Orchard People

[00:00:00] Introduction to Fruit Flies
Susan: Now I'm not a big fan of fruit flies. Those little flies find their way into my home and hang around in my kitchen. They're so tiny, but they still really bug me. But the thing about those fruit flies is that they're really only interested in old rotting fruit.
They stay away from the young fruit growing on my fruit trees, and I'm really happy about that. But those kitchen style fruit flies have relatives who are much more destructive.
[00:00:32] The Menace of Spotted Wing Drosophila
Susan: They're called spotted wing drosophila, and they can destroy crops including cherries, apricots, raspberries, and elderberries.
Spotted wing drosophila is a fruit fly that's native to Southeast Asia, but it's found its way to North America now, and it is really enjoying our warming climate. So what does it do? The female flies inject their eggs inside the fruit on your fruiting trees and shrubs. The fruit then becomes contaminated with larvae and it goes soft and mushy.
The end result is cherries or apricots or raspberries that are inedible and need to be thrown away. These flies are so small, you might wonder how much damage can they really cause. Spotted wing drosophila multiplies fast and can destroy crops quickly. So growers are scared. But my guest today has discovered that with a few simple tools you can make your fruit trees, orchards, and food forests less appealing to this damaging fruit fly.
[00:01:40] Meet the Expert: Nikki Rothwell
Susan: So my guest today is Nikki Rothwell PhD and she's from Michigan State University's Northwest Michigan Horticulture Research Center. Her research interests include tree fruit training and irrigation systems, pest management strategies, and horticultural modernization. Now today, we're going to talk a lot about the role correct pruning plays in caring for fruit trees.
Do you know how to prune your fruit trees? Did you know that fruit tree pruning is really very different from pruning native or ornamental trees?
[00:02:14] Understanding Spotted Wing Drosophila
Susan: So let's learn about spotted wing Drosophila. Nikki, welcome to the show today.
Nikki: Thank you so much for having me, Susan. It's a pleasure to be here.
Susan: Tell me something.
Where did your journey begin with this fruit fly? How long have you been studying it and how bad is the problem?
Nikki: Yeah, so we were a part of an early response detection program with the Michigan Department of Agriculture and Rural Development that we've been on the lookout for this pest for quite some time, and we found it in Michigan, in the state in 2010.
So we've been really dealing with this for quite some time now, and it feels, quite frankly, like time has flown and trying to, hurry and come up with something that resembles some kind of a solution for our cherry growers.
Susan: It's considered a fruit fly. What makes it part of this fruit fly kind of family?
Nikki: Yeah, so it's a drosophila species, so there's lots of different kinds of fruit flies. The ones most commonly that we see are apple maggot and cherry fruit fly in orchard systems, but that's actually a Rhagoletis species. So those guys seem like a piece of cake. So those guys can control, usually have one generation per year, where these Drosophila species, they're known for their work in a lot of genetics research.
And the reason they do this genetic research on these flies is they can really move through generations under these optimal conditions. So in our laboratory, we rear them. We have a county to do a research, and we can see that they can go through one generation every seven days. And I've actually even seen it in the lab even under seven days.
So one week you can get a new generation. Yeah, they're small, but their numbers can grow really quickly. And so every seven days you'd have a generation and you could just think about the numbers that you would be fighting in your orchard system. And so that's why we always talk about this pest is really a numbers game.
And it's all about getting your numbers low 'cause if you don't, there's not a lot of hope that you can stop an army of like billions of these Drosophila species.
Susan: Unbelievable. One generation in seven days. So over a growing season, you'd have how many generations?
Nikki: That's something we're trying to figure out.
So if your listeners ever think about Michigan, we're not the warmest place. So we have winter and I think winter does a number on 'em. So I think the populations get knocked off in the winter time. But the other thing is we have Lake Michigan to our west, so we have a lot of snow cover. And so I think that the numbers go down in the winter and then it creates kind of a bottleneck.
But I think the adequate snow cover protects them. And we've done some trapping all year long. So we'll trap 12 months out of the year and we'll strap on our snow shoes in the winter months and it's unbelievable. But I can catch 'em in January, February, March. Usually the only months I don't catch 'em are April and May, and now we're starting to catch 'em in May.
