Oregon scientists will use parasitic wasp to control invasive pest damaging fruit crops

Starting next month in Marion county, scientists at Oregon State University will release a parasitic species of wasp to control an invasive pest that infests fruits as they ripen, costing half a billion dollars worth of crop damage each year in the U.S. The spotted wing drosophila, a fruit fly native to Southeast Asia, was first detected in Oregon in 2009 and costs the state’s blueberry industry alone more than $10 million in losses annually. It took scientists more than a decade to gain approval from the U.S. Department of Agriculture to release the parasitic wasp, which is also native to Southeast Asia, in Oregon and other sites along the West Coast this summer. Vaughn Walton, a horticultural entomologist in the College of Agricultural Sciences at Oregon State University, joins us to explain the latest efforts to control this invasive pest in Oregon and the West Coast.

Note: The following transcript was created by a computer and edited by a volunteer.

Dave Miller: From the Gert Boyle studio at OPB, this is Think Out Loud. An invasive kind of fruit fly that damages a wide variety of crops is estimated to cost the agriculture industry in this country half a billion dollars a year. Now, a group of scientists is about to fight back with the help of a tiny wasp that has a taste for this fruit fly. Vaughn Walton is one of those scientists. He’s a horticultural entomologist at OSU’s College of Agricultural Sciences. He joins us to talk about intentionally introducing one species to control one that was introduced accidentally. Vaughn Walton, welcome.

Vaughn Walton: Thanks so much for the opportunity.

Miller: When did farmers or scientists first find and identify this fruit fly?

Walton: It’s kind of an interesting story. August 2009, I got a sample through Dr. Bernadine Strik, who’s a horticulturist here in the college, from a grower just outside of Corvallis who had sent me some berries. They said “well these berries are softening and there’s some flies in this.” We reared those out early in the week, I think it was on a Monday or a Tuesday, and the adults emerged from those on a Friday. I phoned the grower back, and I said to the grower, “this is just a drosophila. You should be picking your berries a little bit earlier.” And the grower is like “I don’t know about that.”

Miller: Just so we understand, you called him and you think it’s just a regular old fruit flies? That’s what you told him?

Walton: That’s right. I said “this is just spoil. Your fruit is overripe.” And this is just my stupidity. I let the grower know the best information you have Monday. I go back to the flies that were reared out, and suddenly we see these spots on the wings. So it turns out that this fly needs two or three days for the males to have these spots on their wings. Started looking into it, found a few colleagues, and we discovered this is drosophila suzukii.

So I had to phone the grower back, and I said to the grower “I am so sorry, I made a mistake. You are the first grower who have recorded spotted wing drosophila their fruit.” And so from then onwards, we just saw it spread very very quickly. First recordings of drosophila suzukii was up in British Columbia within weeks after that. Since then it has caused tremendous damage to our crops here in Oregon, as well as the whole of the country and internationally as well.

Miller: Where is this fruit fly from?

Walton: Our best understanding is that it’s from East Asia. It originates from south China, as well as Japan. And this is also the place where the natural fruits, cherries, blackberries, rubras, those types of berries are growing as well. And this fly attacks those fruits in those areas.

Miller: Is this a more damaging pest than the kinds of fruit flies that you’re used to seeing here?

Linton: Oh yes, it’s really damaging. The difference between this one and vinegar flies, the flies that we know …

Miller: The flies that are all around my kitchen every summer.

Linton: That’s right. Those flies, normal vinegar flies, attack overripe fruit. They don’t have an ovipositor that can lay eggs in that firm fruit. But this fly has a saw-like ovipositor, and it can lay its eggs into ripening fruit.

Miller: The females have an egg deposit that’s like a saw, and it can cut into the firm outside of, say, a blueberry and push its eggs in? And then and then what do those eggs do?

Linton: They lay their eggs directly under the surface, and then within a day, sometimes even within hours, those eggs can hatch. Those larvae will start feeding on that fruit immediately. Within two or three days, those larvae would soften the fruit. It would result in spoilage of the fruit. And then within a one week period, under ideal conditions, this larva will emerge from the fruit, and you’ll have new adult flies that’ll repeat that cycle.

Miller: Putting more eggs in more cherries or blueberries or whatever. What is the estimate for the damage that these flies do in Oregon and around the country?

Linton: So our best estimate is around 10% of crop. So if you’re looking at farmgate value, which is the value that the grower gets for the fruit, as an example in blueberry farmgate value of blueberry is around $100-$140 million a year. It’ll be about 10% of that value. So for blueberry on its own, it would be around $10 to $14 million per year here in Oregon. And that’s only for blueberries. So if you include blackberry, strawberry, and cherry, those numbers go up significantly from there.

Miller: What have farmers been doing over the last decade, or maybe a little bit more than that, to try to control these flies?

Linton: Initially, we worked with growers to find the very best conventional pesticides. And growers have figured that out very quickly. Conventional pesticides to start with managed to to protect their crops very efficiently, protect their markets very efficiently. And that has been the main way of managing this insect up to date.

In addition, there are other technologies that are developing. We looked at cultural controls, we looked at the timing of our sprays. We have behavioral controls as well where we’re manipulating the behavior of these insects.

Several other types of technologies that have not made it to market yet. Sterile insects are being reared and released under experimental conditions. But chemical control, to date, has been the most efficient way of managing this insect.

Miller: Is it working?

Linton: Chemical control works well. It’s got its limitations, one of those being the development of resistance of the insect against the pesticide. It’s affected by rain, so if a grower applies a pesticide and you have rain the next day, it washes all those protective pesticides off the berries. Not all pesticides are very targeted, they’re kind of generalist, so they will affect the environment in a negative way. But generally in terms of protecting from the insect, it works well.

