SWD & Yeasts
Hey there podcast listeners! This is one of many in a series of episodes about SWD research. If you have not listened to my episodes on SWD monitoring, I recommend listening to those before diving into this one. There’s a ton of really interesting work going on comparing and contrasting the biology and behavior of this invasive drosophilid to its native cousins…and it’s a pretty big family. You might be familiar with Drosophila melanogaster if you’ve ever studied genetics. You might be personally familiar with the slew of Drosophila species that make their way into your kitchen from time to time.
Some of this stuff is a little less applicable for right now but I hope to bring some perspective as to why finding a more useful monitoring tool for this pest is so challenging and maybe get you excited about some pest management approaches for the future! A disclaimer that I’ll skim across the top of some pretty deep topics, so a pre-emptive apology if I leave anyone hanging.
So we know that the lures used for monitoring SWD are essentially fermentation odors. The commercial lures we discussed in previous episodes - the Trece and the Scentry lures - the odor components of these lures were actually based on odors from wine and vinegar. Many of these odor components are shared with the odor components from yeasts. You might ask yourself, why are these flies so hot and bothered over yeast if they lay their eggs in ripe or ripening fruit? What gives? Well come to find out, all of these so called “fruit flies” are actually much more motivated by the yeasts that live on fruit than the fruit itself. Don’t get me wrong, they love the sugars in the fruit but those sugars also feed the yeasts and the yeasts are a good source of protein. The flies are into that…so they’re into the odors we put into traps. For a much better explanation, I reached out to someone who knows to explain.
So what we know about drosophila in general and yeast and what we think we know about them is that yeasts are really important for their development. Originally what we thought was the Drosophila suzukii, since it has the different feeding strategy of going for fresh fruit, that it was going to be less reliant on protein and therefore less reliant on yeast which are basically little concentrated packets of protein when you think about relative to the sugars and carbohydrates that you find in fruit, right. But it turns out that protein is still pretty important for suzukii, although they don’t need quite as much as some of these other drosophila do. They have a very similar association with yeast as other drosophila.
Anna: Here’s the point where I would have liked to drop in and explain the difference between yeasts and molds and kind of fit them into the general fungal scheme of things. I’ll be honest with you, taxonomy in this area will get you twisted really quickly…well at least I find it completely disorienting. What you do need to know is that there are all sorts of yeasts and molds living on the surface of just about everything. Plants generally have a whole bunch of fungal pathogens that actually have mechanisms for growing inside plant tissue. But here we’re kind of just talking about the benign species of single-celled fungi just living their best lives out on the surface of plant tissue. Sometimes that plant surface is covered with all kinds of different species of fungi, all putting out their own smells, and sometimes there’s a dominant species that has outcompeted the rest because it’s better at handling certain stressors, like heat or desiccation…or the alcohols associated with rotting fruit. This gets really interesting because Drosophila melanogaster has this long history of association with alcohol production and therefore a long history of associations with us humans… because we’re always fermenting something. It was thought there might be a weaker association with fermentation odors for suzukii because she lays her eggs in ripe fruit and doesn’t really need to wait until that fruit breaks down and rots, the way melanogaster has to…but apparently, they aren’t all that different when it comes to associations with these surface-dwelling microbes.
Kelly: But you could, if you were to boil it down to something simple, the idea is that yeasts and flies, they’re friends. And the yeasts, because they have these huge cells, need insects to move them to fruits so that they can get the sugars. So the yeasts cells are not found in the air like mold cells are. It’s thought that they really can’t get from point A to point B without an insect carrying them. So probably there is an advantage to the yeasts to be attractive to an insect so that they’ll carry them.
And one of the things that we don’t know about that particular interaction with the yeast and the flies is whether the yeast that you’re finding is really something that they specifically want that one, or whether it’s a yeast that happens to be really competitive and available to them. So they’ll take whatever they can get and they go for what is the most common, right?
So the species that we find most is Hanseniaspora uvarum and that is actually a really important yeast also for doing wine fermentations. So a lot of people use that species to start wine fermentations because it outcompetes these other yeasts that they don’t want – to avoid off flavors in the wine. So it’s kind of a cool species of yeast. It’s super competitive. It likes to get into fruit early and it goes for the glucose in the fruit. So these are simple sugars and once it’s consumed all of the simple sugars, other species of yeast come in later and those ones are more alcohol tolerant and that’s where you’re really getting the further fermentation.