And so these populations, winter reduces the number, but I think by the time and last year we had really hot and dry conditions, and I think the populations grew slowly because of that. But it was quite interesting when you look at these exponential population growth curves since we've been trapping, we used to really see that exponential growth that would come out in September, October. And then it was in August. And now it's in July.
So it keeps backing up further and further. And so what's happened in our area, is that it's overlapping perfectly with tart cherry harvest, which is a wonderful host for spotted wing drosophila.
Susan: It's wonderful for the spotted wing drosophila. Not so good for the growers mind you.
[00:06:12] Impact on Cherry Crops
Susan: How much damage would it do, can it do to a sour cherry crop?
Let's say even if you get a little bit of damage from spotted wing drosophila, is that a problem? Maybe one or two cherries or a thousand cherries if it's a big harvest, no big deal.
Nikki: It's a problem in our industry. So the tart cherry industry, and I'll just give you a little refresher for your listeners.
The tart cherry industry is pretty big in Michigan, so maybe other folks around the world don't think of it. But next week is the National Cherry Festival. So we're home to kinda the largest cherry festival in probably North America for sure. And so what we have is we grow 50% of the US. All those tart cherries that are grown in the US are grown within the five county area that surrounds the Grand Traverse region.
If you add in the rest of Michigan, we grow somewhere between 75 and 80% of the US tart cherry crop is actually grown within the state. So cherries are a really big deal for us, and it's part of our agricultural heritage, particularly in northwest Michigan where I'm located. So the thing tart cherry industry has, and this has been on the books for many years, is what we call a zero tolerance policy.
So there used to be inspectors at all the receiving stations and processing facilities. They don't do that as much anymore, but they do take samples out of a tank. So we harvest tart cherries into a thousand gallon tanks of cold water. And then those are brought immediately to either a receiving station and/or a processing facility.
And they get pitted almost immediately because they're so perishable. So in somewhere there's an inspector that grabs handfuls out of these tanks, and if there's one larvae, and it doesn't matter if it's a spotted wing or another kind of insect, if there's one larvae on a truck, the whole truck gets dumped.
Susan: Wow.
Nikki: So that's been a policy that's been in place for a lot of times so growers can't afford that risk. So we don't really know, on the research front, we don't really know what that threshold is or how much loss we can really take. And growers just can't gamble because if they do find even one larva in their load of fruit, everything will get dumped.
[00:08:33] Consumer Concerns and Solutions
Susan: So if I were to buy a bag of cherries from somewhere, let's say, and I pop a cherry in my mouth, I don't even know if there's a larvae in there, do I? Or would the cherry itself be mushy and unappealing right from the start?
Nikki: You're right, these guys are tiny and especially the eggs are tiny.
And then the first instars are also pretty small and they're pretty hard to see with the naked eye. I can recognize but I've been doing this for a little while. I can recognize an orchard when they've got fruit that actually have SWD or spotted wing in there, because that's what happens over time. So the larvae will start feeding on the inside of the cherry, and then the cherry starts becoming softer.
And when you pick it, it's really juicy. And so you're like noticing that your hands are full of juice, and that's not very typical. And so they get really juicy. And then over time, it becomes like basically a sack of leftover remains of larvae, fecal matter and all that kind of stuff.
And they'll look like empty sacks hanging on the tree that just drip liquid out of 'em. But that process takes time for the larvae to feed inside the fruit. And the other thing that I've actually noticed, we have a lot of research trials here at our research station, and we don't spray at all for 'em.
So once an orchard gets infested pretty badly, you can actually smell it before you even walk into the orchard.
Susan: Wow.
Nikki: So it gives off a vinegary odor to it. My summer crew thinks it's like a little bit like vinegar mixed with ranch dressing that's gone a little off.
Susan: Wow.
And what is it exactly that we're smelling? Are we smelling the flies? Are we smelling their frass or their poop? What is the smell?
Nikki: My guess, it's probably a little from the fly itself and it's a little from the fruit when it starts to decay, but it's that combination of that vinegary smell that you gotta have the fly and the fruit.
So like I said, we have a colony here at our research station and our research station stinks from the time we start the colony in the winter all the way through harvest season. So the flies themselves are fairly stinky. So I think it's a combination of them both.
What we've seen in orchard systems, it's quite interesting. You'll see that you'll walk by a tree, the fruit looks intact, looks great, and then you'll go five trees down and you'll notice that this one tree has really been stung up. And there's a lot of SWD in that tree. And I wonder if there's a queue, like a volatile cue, either from the fruit and or the feeding of these insects on the fruit that they don't call in more of their species and then they start singing up.