Miller: But you and you and a lot of other scientists have been working for a while now on a biological control. How do you go about finding a wasp in the first place that’s going to go after this particular fruit fly?

Linton: I teach this class, it’s called Pests, Plagues and Politics …

Miller: I would take that class.

Milton: I tell my students the very first thing you need to do when you find an insect is you need to do an identification. As soon as you do your identification, all of that information about that insect comes up. And so that was what we did first with this fly. And immediately, you find a record of where this fly is originating from, where it causes damage. And so as I said earlier, it’s originating from East Asia. This is where this insect has evolved over thousands, millions of years. And this is also where all these natural enemies occur. And some of these natural enemies are generalist natural enemies, like lady beetles and so on. And some of them could be specialists.

This insect is a specialist. It’s evolved over time to attack only spotted wing drosophila. It doesn’t have any other host. The work has been done under lab conditions here in quarantine, and it is found in conjunction with that insect in places like south China, South Korea, Japan. When you’re looking at those populations of spotted wing drosophila, you go and you pick wild cherries, wild blackberries, or raspberries, or strawberries, you find under those natural conditions that about 30%-60% of that population is parasitized by these specialist parasitoids.

Miller: Can you describe how it actually does this, how this particular wasp goes about its business? You describe the life cycle of the fly, that it saws into the little holes into cherries, deposits eggs there, the larva eat their way out of the cherries or whatever, and they grow into more fruit flies and it continues. So, what’s the wasp’s version of an ideal life?

Linton: This life cycle is very closely tied to spotted wing drosophila. So remember I said spotted wing lays its egg directly under the surface, and you have the larva hatching. That larva is inside of the fruit. And so this parasite will come, and it will sense that there are larvae of spotted wing inside that fruit. It’ll go to those fruit. It’ll go and sit on top of that fruit, and it will put its ovipositor into the larva of spotted wing drosophila within the fruit.

Miller: It’s basically treating the larva the way the fruit fly treats the fruit, putting its eggs inside the larva. Wasps are the most disgusting beautiful animals there are.

Okay, so then what do the wasp eggs do?

Linton: So what then happens is this wasp egg starts developing a very complex process where it overcomes the immune defenses of the pest insect. It basically takes up the whole inside of that larva. Eventually the spotted wing larva will die, drop out of the fruit, and this larva of the parasite will emerge from it as an adult. And this happens more or less in the same timeline as the pest insect life cycle as well.

Miller: This is fascinating and I can see how this would be good for controlling the population of fruit fly. But I got to say as a consumer, I don’t think I want to eat the blueberry that sequentially first had a fly larva, and then a wasp egg eating that larva. So what’s the plan for releasing these without still having a bunch of fruit get pretty gross in terms of human concern?

Linton: That’s a question that I think a lot of people have. The idea behind the release of these parasites is that we release them in surrounding vegetation. And the reason why we do that is because of spotted wing drosophila having a very, very wide host range. So if you look at any population of spotted wing drosophila, about 90%, even higher than that, does not occur inside the crop. It occurs outside of the crop. And so the idea behind releasing this is you’re releasing it outside of the crop. It attacks the host outside of the crop, and significantly reduces that population. And then ultimately, that lower population in the outside of the crop results in way, way lower pest pressure in the crop. You will have way way less eggs laid inside the fruit in the crop that we’re eating. And then the grower still will continue doing some of their management techniques. They will use way less pesticides, produce a crop that is cleaner and safer to us as consumers as well.

Miller: I have read that there are examples from history of humans introducing species to control what they see as pests, and then having that backfire. In Australia, cane toads were brought in to go after beetles, but that didn’t work. And now 90 years later, the toads are a really big problem in their own right. What do you have to do, and other members of the team, to make sure to the best of your ability that something like that won’t happen? Some kind of unforeseen circumstance where the solution causes its own new problems?

Linton: You look at that example of cane toad, and we’ve learned very expensive lessons from there. One of those lessons are that you need to select your biocontrol agent very, very carefully. And so, one of the reasons why it took us about 10 years to get a release permit for this insect, is the testing has been very, very rigorous. So when the initial foreign exploration had been done, about three or four parasites were found. Some of them were more of a generalist, and some of them, including this parasite, is a specialist. We determine that by looking at whether this insect can lay its eggs in other insects. Does it attack other insects, or is it very host specific?

This specific insect is very host specific. It targets only drosophila suzukii, only the invasive insect. And so when drosophila suzukii populations go down, this insect population will go down significantly as well. And what you’ll have in the end is hopefully such a low level of spotted wing drosophila, and it’s been shown in the past on other insects, that it kind of disappears off the map. It becomes a non issue. It’s still there, but it’s there at very, very low levels, and it’s not affecting any other organisms including us as human beings.

Miller: Where are you going to be releasing these wasps?

Linton: So the release of these wasps will start here in Oregon, in the Willamette Valley. We’ve got several trial sites that have been set up here. But it will also expand throughout the whole of the state where we’re producing these crops, including the Hood River area, including southern Oregon, eastern Oregon. And Oregon is not the only state where we’re releasing this insect. These releases are being done throughout the whole of the country. They’ve actually been done in Europe starting about a year ago already.

We’re very, very excited about this. We believe that in the long run, it will establish, and that it will significantly reduce these insect based numbers.

Miller: Establish, meaning the thinking is you won’t need to regularly release new wasps, but you’ll create some kind of self sustaining population where they have just enough of the fruit flies to to maintain some kind of reservoir?

Linton: The idea is that they will adapt to the environment here. If you’re going to need to do continuous releases, it’s going to cost a lot of money. What’s been shown in the past is with biological control like this, these insects will establish, and they will naturally control those pest insects. It’s been done in the past on multiple insect species, also here in Oregon itself as well.

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