Well I hope you listen back to our previous episodes on BMSB before you jump into this one. If you do, you’ll hear me kicking back and letting Nicole Quinn do my job for me, explaining the basics on the invasive brown marmorated stink bug, or BMSB. I’m going to go ahead and do that again and let Nicole explain her work studying a teeny, tiny, non-stinging, parasitic wasp from Asia:
Anna: Do you want step back and just tell the whole samurai wasp story from the beginning?
Nicole Quinn, USDA-ARS: Yeah sure, so samurai wasp, Trissolcus japonicus is the name. It’s a parasitoid of stink bugs. It really likes brown marmorated stink bugs specifically in their native range. And what it does is that it attacks the eggs of the stink bug. And by that I mean that it lays its eggs within the stink bug eggs and then a parasitoid comes out instead of the stink bugs. That’s what parasitization is. It’s really similar to predation functionally. It kills it completely so it’s not a parasite, it’s a parasitoid, so it kills the host rather than living with it. So anyway, in the native range, samurai wasps actually kill about 80%, are bound to parasitize 80% of wild egg masses of brown marmorated stink bugs, so it’s very effective in there.
So after brown marmorated stink bug invaded here, obviously there was a lot of interest in defining these effective natural enemies of brown marmorated stink bug from their native range. And this is kind of one of the principle underpinnings of what they call classical biological control. So the idea is that the reason why basic pests are a problem is that they come to a new area where they don’t have any of their natural enemies and the environmental conditions are right and they flourish because there aren’t any checks on their population. So by going to their native range and finding out what attacks them there, the idea is that you can identify those insects or whatever attacks them is, bring them to the invasive range, release them, and even it out.
The other question we always get about biocontrols is: will this get rid of the stink bugs. The answer is no. Even in their native range, there still are stink bugs, there’s just not as many.
So they found this parasitoid and they brought it to the US to test it in quarantine for host specificity testing, you want to make sure that there’s not an unacceptable risk of non-targeted effects. You don’t want this introduced natural enemy to attack something that you don’t expect or attack something that you do expect too much.
Anna: I’m going to jump in here – not so much to explain something any better than Nicole did – but to put some historical context to host specificity testing. I’ve lived in many areas of the country and pretty much everywhere has some kind of urban myth about the origins of Asian multicolored lady beetles, Harmonia axyridis. You know, the lady beetles that come in your house in the winter? People have heard the government was to blame, that this university or that university did it. A whole lot of finger pointing going on and muttering under the breath as our nation’s vacuum cleaners remove lady beetle carcasses from window sills.
But it’s true, there have been purposeful, and some accidental, introductions of Harmonia collected from outside the US for more than 100 years. The first releases occurred in California in 1916. The most recent purposeful release was a USDA program in the late 1970s to manage an aphid crisis in wheat. Wheat is ok now…but releasing such a voracious predator had many negative repercussions – not just in our nation’s windowsills – but on native lady beetle populations, who were outcompeted in many circumstances, even preyed upon by Harmonia. Furthermore, there’s a few examples of crop damage due to Harmonia feeding when they run out of aphids and other small insects to eat. Nowadays, we just don’t do area-wide releases of generalist predators like this. The potential for negative non-target impacts outweigh the potential benefit.
You might ask yourself, what are the potential negative impacts of killing stink bugs? Back to Nicole:
Nicole: So the example I give is the spined soldier bug—it’s a stink bug—but it actually eats a lot of other pests in the US. So it’s a very important natural enemy in various other systems especially in things like soybean and some other crops. And so there was a lot of concern about potentially introducing a natural enemy that could harm biocontrol in these other systems. You want things to be in balance as much as possible. And so basically they did those no choice testing and found that Trissolcus japonicus, samurai wasps, will attack a lot of stink bugs, in the lab anyways in these little tiny vials where they don’t have any other options. But if you put them in what they call dual choice testing, so you put brown marmorated stink bug egg masse in the vial along with a Podisus stink bug egg mass, and then you put a parasitoid in there, and you let it choose, usually it will choose brown marmorated stink bug a vast majority of the time. And this is true of pretty much all of the US stink bugs that they tested. They do other measures too to kind of see how this parasitoid would perform under different scenarios and with different insects.
It’s still hasn’t been technically approved for release. I don’t want to jump the gun on that whole story. But they are testing it and the testing looks good. It looks like it’s pretty specific. So then they’re going through all of this testing and while this is going on people are monitoring for natural enemies of brown marmorated stink bug that are here in the US already. So like native parasitoids and things like that, and predators and trying to see if maybe some of those could basically step up and provide some more control.
Incidentally when they did this, they actually found this Trissolcus japonicus samurai wasp emerge from an egg masse that they put out as a sentinel egg.
Anna: I’m going to jump in again because this incidental introduction of the samurai wasp, or Trissolcus japonicas – actually the scientists that know it really well call it TJ – This incidental TJ introduction was a real fly in the ointment for the folks who had brought this insect from Asia to quarantine facilities in the mid-Atlantic. Did they accidentally release an organism without the proper non-target testing? What happened here? Nicole will explain but that also raises the question, how do you know if teeny tiny parasitic wasps are out hunting where you live, or where BMSB lives.