Susan: Oh.
Nikki: We have some data that's quite neat. I've actually shown they do prefer fruit that's already infested.
So we put infested fruit into these kind of arenas and then the females can go and they can lay eggs in fruit that has larvae already in there, or nice intact fruit. And they'll always choose the fruit that's already infested.
Susan: Wow.
Nikki: And then we've actually watched, we've done timing trials too. So we actually watch her. She'll spend physically more time on fruit that's already fested as well.
Susan: Interesting. Wow. we've got an email here from Donnie from Connecticut. Hey, Susan. I was wondering what's the best organic way to care for our trees?
What fertilizers and ways to manage pests organically. Thanks. So we're gonna go into lots more detail about how to manage these particular pests, but I'm wondering, do you have, Nikki, any comment about the importance of feeding your tree properly so it can fight off pests like this? Or any thoughts on that?
Nikki: It's a good question. And we've done a lot of this work more on the disease front, just because I think that healthier trees can fight off disease much better than sickly or unhealthy trees. Unfortunately, I don't think that spotted wing or SWD really responds that well to the health of the tree 'cause we've seen growers that have walked away from orchards, basically they threw in the towel because of whatever reason. And those fruit are still perfectly fine.
And so I feel like the fruit itself, if there's fruit on the tree and you're not managing or spraying for it, then I think that SWD will still get in there.
We do have two products that are labeled organic on the US side that will work against SWD.
Susan: Oh yeah, you're talking about sprays that will work against them.
So what are they called?
Nikki: Oh, it's a spinosyn and so it is labeled for organic production.
And it does work. So that's the thing that's quite interesting about a lot of our organic materials that we spray against insect pests. Sometimes they don't work as well as we'd hope, and so we do see growers that are successfully managing this pest organically in northern Michigan. The problem is though, again, if those populations rise later in the season and let's say your harvest gets delayed, that's where we run into trouble is again, you just can't beat down all those billions or millions of flies in an orchard.
They can overwhelm even the most conventional type spray programs they can find. We've done tons of research where we've actually bagged certain areas on trees so they're not sprayed. And then we'll remove those bags and they can find those unsprayed fruit in a block of 10 acres of cherries.
Susan: Wow.
Nikki: So yeah, they can basically find any hole in a spray program, whether it's organic or conventional.
Susan: I'm gonna ask you a couple more questions. First of all, give me an idea, what is the extent of the spread of this insect in North America?
Has it come to Canada? What states might you find it in? Is it spreading?
Nikki: It is spreading. So I was out in Utah. So we have a tart cherry area that we grow out in Utah. But the thing that those guys don't have, which is different than a lot of areas in Canada or Michigan and the east is they're growing a lot of this fruit in the desert.
So our main areas where we grow sweet cherries, BC and into Washington, a little bit down into Oregon. A lot of those guys, they're growing fruit in a desert. So once those cherries are harvested, there's really nowhere for those flies to go to build up those populations. Or here in the east, like Michigan, New York on in to Ontario, the problem we have are woodlots, and those woodlots have autumn olive, they have mulberry, they have blackcaps, they have wild raspberries, and those are all hosts for spotted wings.
When it gets hot and dry out in the west, as soon as the cherry harvest is over, those populations just drop. Where here in the east, the populations just keep building and building until the first frost, essentially.
Susan: Uhhuh.
Nikki: So they are around. I actually heard some colleagues from OMAFRA this year that they're catching huge numbers in their traps already. So there are folks in Ontario that are monitoring for this pest as well.
Susan: Wow. So interesting that it's spreading. It's not a good thing.
The other thing is, I understand that sour cherries are appealing for these flies, but do they also go for sweet cherries, or what are the other fruits that they like destroying?
Nikki: Yeah, so they will go to sweet cherries. We've done a lot of choice and no choice tests where we release flies in different types of arenas, and then we look to see where they're going, what they're preferring, and they will attack sweet cherries.
I think that the issue is, where I'm from, in the northern part of the state here in Michigan, we are basically done with sweet cherry harvest before the populations get big. But over time, I feel like this year might be really optimal. They really like high relative humidity and we have just had tons of rain.
So I feel like this year might be a year that we could have issues in sweet cherries.