So originally we really thought this is cool, this yeast species is a yeast species that basically has the same kind of habitat as the flies were going for, where it’s going for really early fermentation, rather than later stages like other drosophila. And one of the reasons we’re really interested in this is because if these yeasts are attractive to flies, they could be used for monitoring traps, particularly when we think about the fermentation based lures that we’ve been using, we attract all different kinds of flies and basically any insect that likes rotting fruit will go into those traps. So we were hoping to develop something that was more specific for Drosophila suzukii, and we were working with a lot of chemical ecologists and people that were interested in trying to figure out what specific compounds in these yeasts were attracting the flies.
Anna: So did you find any difference in the attraction of flies to the different yeast species that you study?
Kelly: Yeah, we did say that there were differences in their attraction between the different fly species and that yeasts, different yeasts, produce different smells also, so they’re able to distinguish different yeasts.
Anna: Oh cool. From your personal experience, like your own nose, do you think that H. uvarum smells different than like regular baking yeast?
Kelly: Oh yeah. It’s interesting. Well, you don’t actively go around trying to smell yeast, but my lab sometimes especially when you walk into it and you’re like woah, this is a smell. But the different yeast species not only do they smell different they smell different depending on how old they are so a lot of them go from having kind of a fruity smell that’s pretty pleasant to more of a dirty sock smell as they age.
Anna: So Kelly mentioned several practical applications that her lab and many others are working out, based on what we know about yeast-fly interactions. I’m going to drop in on a few folks putting this information to work.
Cesar Rodriguez-Saona, Rutgers University
Cesar: Insects rely on chemicals for their communication. Chemical Ecology studies what those chemicals are and what role they play in insect communication. We try to exploit that kind of communication for pest management. Most of those applications have been to develop monitoring tools, chemical attractants. We also want to use these tools, not only to monitor, but to also to see if we can control them. For example, sex pheromones have been used for mating disruption. We’re also working towards attract and kill. Also, we can combine multiple chemicals that work differently – like attractants and repellents – in approaches like push-pull.
Manipulating insect behavior can be challenging, because insect behaviors take a lot of different steps. Throughout those steps, insects will use different chemicals. Manipulating insect behaviors can be challenging in nature and they can be context dependent.
Anna: Cesar has studied the chemical ecology of many pest insects, including SWD. He and I had a much longer conversation about some of the challenges we face in manipulating SWD in the field – that might need a whole other episode – but, long story short, SWD is a case study in the challenges he just mentioned. Our lures are regularly outcompeted by the real thing, as we see the numbers of flies in our traps change when there is lots of ripe fruit in the field. Also, the behavior we’re trying to manipulate is egg-laying, which is stimulated by a whole different set of cues than finding food, which is stimulated by these yeast-associated odors in our lures. What we’ve found out so far, in attempting mass trapping of SWD, we’ve learned that we can kind of manipulate where SWD lays their eggs…but it tends to be in the plants where we put our lures. This is a phenomenon we’ve talked about before - in plum curculio attacking apple, where managers were targeting chemical management to trap trees where aggregations were created with pheromone lures.
Cesar: Yeah, so I’m working on a paper right now with some of our results in blueberry. I’m calling them a trap bush of course, because its not a tree, but it’s the same concept. Baiting the bush to see if you can aggregate the adults to certain areas.
Anna: Stay tuned for more in terms of SWD behavioral manipulations. There’s lots of interesting stuff coming from applied chemical ecologists that might be useful in keeping SWD infestations at bay, particularly in the early weeks of blueberry crops.
For something even further out into the future, I touched base with Antoine Abrieux, who had been investigating methods of using RNAi for SWD managment in Joanna Chiu’s lab at UCDavis – check out her website at clocklab.org for some very cool stuff about circadian and seasonal rhythms. When we spoke in the early months of 2020, he was starting a new gig with a new start up company, working on ways to use yeasts to deliver this brand new kind of biopesticide to its target.
Anna: Can you talk about your project or will it be proprietary?