There’s a few ways. If you know the species you’re looking for, you can use yellow sticky cards. All sorts of insects are attracted to the yellow color and get themselves stuck. You take those cards back to the lab – well first you get a whole bunch of training on how to identify parasitic wasp species – then you take these cards back to the lab and look at them under a microscope and try to find the species you’re interested in.
If you don’t know what you are looking for, if you just want to know what is attacking the eggs of a pest species, you get yourself a colony of that insect, produce some eggs and use sentinel egg to census the egg parasitoids in your neighborhood. Back to Nicole:
Nicole: So what a sentinel egg is, to explain, it’s basically an egg mass that you put on a piece of cardstock and usually there’s a piece of double sided tape that holds it in some sand that makes it so the parasitoid doesn’t get stuck to the tape. The other thing is parasitoids are very small. It’s about the size of a poppy seed, or a period at the end of a sentence, I would say. Some parasitoids in general can be bigger but usually they are tiny.
Anyways they have these sentinel eggs they put out. They put out one of these egg masses and they bring them back into the lab and wait to see if something will happen, if something will emerge. What they found was that this Trissolcus japonicus actually emerged from one of these egg masses. And again, they had not released the samurai wasps here in the US at all. This was just, they happened to be monitoring for some of these other parasitoids that might be out there and they found this. And so there was a lot of excitement and some mild concern. People wanted to make sure that it hadn’t escaped from the lab or something. But they did DNA testing and they found that it was not the same. The microsatellite data, the DNA was different enough from the lab colony that it was not from the lab colony. So it found its own way here accidentally, just like the stink bug did.
No one knows how or when or why. People do really think that it wasn’t one of those things where it had been here for a long time because there had been a lot of monitoring of natural enemies. But anyway, they basically found this parasitoid and that kind of changed everything for biocontrol of brown marmorated stink bug here in the US. A lot of research programs really began to focus on this parasitoid.
What was interesting was that it was really only detected, at first, in one state. It was detected first in Maryland, but then it subsequently was detected in a lot of other states. Every year a few more states get added to this map. This would be another map that I would have, this was a presentation. But basically it’s in many of the states where brown marmorated stink bug is found at this point.
Anna: Nicole and I had a much longer conversation about how we would go looking for TJ, if we wanted to know if it had made its way up to New Hampshire. An important thing to know about both BMSB, BMSB eggs, and the egg parasitoid is that you’re much more likely to find them way up high in tree canopies. I highly recommend taking a look at some of the sampling strategies deployed by Nicole and her colleague. They rigged pulley systems and stuck long bamboo poles up in wooded borders of orchard – there were entomologists running around with bucket trucks - all to get a better sense of population dynamics of these bugs. Not easy work.
But something I was naturally curious about was how our winter weather might influence TJ?
Anna: So I know that we don’t know a lot about the biology of the wasp, but my first question being I’m in a cold climate and thinking more cold climate, is that it’s a parasitoid and the stink bug overwinters as an adult. Do we know how the wasp overwinters?
Nicole: It is thought to overwinter as an adult but no one has ever found it overwintering. There’s another species in Asia that they’ve recovered overwintering under tree bark. Anyway there’s another species in Asia that has been found overwintering under tree bark. And so they think that Trissolcus japonicus probably does the same thing. We’ve tried to do some lab studies to see if there are different overwintering substrates that Trissolcus japonicus liked, but there were so many issues with that study. So I don’t know, someone needs to work on that some more.
Anna: I personally have a lot of experience failing at keeping insects alive in overwintering studies so I have a lot of sympathy here!
I do hope someone picks up on this question, both because it will be a tremendous scientific feat and because I want to know if nature will take its course on this invasive pest before it becomes a problem in New Hampshire. Peter Jentsch has been busy with a project releasing TJ with the help of citizen scientists. For more on that you should check out his blog. There are several other potential organisms that are looking like important natural enemies as well – shout out to Ann Hajek’s lab at Cornell - they’re studying a microsporidian infection that might be working to naturally controlling outbreak populations of BMSB in eastern North America. More to come in this area. But I asked Nicole how confident I should be that natural enemies will do our pest management jobs for us?
Nicole: It’s one of those things. The idea with this parasitoid is that hopefully it will reduce the stink bug population enough that overtime it will become less of a problem. And by that I mean it will cause less injury to crops and that overtime that means ultimately that growers can spray less, which is what the public a lot of times is kind of interested in. What does this mean at the end of the day? It will be easier for growers to run their operations. There will be less pesticides and things like that going into the environment.
That’s it for now. Thanks to Nicole Quinn (of USDA-ARS). Thanks to all of you who have stuck with me on the this BMSB adventure. I really took the name of this podcast seriously on the topic of BMSB! And a special thanks to Jason Lightbown, who wrote and performed our theme music.