We trap every year, so we have strawberry growers here and we trap in strawberries, and I really haven't seen 'em in strawberries. But again, I think it's a function of population size. What we have seen a lot acRoz the state and actually everywhere in the east, raspberries. They love raspberries.
And it's funny, I have a lot of homeowners that call and say, oh, I've heard of this bug, I don't have it. And I think it's just 'cause they can't see it or they're eating 'em and they don't know it. So raspberries are probably the far and away most favorite thing that they could eat. And so raspberries, they'll also get leaky.
And then we used to have growers, especially You-Pick operations, that would have fall bearing raspberries that industry's basically gone within the state just because they can't keep SWD out of fall bearing raspberries.
Susan: Unbelievable. That is so scary. Now I understand apricots, which I love growing in Toronto, they're vulnerable as well.
Nikki: They'll get some, but you know what? I really don't see them. What we've seen, we've done a lot more work in peaches because peaches are later in harvests than tart cherries in our area. So they really like the suture line on a peach, and I'm assuming on an apricot, but we'll see a lot of eggs, but we've never really been able to get a lot of larvae or adults out.
So I think the fuzziness will prevent them and the firmness. So I think our data show that, as soon as you get below five pounds of pressure on a peach, that's when it's over. So my assumption would be the same for apricots. Same with plums. They will attack plum. But they really don't like 'em until they get really quite soft.
Susan: We got an interesting question here from Aman. hi Susan listening from El Paso, Texas. Love the show, but what a creepy topic. It's so creepy that you may not be able to see these flies on fruit with the naked eye. Does washing cherries aggressively as a consumer fix this? Thanks. I think that's a good question.
Is there anything that we consumers can do?
Nikki: Washing is probably not gonna help, but what we've done and so the other fruit, I should have mentioned, blueberries. They really love blueberries too. And we have quite a big industry in Michigan with blueberries, where they've been seeing some challenges.
So as a consumer, this is what we've done. So we buy a lot of Michigan blueberries and cherries especially, but when you buy fresh blueberries or any fresh fruit, you can actually let 'em sit out for a little bit and then you can put 'em in some warm water with like maybe a brown sugar solution.
The larvae do not like to be disturbed. So when we do our research, we always do this brown sugar test where we kinda gently squish the fruit, which obviously as a consumer you don't want squished fruit. But if you put 'em into, like brown sugar water ratio, it's just it's just so sweet.
They just don't like that irritation and they'll float or they'll come out of the fruit. Washing it won't help because the larvae are actually feeding inside the fruit, which is also a reason why insecticides don't help. A lot of times too, once they're in the fruit, it's too late. So brown sugar solution, they will come out when they're irritated, but sometimes when they come out and they're floating around, a lot of times the fruit, frankly, isn't as appealing as it was before.
So yeah, I actually just don't tell my daughter about the raspberries and just let her eat 'em.
Susan: Obviously it's not gonna kill us if we have some, I guess it's a little extra protein, right?
Nikki: Yeah, exactly. And the other thing is too, I think, not that I'm advocating for eating insects, although I'm an entomologist, I feel like yeah, an occasional larvae here and there probably won't be really bad.
But once the infestation is bad enough, you're getting fruit that has off flavors, off taste, and you'll be able to see that. And you'll be able to taste that. I think the consumer will be able to.
Susan: Okay. So I think we've got a really good big picture of what the problem looks like.
[00:21:00] Research and Modernization Efforts
Susan: I want to talk about the solutions that you've discovered.
So Nikki, tell me a little bit about how you started researching solutions to this problem.
Nikki: Yeah, so that is an ongoing research project. I think I told you when we joined on the phone that we're setting up our trials this year for 2019. I was starting to think about the tart cherry system or the sour cherry system. So it's frankly a quite old system that we've been trying to work with growers on maybe how to modernize it.
So our tart cherries are grown on a standard root sock. So if your listeners don't know, the bottom half of the tree is the root stock, and then we graft a scion or a variety to the top. And that root stock is called the Mahaleb. Very old. It's a very old rootstock. And actually the Montmorency cherry is the majority of our industry within north America.
And the Montmorency is an old French variety that's 250 years old. So the way we grow these trees is we put 'em in the ground. We have huge wide spacing, so nothing like espalier trees, there's nothing high density about this. So they're about 20 by 20 feet and we put 'em in the ground and then we keep the fruit off 'em so the tree grows and fills its space.
