A tiny fly that lays its eggs in still-ripening fruit has begun to wreak havoc on small fruit crops in the Chesapeake Bay watershed, leaving growers few options beyond additional pesticides for controlling it.
The newcomer is the spotted wing drosophila (Drosophila suzukii). Originating in Asia, the fly made its way into the United States in 2009. It has traveled up both the West and East coasts, and appeared in Virginia in the summer of 2011. It has since showed up in Pennsylvania, Maryland and Delaware.
As with any new pest, researchers are scrambling to provide growers with alternative or organic methods for protecting their plants while equipping them with the best tools available for now.
“When we have a new species, we don’t have the answers right in hand,” said Doug Pfeiffer, fruit entomologist at Virginia Tech University. “All the entomologists are working on the same long-term goal: to reduce insecticide use. We’ve just had this setback.”
Unlike the average fruit fly, the female spotted wing drosophila has a saw-like ovipositor that she uses to cut into soft-skinned fruit, planting her eggs into the berry or grape before it’s ready for harvest. The growing larvae are not obvious to the naked eye as they eat away at the inside of the fruit, turning healthy looking produce from the market into mush in a customer’s fridge.
The fly, often referred to as SWD, has proved devastating to some small berry crops in the Pacific Northwest and now appears to be established
in many fruit-growing regions around the country.
In the mid-Atlantic growing region, the bug joins a long list of other pests and environmental factors, like humidity, that already make growing produce without the use of pesticides difficult.
The brown marmorated stink bug became a significant agricultural pest in Virginia in 2010, though researchers think it first began appearing in 2004. It’s been known to snack on and damage around 350 species of plants, including key agricultural crops like soybeans.
The invasive stink bug has had a greater impact in the Chesapeake Bay region than anywhere else in the United States, where it was first spotted in Pennsylvania in the late 1990s. It is considered a severe agricultural pest in every watershed state except New York.
On a fall day, the almost flat, arrow-shaped bugs form thick clusters on trees and the walls of homes in some areas of Virginia, particularly near farm fields with wooded buffers, said VTU entomologist Tom Kuhar.
This stink bug is susceptible to the existing arsenal of pesticides used to ward off other stink bugs, but “the sheer number of them was something we hadn’t dealt with before,” Kuhar said.
But the types of insecticides that are effective against the bug are not the type entomologists like to use. Broad-spectrum insecticides like neonicotinoids and organophosphates will kill the bug, but they also kill pollinators and other beneficial insects.
“We’ve made a lot of headway with IPM (integrated pest management), and then you get something like this that comes through with no good answer,” Kuhar said. “You’ve got [spotted wing drosophila] doing the same thing to fruit with no good answer except spray. It’s setting back the work we’ve done all these years.”
As for its impact on specific crops, Pfeiffer said there’s really never been a pest as bad as the new fruit fly.
“On very vulnerable crops like raspberries, growers may need to spray weekly or more often. In my career, there’s never been anything like this on these crops,” he said.
Berries account for a small percentage of total agricultural acres in Virginia, but grape growing for the state’s burgeoning wine industry is on the rise. Around 2,500 acres of grapes were harvested in 2008, according to the latest agricultural statistics, with almost all of them going into wines.
Virginia ranks fifth in the country for the state with the most wineries at 240 and is tied with Texas as the fifth largest wine-producing state. The industry contributes almost $750 million to the Virginia economy, according to a 2012 economic impact study.
The new fly poses a particular threat to wine grapes. Its larvae’s eating habits inside the fruit can cause the grapes to rot or break down, creating acetic acid or vinegar. While crushed grapes may not show the presence of the pest, a change in the juice’s acidity can present problems for winemakers.
“The bottom line is that the SWD spoil the grapes by depositing their eggs in them, and this results in sour rot,” Bill Tonkins, vineyard manager at Veritas Vineyard in Afton, VA, explained in an e-mail. “Too much sour rot imparts volatile acidity in the wine.”
Tonkins, who’s president of the Virginia Vineyards Association, said growers didn’t know much about the pest until Pfeiffer alerted them to its presence.
Christine Vrooman, owner of Ankida Ridge Vineyards in Amherst, VA, said it didn’t take her long to find the SWDs in her vineyard after she started looking. She sent Pfeiffer pictures to confirm and, sure enough, “our days of little to no insecticide this season were finished,” she wrote in a recent newsletter.
Vrooman has since been working with Pfeiffer and his researchers to find more organic solutions for the pest. She experimented with using essential oils to ward them off, but the treatments didn’t have much effect.
Vrooman’s team had some success with an organic, clay-based product called Surround, but she was waiting to see how the product might affect the winemaking process.
Pfeiffer said organic controls will continue to be a challenge for this pest, but research is ongoing.
A coalition of researchers on the East Coast is looking at a series of biological controls, including using natural enemies or pheromones to draw the pests to trap trees, non-fruit-bearing trees that have been sprayed.
Similar methods have begun to work with the invasive stink bug. Kuhar said some parasitoids that naturally attack the existing species of stink bugs in Virginia have begun to attack the new species. Researchers have brought the stink bug’s natural enemy from China for experiments in quarantine that will determine whether releasing it would be a helpful control.
Research to see whether other host plants, such as sunflowers, can lure the stink bug from valuable crops and into traps or sprayed areas is under way as well.
Kuhar said discoveries that stink bugs first attack the outer edges of a field have allowed growers to contain additional sprays to those areas, reducing the overall use of insecticides.
He said the pest has hit organic growers the hardest as they are left with few options for controlling it. One organic vegetable grower in Rappahannock County found the most egg masses of the stink bug researchers had ever seen on a single tree on the property.
“The potential is a vast majority of our agricultural crops in Virginia could be impacted. Right now the only option is really chemical control, which means increased spraying of things that you might not have sprayed much before,” Kuhar said.
It’s not clear the effect such an increase in pesticides might have on water quality, or when it might appear. A coalition of 25 organizations called the Maryland Pesticide Network is working to better track the use and impact of pesticides in the state and watershed.
Ruth Berlin, executive director of the network, said pesticide use often increases with the introduction of new invasive species, and there’s not specific knowledge about how that impacts the ecosystem.
“Unless you know when it’s sprayed and where, you can’t really assess it,” Berlin said. “I’m not saying don’t use it; I’m saying we need to understand. We’re protecting one thing and possibly destroying another.”