In 2008, a fruit fly, known as the spotted wing fruit fly, made its way from Southeast Asia to the mainland US, likely piggybacking on fruit shipments. The insect was first discovered in raspberry fields in California and quickly spread to other states.
Unlike the common fruit fly, which is attracted to rotting food, spotted fruit fly prefers ripening, healthy fruit. Using a serrated, tubular organ, the females cut through the rind of the fruit and lay 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 pests and beneficial insects. But scientists are working on new solutions that could one day replace or at least limit the need for chemical spraying.
In Oregon greenhouses, researchers at the U.S. Department of Agriculture began testing one such approach last month: sterilized male flies. The gene-edited bugs, created by St. Louis-based biotech company Agragene, aim to suppress wild fly populations. The idea is that if released into the environment, the sterilized males would mate with wild females, resulting in a dead end for fertility. “We see this technology as a way to provide healthier fruits and vegetables without doing much damage to the environment,” said Bryan Witherbee, CEO of Agragene.
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 in 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 doing the damage,” said Stephanie Gamez, director of research and development at Agragene.
But before the company can release genetically modified insects into the open air, it must first test them in closed greenhouses. Working with government researchers, and with USDA approval, the company is testing how well the processed males can reduce raw fly populations in greenhouses and prevent damage to the blueberries grown there. The experiments will last two to three months.
The company is now applying to the agency to conduct field tests next year. Ultimately, Agragene’s plan is to sell small cardboard boxes of sterile male pupae – the stage just before flies reach adulthood. At this stage, the pupae are cocooned and immobile, making them easy to transport to farms. (Witherbee says the company tried to ship live adult flies, but some of the insects died in the process.) Boxes would be placed in fields, and when the adult flies emerge, they’ll look for females.
Witherbee thinks it will take a ratio of four or five sterilized males to each savage to wipe out a population, and the severity of a field’s infestation will determine how many bugs need to be released. Because the males are sterile, no offspring are produced when they mate with females. Drosophila with spotted wings only live for a few weeks, so if the first generation dies, repeated releases of worked males are needed to keep the population down. Witherbee says the company will start with weekly releases in the greenhouse experiments, but multiple releases in a shorter period of time may be required in the field.