And we do not harvest these trees until years, around five, six, even seven.
[00:22:28] Mechanical Harvesting of Cherries
Nikki: And the reason we do this is the majority of the industry mechanically harvest their cherries. So they actually use a cherry shaker. So the cherry shaker has a clamping mechanism where it actually clamps onto the trees and it gives them a couple good shakes and the fruits fall from the trees onto some kind of catch frame, and then they're harvested into those tanks of water.
So it's a system that's quite old. It was quite revolutionary when we came up with the shaker because a lot of systems that are still hand harvested, tart cherries were mechanically harvested. So we used to have lots and lots of people that would come to the Grand Harvest region every summer 'cause all those little tiny beautiful red cherries were hand harvested at one time.
So the system is very old.
[00:23:12] Impact of Temperature and Humidity on Spotted Wing Drosophila
Nikki: So when I think about spotted wing, they're in the literature, they really don't like hot and dry temperature. So remember when I was talking about out west, once those cherries are gone, there's nothing really for them to reproduce in. So in hot and dry situations, I think that they will move from the crop of choice into the woodlots where it's cooler, it has more shade.
And then we started looking at our tart cherry canopies. And I thought, my gosh, those are giant. Why would those females need to be moving out of the canopy to those woodlots if they have a nice shady area that the temperature's not super hot and that the relative humidity is probably higher? So we started to test that.
Is the relative humidity higher in a canopy than outside the canopy?
[00:24:00] Pruning Strategies to Reduce Infestation
Nikki: And then we started looking at different pruning strategies. So how much wood do you need to remove from the canopy to actually decrease your relative humidity, and then also decrease your temperature.
So we started to do these different pruning treatments, and yes, if we remove six to ten limbs of a tart cherry tree on an annual basis, we will reduce our larval infestation by 40%. And that's with no insecticide applications at all.
This was exciting, Susan, because it was like there was some kind of light because that's what this patch has been.
It's actually, this patch has been very har heartbreaking in a lot of ways 'cause there seems to be nothing you can do to really stop it. And no growers wanna spray more than they have to. And so we wanted to try to come up with a more holistic approach to managing this pest rather than just telling growers to jump in the sprayer and go spray insecticide.
Susan: That's incredible. So basically what you're doing is you're opening up the tree, you're increasing air circulation to dry it out, and you're letting the sun in to make it way too hot. It's good for the fruit because the fruit ripens, but it's way too hot for the fruit flies for the spotted wing drosophila.
Letta hang around in there. So basically you wanna make it too hot for the flies.
Nikki: Exactly. And I want them to leave the crop because if those females just stay in that nice, comfortable canopy, I think that all her resources are really devoted and putting into laying eggs rather than traveling back and forth to wood lots.
And so anytime we can get 'em out of the system or reduce the numbers in the system, the better. So we were really excited to see that.
[00:25:45] Innovative Tools for Pruning
Nikki: Where I really wanna take this, is six to ten limbs. Every grower has a different pruning strategy. We were out today, we have this really sweet new machine. It actually looks like a light saber, but it measures the amount of light in the canopy.
And so we're trying to come up with, how much is good enough pruning, six limbs to one grower, maybe 10 limbs to another grower. So I would like to get it down to the percentage of light that's infiltrating the canopy. What's really good for SWD or spotted wing control. So we have this new machine that we were out just trying to test, and it basically measures the light in the canopies and we're gonna measure light in the different pruning treatments.
And then, maybe this isn't fantasy, but somewhere down the line, is there an app on your phone that measures light? And growers can go out and look at their canopy and say, oh yeah, this is open enough. Or the humidity is low enough to really minimize SWD in the canopy.
Susan: Oh, that is so exciting.
[00:26:47] Listener Questions: Winter Survival and Consumption Safety
Susan: We have an interesting question from Bev. She writes, hi Susan. I'm Bev. I'm listening from Hornepayne in Northern Ontario. I love the show. A question is, are these flies surviving the winter in fallen fruit? If the orchard was cleaned up, would it help in getting rid of this fruit fly? Thanks so much.
Nikki: That's an awesome question. And so we actually have a research project that's going to be dedicated to that. So the other thing that we've been finding is they over winter on a lot of different things, we can find 'em on mushrooms, we can find 'em in pine sap, so we can find them on apples. And what we're really interested in right now are cull piles.
