GRAND RAPIDS, Mich. (WOOD) — Most Michigan fruit growers are quite familiar with the spotted wing drosophila, an invasive fruit fly that can wreak havoc on several crops if not tended to carefully. After years of research, Michigan experts believe they may have a solid solution: introducing another foreign insect.
Spotted wing drosophila — or SWD in the entomology world — is native to Asia. The tiny fruit flies have a saw-like appendage that can cut through thin fruit skins, lay their eggs and then allow the larvae to feast on the fruit as it grows.
They were first spotted in Michigan in 2010 and had taken a firm hold by 2012. According to Nikki Rothwell, the director of the Northwest Michigan Horticultural Research Center, it’s been a problem ever since.
“Once the larvae are in the fruit there’s nothing you can really do to kill them. So you’re really targeting the adults,” Rothwell told News 8. “Because it’s a fruit fly, the real stressor for us is the reproductive capacity of this insect.”
Rothwell’s team has been studying SWD for years. She said SWD typically age from larvae to adult in seven days, but in the lab under ideal conditions, the insects have reached adult stage in five days.
“You can start out at very low numbers, but then over time they can have multiple generations and we can be dealing with millions of pests,” she said.
Michigan’s apple growers don’t need to worry about SWD and neither do the state’s wineries, but SWD can impact several harvests, including cherries, blueberries, blackberries, raspberries and strawberries.
In 2016, an estimated 20% of the state’s cherry crop was lost to SWD. Another big loss? The fall raspberry market.
“The industry that probably was hit the hardest, though it’s a very small industry, is the fall raspberry industry,” Jim Nugent told News 8.
Nugent is a cherry grower who previously led the Northwest Michigan Horticultural Research Center and had served as the chairperson for the Michigan Tree Fruit Commission. He remains plugged in to growing trends across the state.
“(There was) a lot of it in southeast Michigan where they have pick-your-own operations or direct markets. But that industry has really been devastated and a lot of growers have just given up on fall raspberries,” Nugent said.
That lost harvest means big bucks for farmers.
“Those used to be a real gold mine for farm marketers because they were fairly easy to grow. They didn’t have a lot of inputs and they get quite a lot of money for a pint or a quart of raspberries,” Rothwell said.
For now, many farmers have turned to insecticides to keep their crops healthy, but it is something that needs to be done routinely to take out the rapidly producing SWD. Rothwell’s team has also found some methods to help keep SWD away — including heavier pruning and keeping orchards mowed — but nothing to effectively limit the insect’s population.
USING A FOREIGN PREDATOR ON A FOREIGN PEST
Instead of turning to chemicals, researchers have also looked to the natural world to see how the ecosystem handles this pest. That’s how they landed on the samba wasp.
“We need to have nature help us bring these populations down,” Nugent said.
Let’s be clear, this isn’t your typical wasp. They are about the size of a grain of rice and don’t have stingers. It is a parasitoid, meaning it is an insect that lays its eggs inside of another. Rothwell describes the unappetizing process, likening it to a horror story.
“Basically, that wasp egg hatches inside the SWD and then the wasp larva eats the SWD from the inside out,” Rothwell said.
The two key takeaways from the research on the samba wasp is that it can be an effective solution to tamp down SWD — studies estimate between 20% and 65% — but it’s little threat to our ecosystem.
“It’s very specific to SWD. It’s not going to just lay its eggs in random fruit flies, it’s going to lay its eggs in SWD. It’s a very specialized wasp or parasitoid,” Rothwell said.
Said Nugent: “We have 18 native species (of fruit flies) in North America. Scientists can arrange them genetically and know where SWD fits kind of within the genetic variability of drosophila. And (for the samba wasp), it had no effect on 16 of the 18 natives and only had a slight effect on the two that were most genetically close to SWD.”
Rothwell’s lab in East Lansing has been working for several years now to grow its own colony of samba wasps and have figured out the ideal variables to help them thrive. They will continue to grow the colony and perform select releases to try and introduce it to the ecosystem.
“It seems like baby steps right now, but I feel like as an entomologist it was exciting,” Rothwell said. “I think it’s going to be really great for growers. It’s a fun story to tell from a science perspective but also a real-world perspective.”
The problem is still a long way away from being solved, but Nugent believes scientists are on the right track.
“All of us as growers and scientists have our fingers crossed that this is going to become established and help us, help nature reduce the population of this pest so that our efforts as growers have a better chance of being successful,” he said.