Antoine: Yeah, I can talk about it broadly…
Anna: So the Chui lab had been investigating RNAi, or RNA interference, as a potential mechanism for managing SWD. A VERY rudimentary explanation for what’s going on here: scientists identify an important gene for SWD success, they produce a tiny piece of double stranded RNA that would snip out that important function - here they were looking at important gut functions - the fly ideally would consume that double stranded RNA, which snips out that gut function, and the fly would not thrive or would die because that gut function was disabled. Coincidently, double stranded RNA are produced by growing them in yeasts. Back to that later. But this approach can be extremely selective, meaning that it would only effect SWD and no other species. However, RNA is pretty darn ephemeral. It’s just a string of nucleic acids afterall and not something that will last too long on it’s own. Not an ideal active ingredient if you’re interested in long residual efficacy.
Antoine: So basically this approach, RNAi have been used for pest management but the drawback is that it’s not stable in the field and it’s really hard to deliver it to the pest. The idea of using the yeast to produce it, and there is this mutualism between the insects and yeast – the yeasts need the insect to spread them around and the insect needs the yeast because it’s part of their diet and during it’s evolution yeast have been emitting these volatile compounds – we’re actually relying on that to enhance the attractiveness. We’re going to use the yeast, which will be ingested by the insect and ultimately delivered to the gut.
Anna: And if you look at the stuff that Kelly’s doing, it sounds like there’s some specificity.
Antoine: Yeah, and ultimately we can think about trying to engineer this into other yeasts. Right now we’re just using Saccharomyces cerevisiae, which is the basic yeast, like the baker’s yeast or brewer’s yeast, but, yeah but maybe we can pick specific organisms to target insects.
Anna: That’s some very cool stuff, but certainly not ready for application. Let’s wrap up with one more potential application that might be ready for adoption now. If these flies are attracted to the yeasts on the fruit, moreso than the fruit itself…can we make the fruit less attractive to the flies by reducing the amount of yeast on the surface? Yes we can! I talked to Rufus Isaacs of Michigan State University for more on what they’ve found using crop sanitizers to clean up yeasts on blueberries to do just this.
Rufus: last year we tested JetAg, another one called SporeQuell, and it did decrease infestation. It’s nothing like a chemical broad-spectrum insecticide, but maybe a 20-40% reduction, which is certainly part of the overall tactics for getting this pest controlled. It’s interesting, with this insect. In our trials we’re often looking at what level of infestation did we have? Is it significantly different lower, as a way to measure success. But given the way we’re managing this pest, if you could delay the need for that broad-spectrum insecticide for a week, that’s really significant. Environmentally, economically.
One of them, I think JetAg, actually has on its label that it’s a fungicide. Some farmers have been using this at harvest to control things like fruit rots. Knowing that it might give you a little benefit for SWD is helpful.
Anna: Do you know if there’s any benefit to using crop sanitizers in terms of post-harvest? Would this benefit shelf stability?
Rufus: We tried a small trial that didn’t turn out to answer this question last year. I know this is something that farmers will do and are happy with what it does for fruit rot quality, related to keep their fruit healthy and looking good and marketable.
Anna: Back to Kelly for more on the microbes that live on fruit in the field and potential for pest status of the fly as well as other plant pathogens.
Kelly: Yeasts are not generally pathogenic, so usually not a concern, but we have been isolating filamentous fungi, or molds, off of suzukii. Some of these are pathogens and there might be some interactions there too. It’s possible that, since SWD was introduced, we’re seeing different pathogen pressure and changes in different types of pathogens.
In Maryland, we seeing more Cladosporium infections. Now, we’re not sure if this is because we just hired a plant pathologist who started looking for it, or if we’re seeing more of it because SWD is causing more wounds and Cladosporium is facilitated by wounds. In a pick-your-own setting, its often the molds that are much more obvious and more important to the consumer. Having that fungal control can be really important.
Anna: All right! So we covered a whole lot of ground in this one but there’s a lot of potential for all of these ideas to come together some time in the future. For right now, I think I’m convinced that a foliar application of a crop sanitizer might be worth considering. Rufus mentioned JetAg and SporeQuell but there are other sanitizers that are registered for use on fruit crops as a foliar application, so check those labels. It might buy you a week or so during blueberry harvest before you initiate sprays and it might clean up any molds and other fungal pathogens that might be lurking out in the field too.
Thank you to Kelly Hamby at UMD, Cesar Rodriguez-Saona at Rutgers University, Antoine Abreiux and Joanna Chui at UCDavis, and Rufus Isaacs at Michigan State University. And of course a big thanks to Jason Lightbown, who wrote and performed our theme music.