Let's say you have a cider mill or something and you have all this leftover fruit, will they reproduce in those areas? So we're trying to figure that out if there might be hotspots, but I think the more you can do to clean up the fruit is probably good. But then they're in tricky places like pine sap in the winter.
And then on mushrooms, so in a wet fall, there's just a lot of mushrooms laying around there, as well.
So it's not foolproof by any means, but anything you can do to reduce the populations or the chance for them building those populations, the better.
Susan: It's a piece of the puzzle. Yeah, so we've got an email now from Ben. Hi, Ben here from Santa Rosa, California. So you are saying if we consume these flies, even at a large quantity, since I absolutely love cherries, that they will not hurt me. I am worried now. Thanks. Okay. Should Ben be worried?
He's in Santa Rosa, California. He eats a lot of cherries.
Nikki: So no, we shouldn't worry. Our growers are doing the best they can to balance out, managing the pests and also trying to do holistic approaches. So I think they're doing the best job they can. The other thing that I think is, if there is infestation, those usually will get tossed and won't go onto the market.
And then the other thing is, I feel these approaches that we've been talking about pruning, and then there's some other things we've been doing with mowing is reducing the population. In California, we've actually had some colleagues come out and speak to our growers. They have a real issue with cull piles from citrus.
So where they have problems with spotted wing is where they have cull piles from citrus, it breeds in the citrus and then it moves to the adjacent sweet cherries. So in California, they found that if they remove those cull piles of citrus, they drastically reduce their SWD populations. So there are growers all acRoz the country, growers, researchers, trying to come up with these approaches that hopefully aren't just insecticide based.
And like I said, if there are larvae in fruit occasionally, most of the time it'll be a very minimal level of detection. So I don't think anyone should give up their cherry habit at this point.
Susan: Oh, that's good. I'm relieved. So we also had an email from Roz from Ottawa, Ontario.
[00:29:56] Barrier Covers and Biodiversity
Susan: Roz says, does your guest have any experience with or comments about barrier covers?
For example, knit covers to combat SWD. These covers seem like an interesting idea, but I suspect the timing of when they're put on the fruit tree is critical. If the covers are put on too late or are not sealed properly, you would risk trapping. The fruit flies inside with the fruit. I also wonder if these covers could keep out the fruit flies, but affect the ripening of the fruit since the size of the mesh is considerably smaller than bird netting.
So that's an interesting question from Roz. And I did see online that there are literally these zipper covers that you can zip on top of your fruit tree. what do you, have you heard about those and what do you think about it?
Nikki: It's a great idea, especially I would say this is particularly a great idea for a backyard gardener or a backyard orchard owner, just because you don't have the sheer number of trees.
So we have 32,000 acres of tart terries in the state of Michigan, and our trees are somewhere, by the time you have a 15-year-old tree, you're talking about 18, 20 foot in the air. So netting is not possible there. However, netting does work. We brought in a speaker from France that they are netting a lot of their cherries in France and then the Italians are also netting a lot of their cherries.
So we had a speaker, he's a breeder and so he really is trying to be breeding a lot of these. And his focus for his research has totally changed. And now he's breeding for architecture that fits under nets. So they're receiving a premium price for this fruit still in Italy where it makes sense to cover. Because that's the problem. It's not very economical, especially on a bigger scale.
So that's the issue with that. But I think in Italy, he seemed to think that it was still worth it for them to try to cover and make a physical barrier to keep that fruit fly out.
Susan: I also have an email from Tisha from California.
I would like to suggest to the owners of orchards that they all watch the new major award-winning documentary called The Biggest Little Farm. It shows why the fruits get ravished (not enough diversity in plants, animals, and insects). And it explains how to balance the system so all life flourishes beyond expectations.
The film is playing in small theaters and will be on Netflix soon. Please watch it. This movie is inspiring all those that watch it. It will help motivate all of us to take part like me, smiley. Tisha's talking about biodiversity. Do you think that could play a role? Like maybe there is some natural predator that another fly or another insect that will eat the SWDs?
