Scientists release hundreds of wasps to save Willamette Valley fruits

In 2009, a berry grower in the Willamette Valley sent Oregon State University entomologists a sample of berries that were being attacked by what appeared to be common fruit or vinegar flies.

But unlike a typical fruit fly interested in soft, rotting produce, something was getting into even hard and partially ripened berries, ruining them for harvest. 

“We misidentified them initially,” said Vaughn Walton, an entomologist and professor in the College of Agricultural Sciences at OSU. “We said ‘It’s a general vinegar fly and you’ll just have to pick earlier.’”

Two days later, the scientists noticed spots on the wings of the flies that the grower had sent in.

“We phoned them back and said, ‘You’ve got spotted wing drosophila,’” Walton said.

Spotted wing drosophila is a fruit fly originating in Southeast Asia. It arrived in California in 2008 and shortly after moved up the West Coast of the U.S. and into Canada.

Within three years, it had spread to every state in the country. 

The flies attack hard and ripening fruit as opposed to overripe fruit. They’re partial to cherries, peaches, figs, strawberries, raspberries, blackberries, wine grapes and, especially, blueberries. Adult females lay up to 600 eggs per year within ripening fruit. The larvae eat the fruit from the inside out as they emerge. 

Nationwide, spotted wing drosophila cost farmers about half-a-billion dollars a year in pesticides and crop losses, according to the U.S. Department of Agriculture. 

In Oregon, they’ve been costing blueberry farmers between 10%  and 50% of their crop in any given year, according to Walton. Just a 10% loss constitutes between $10 million to $14 million each year for Oregon blueberry farmers.

Walton said at first, growers used more pesticides to combat the flies, but insects eventually become resilient to such chemicals, so he and scientists from OSU and the University of California at Berkeley set out to find the flies’ natural enemy.

In 2011, the scientists went to South Korea to collect fruit from the mountainous region of the country. They discovered within those fruits several parasitoids – insects that lay their eggs in other bodies – destroying the host. It was a parasitoid they’d need to kill the fruit flies. 

The scientists experimented for several years in collaboration with the Oregon Department of Agriculture and the U.S. Department of Agriculture. They eventually found one parasitoid that would only attack the spotted wing drosophila: a tiny wasp called ganaspis brasiliensis. 

“They only lay eggs in the spotted wing drosophila,” Walton said. “If it’s not there, you won’t have these wasps.”

They will not attack any other insects, and will not sting humans.

More than a decade after their researchy began, the scientists got approval from the U.S. Department of Agriculture late last year to raise and release the wasps in Oregon. 

In June, scientists released the first couple hundred wasps in Marion, Yamhill and Multnomah Counties. Most of the fruit flies live in habitats surrounding fruit crops, so the wasps are being released into areas surrounding orchards.

According to preliminary data, the wasps reduce fly populations by up to 65%. And across the country, scientists are beginning to experiment with the wasps to protect vulnerable fruit crops.

“It will ramp up over time,” Walton said. “We have teams up and down the West Coast rearing them.” 

In Oregon, three government laboratories are in charge of the wasp incubation and release. 

Over summer, the scientists are expected to release about 1,000 wasps in Oregon orchards.