prevent damage to blueberries being grown there. The experiments will last two to three months.
The company is now applying to the agency to do field tests next year. Eventually, Agragene’s plan is to sell small cardboard boxes containing sterile male pupae—the stage right before flies turn into adults. At this phase, the pupae are cocooned and immobile, making them easy to transport to farms. (Witherbee says the company tried shipping live adult flies, but some insects died in the process.) Boxes would be placed in fields, and when the adult flies emerge, they’ll seek out females.
Witherbee thinks a ratio of four or five sterilized males to every wild one will be needed to quash a population, and the severity of a field’s infestation will determine how many insects to release. Since the males are sterile, no offspring are produced when they mate with females. Spotted-wing drosophila only live a few weeks, so once the first generation dies, repeated releases of edited males would be needed to keep populations down. Witherbee says the company will start with weekly releases in the greenhouse experiments, but in the field, multiple releases in a shorter period of time may be needed.
“If you release enough males over a long enough period of time, you can eliminate the pest,” says Omar Akbari, a professor of cell and developmental biology at the University of California, San Diego, who originally developed the gene-editing technique for flies and licensed it to Agragene.
Using sterilization as a way to control insect pests isn’t a new idea. Since the 1950s, the US government has used it to tamp down screwworms, parasitic flies that feed on the flesh of livestock. Sterilized male screwworms are released to mate with females, which lay eggs that don’t hatch. This technique has led to their eradication in North and Central America. But those male insects are made sterile with radiation rather than genetic engineering. The downside is that high levels of radiation impair their ability to reproduce, meaning dozens of insects are often released for every wild one.
Meanwhile, researchers at North Carolina State University are developing an alternative control method that wouldn’t require repeated releases. In the lab, they used a technique called a gene drive to make female offspring unable to reproduce. Gene drives are designed to preferentially spread or “drive” certain genetic traits throughout a population, overruling the rules of heredity.
In a paper published earlier this month, entomologist Max Scott and his team described using Crispr to inactivate a region of the doublesex gene, which is required for female sexual development. The researchers injected Crispr into fly embryos, along with a fluorescent protein so they could track which flies ended up with the desired change.
When the flies matured, the researchers mated the engineered insects with wild ones that didn’t have the mutated doublesex gene. Normally, about 50 percent of offspring would be expected to inherit the change. But with the gene drive, 94 to 99 percent of the flies’ offspring ended up with it. Females that inherited the mutated gene were sterile and unable to lay eggs. But male offspring, which also carry the change, remain fertile. Unlike Agragene’s approach, the males go on to breed and pass on the trait to subsequent generations. This allows the gene to continue spreading—and knocking out future generations of flies—without the need to release more insects.
“If we're trying to do population control, you really want to target a gene that's required for female reproduction, because it's the female that is producing the next generation,” Scott says.
The researchers used mathematical modeling to predict how well the gene drive system would suppress populations of flies kept in laboratory cages, and found that releasing one modified fly for every four wild ones could tank populations within eight to 10 generations. (Each generation lasts about two weeks, so crashing a lab population could take around 20 weeks.) Now the team is conducting experiments using cages of real gene-edited flies to learn whether the gene drive will suppress their numbers as the modeling predicts.
Researchers elsewhere are developing gene drives as a way to control disease-carrying mosquitoes, but none of these insects have been released yet in the wild. “There are a lot of risks associated with it, because it’s a technology that you can’t control once you release it,” says Akbari, whose lab has also worked on gene drives for spotted-wing drosophila. One risk is the possibility of spillover: If the target species mates with a closely related one, it could transfer the gene drive to a native or valued species.
And there’s a chance that the gene drive could stop working. Crispr works by recognizing a specific site in the genome. Any flies in the wild that have natural variations at this site wouldn’t be affected by the gene drive. “It’s one thing to work in a lab where there’s probably not a lot of genetic variation within your lab population,” Scott says. “But if you go out into the field, you’re dealing with huge populations and a lot of genetic variation.”
Hannah Burrack, an ecologist at Michigan State University who works on pest management, says the gene drive work is promising, but it’s far from being deployed by farmers. US regulations on gene drives are unclear, and scientists agree that work in this area should proceed cautiously. Still, she sees certain advantages: “The promise that Crispr has is that instead of having to overwhelm the population, you could potentially release small numbers of flies and rely on the deleterious gene to move through the population.”
She says the sterilized male approach would likely require large numbers of released flies, so it may not be practical for an individual farmer to do. Instead, she thinks repeated releases of gene-edited males will probably need to be done by a large group of growers or a government.
One concern with both approaches is how eradicating the fly might affect other species in their surrounding environment. For example, would getting rid of these flies deprive another animal of a food source or allow for the rise of other nuisance species?
Since the spotted-wing drosophila is invasive and arrived in the US fairly recently, Burrack thinks the ecological impact of wiping them out would be minimal. “It becomes a risk-benefit comparison,” she says. “We’d be comparing the presumably small risks associated with reducing the abundance of this invasive species from the environment with the risks associated with the status quo management right now, which is more pesticide use.”