Nikki: We're looking for that. There's quite a big team of North American researchers that are back in Asia. They're trying to find a pest, which we all know that can turn out to be very troublesome if we select the wrong natural enemy or predator. So there are some things in USDA quarantine right now, but in terms of, diversity within like a backyard farm or a farm itself, I think that probably there is something to that when you have lots of acres of, let's say tart cherries, they don't have to fly that far to build those populations. So diversity may be able to help. The problem with spotted wing is that it does attack a lot of soft flesh fruits, and so you'd have to really think about what you'd put on your farm to really diversify it enough that you would have things that it wouldn't like.
So my guess is that, it'll get into strawberries and raspberries, and it will get into soft flesh peaches and plums and apricots and tart and sweet cherries. So that leaves kind of apples and pears, if you're thinking about fruits that they don't really like or they don't infest. Oh, the other one up here is that we grow a lot of wine grapes, so they do not like viniferous wine grapes.
We've tried to infest viniferous wine grapes for many different trials, and there's something about wine grapes that they do not like.
Susan: Wow. Okay. So that's good. So keep an eye when you're creating a more biodiverse environment, just make sure you're not just giving them a whole selection of things that the spotted wing drosophila likes.
You wanna give them things that they don't like.
Nikki: They like what we like.
Susan: Yeah. Yeah.
Nikki: If you were gonna grow things in your backyard, you just think of all those fruits that we just named, and that's probably what everybody wants to grow in their backyard.
Susan: We've got an email from Carmen.
Hi Susan. Just tuned in. A few minutes ago, I had to really listen to you and your guest as I thought you were talking about STDs for flies. Not SWD. Hilarious. I heard you wrong. Love your show from Pueblo, Pablo, California. Thank you, Carmen. That's so cute. Nope. SWD is the short form. It's a short and snappy way to say spotted wing drosophila, which trust me, I've said it a few times during the show. It is a mouthful, isn't it?
Nikki: Yeah, it definitely is. And I kinda like that analogy a little bit. It does feel like it is something really bad that you don't want, like you would think of an STD.
So maybe there is something to that.
Susan: Maybe there was something to that. You're right. Yeah.
Nikki: Exactly.
Susan: Okay, there's a few more things that I want to explore with you.
So Nikki, another thing that you explored. We talked about pruning and how essential it is to opening up the canopy, bringing in the air, and bringing in the light, and bringing in the heat to keep the spotted wing drosophila away.
[00:35:47] Mowing and Orchard Floor Management
Susan: But mowing was also part of the solution and that really surprised me. Tell me what role the floor of your garden or orchard plays.
Nikki: It is kinda amazing really when you think about it, 'cause I wouldn't have probably put those two together. But again, I was looking at our system and we do a lot of sod row centers in all of our orchards.
And actually that's fairly common practice that prevents erosion, all that kind of stuff. So there's a lot of sod row centers in there, but sometimes growers don't mow them that often. And you kinda are in there and you're up to your like knees in grass and weeds and things. And I start thinking.
Is there a role that this grass or these weeds, when they grow up, does it contribute to the overall humidity of the entire orchard system? And we found, frankly, it does, which it blew my mind that what you can do on your orchard floor is actually affecting insects and how they behave in a canopy that's 15 feet off the ground.
But we had these data loggers in our orchard. We've repeated this for multiple years now. And we find that we actually make the orchard more humid when we don't mow, compared to just mowing every two weeks. And then the other thing we did, early on, is we started to look at different covers in an orchard.
And then the other thing we did is we actually tried to herbicide everything down so there'd be nothing green in the orchard, which I'm not recommending by any way, by any means. But we wanted on the research front to see if we herbicide anything that was green, would we reduce SWD or the other thing we did was tilling.
So we use the herbicide on the side row centers at the beginning of the season, and then anytime we saw something green, we came through and tilled it, and there were no statistical differences in our data from mowing every two weeks, and these more kind of quote unquote drastic type measures. So I just tell growers they just gotta mow every two weeks.
Keep your grass short. Don't let everything go long, and it'll keep the orchard less humid, you'll also raise the temperature a little bit, which will really affect how you manage SWD. So it is surprising. We've crunched the data multiple times 'cause I was like really? At what we're doing on the orchard floor really affects the canopy.
So that was another thing that growers can do, again, with that holistic approach in mind.
Susan: So I'm understanding that if you don't mow, you are somehow adding more humidity to the orchard. But have you actually proven that humidity means that there's higher numbers of spotted wing drosophila?
Did you make that connection or you're just saying, look, if it's more humid, they're gonna like it better?
Nikki: It's a good question. So we've actually tried to vacuum insects up off the floor to see if there was actually physically more SWD using this kind of vacuum.
But the problem is, I can never get the airflow of the vacuum to not destroy the teeny little flies. So we're still working on that. But what we actually showed with the data is that we had lower numbers of infested fruit where we mowed compared to where we didn't mow. And then we show that the relative humidity, it was much more humid where we didn't mow.
So it's maybe not a direct comparison, but it's pretty close. But we did try to go see if we can get those flies off those different types of orchard floors and we just really couldn't find anything.
Susan: Yeah, I find that really interesting because one of the things that I am interested in, and I'm totally passionate about biodiversity in an orchard environment or in your backyard garden, and there are people who talk about beetle bumps.
I'll talk about it with somebody who's very, knowledgeable about it in a future show. But these are like leaving tall grasses in your orchards so that predatory beetles can hang around in these beetle bumps, and then they crawl out and then they eat their insect pests and then they go back. But I guess that would be similar if you have a strip of tall grasses in your orchard. That may actually be good for the SWD?
Nikki: Yeah, it might be. I've had growers that have taken some of those data a little too literally, and they think that the SWD is hanging out in those tall grasses, and we haven't shown that yet.
So I have a lot of growers that called me and said, Nikki, I think I should spray my grasses. And I was like, no. That's not what we're saying here. So we still have more work to do with that. We do that with mites, two spotted spider mites. We used to let grasses grow up in the orchards just because the mites would hang out on these tall grasses rather than moving into the tree.
And we always saw 'em. As soon as you mowed your orchard, they'd move from your grasses in the middle over to your tree.
So there was some reasons we did let our grasses grow long, and it was really for two spotted mites. You would have less problems in your trees 'cause they'd be hanging out on your grasses.
Susan: Aha. Very interesting. So maybe there's a balance. What you were talking about is the orchard floor, your garden, the ground of your garden. But having a strip of these grasses might actually be useful, and grasses rather than weeds, specifically and carefully chosen and planted. So again, it brings us back to that lovely biodiversity piece that we got the email from Tisha in California.
So is there anything else?
[00:41:13] Future Research and Hope for Growers
Susan: Where do you think the research is gonna go into the future and how hopeful are you that we are actually going to be able to defeat this nasty little fruit fly?
Nikki: I am more hopeful, Susan, than I would've been like probably five years ago. This pest really gets people's emotions up just because you feel like you're throwing everything but the kitchen sink at this thing, and you're still losing. So it's been really heartbreaking and I feel we have a lot of growers right now that are struggling for a lot of different economic reasons.
This is not new to farming in general, but this SWD just seems like just another nail in that coffin that just makes it seem so difficult. So I'm more hopeful for things down the line, that we might be able to use something like a sterile male release, or there might be some type of predator or parasitoid that we might be able to use.
So we are screening a lot of predators and parasitoids that are already here in North America and that we're not necessarily bringing everything in from Asia. I feel like there's more hope than there used to be. But it's almost like you're rushing to try to come up with a solution and you're putting things, like making recommendations to growers as fast as we can get the data and get it out to 'em.
So it feels like it's been very emotionally taxing on all of us. And so I feel like there is some hope with some of these practices that things that growers are doing already. So I'm not telling growers that they should do something really wild and crazy. I'm just saying, hey, maybe we should just prune to open the canopy.
And that makes things feel more hopeful.
Susan: Oh, absolutely.
Nikki: Hope is. Yeah. We need that right now.
Susan: We need that. We need that right now and, these are simple measures and I think it's really fantastic that you thought to research it.
[00:43:08] Conclusion and Farewell
Susan: And Nikki, thank you so much for being on the show today. It was really fun to talk to you and it's really exciting to learn about the things that you guys are doing, and how it's helping all of us to be better growers and to grow better fruit. And it's super exciting.
Nikki: It's been a real pleasure, Susan.
I'm glad you found me and it was so fun to be on your show, so I wish you the best.
Susan: Thank you so much. Goodbye for now.
Nikki: Goodbye.
Susan: That was Nikki Rothwell, PhD from Michigan State University's Northwest Michigan Horticulture Research Center. That's it for today's episode of the Urban Forestry Radio Show.
Thank you so much for tuning in. It's been wonderful to have you as a listener, and I hope to see you again next time.