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USDA researchers find new hope in the
fight against Stinkbugs!


Kim Alan Hoelmer, Ph.D.
Research Entomologist / Location Coordinator
USDA-ARS BIIR (Beneficial Insect Introduction Research Unit)



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Mike McGrath Introduction: Dr. Hoelmer, welcome to You Bet Your Garden.


Dr. Hoelmer: Thank you, Mike.


Mike McGrath: I am a huge fan of beneficial insects and research in general and one of the things I always tell people whenever we have one of these imported pests that shows up and gets out of control, one of the ways that we, as a nation, react to that is the USDA sends researchers over to the pests' home land to watch what eats it there and then they try to figure out what predator or parasite that they can safely bring over here. Is that what happened with this new stink bug that is causing so much trouble in our country?


Dr. Hoelmer: That's right, as soon as the stink bug became obvious as a pest in Eastern Pennsylvania a few years ago and it was identified as an Asian invader, we started to do research on it and we recognized that it had a potential of developing into an important economic pest of various crops in the U.S. based on what we knew it fed on in Asia. At that point we initiated some background research to find out what sort of natural enemies attack it in Asia and what kind of impact they have. At the same time we began to look at native natural enemies here in North America to see whether they would step in and do the job.


Mike McGrath: We have native stink bugs, right? We've always had stink bugs.


Dr. Hoelmer: That's right, quite a few different species.


Mike McGrath: Is it the Marmorated stink bug?


Dr. Hoelmer: A Brown Marmorated Stink bug, yes. A fancy word, basically meaning "marbled".


Mike McGrath: Our stink bugs are no pleasure and for the first couple of years this bug was around, because as you and I discussed earlier, I live in ground zero, I live in a county in Pennsylvania where this bug was first discovered, mostly it was an annoyance coming into people's houses to hibernate in the winter time. And I have to give you guys all the credit and all the chops in the world because the very first articles I saw about it said that this was a hideous fruit pest crop in I believe Korea and that was the potential for danger over here but we didn't see it and then last year, last summer, it reaped havoc in northeastern gardens.


Dr. Hoelmer: That's right, it's not uncommon for invasive insects to take a few years for their populations to build to the point where their true impact becomes evident and we think that's probably what happened with the Brown Marmorated. Clearly, it had been present here for a number of years and populations very gradually grew. In recent years, they've been growing faster and faster because there are more and more of them.


Mike McGrath: But you weren't caught napping, you were looking for its enemy when it showed up.


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Dr. Hoelmer: Yes, we began to think that a classical biological control program might be what we needed. We first needed to determine if there was going to be a real need for it before we began to put our time and energy into it.


Good Bugs to the Rescue


Mike McGrath: When we talk about beneficial insects and natural predators, everybody knows the lady bug. I don't think enough of them know that it's the larval form of the lady bug that really chows down on aphids and other soft-bodied pests. One of my favorites is the lace wing family, whose really similar looking larva are so notorious. Isn't one lace wing known as the aphid lion?


Dr. Hoelmer: That's right, the immature lace wings are very important predators.


Mike McGrath: Another of my favorites is the broad family of what we call mini wasps who parasitize caterpillars. Every tomato grower in America, I don't think any of us have ever seen the wasps, they're so small. But, we've all seen the after effects on a tomato horn worm when they have those white spines down their back.


Dr. Hoelmer: This is exactly the kind of natural enemy we are looking at as a predator or natural enemy of the Brown Marmorated Stink bug. We are looking at parasitic wasps that specialize in attacking the eggs of the stink bugs.


Mike McGrath: Now we want to make clear to our listeners, because you say wasp and people start freaking out because they've been stung by paper wasps, hornets, and mud dauber wasps, these things are almost impossibly tiny, right?


Dr. Hoelmer: Right, most people are well aware of the big wasps that sting, but most wasps actually are these parasitic types of wasps. Nearly every insect has a natural enemy that is a parasitic wasp of some type and usually they are so small that we do not even see them. Almost every insect in our environment here has parasitic wasps that attack it. They are around us in the environment everywhere, all the time, and most people don't even know it. So, we are not talking about something that is unusual or rare.


Mike McGrath: No, and literally the line on these things, which I believe is true, is they are typically the size of the period at the end of a sentence in a newspaper story.


Dr. Hoelmer: Many of them are extremely small and with the wasps that we're looking at, which attack the stink bug eggs, a stink bug egg is very tiny, maybe a twenty-fifth of an inch or so in size, and the wasps are just as tiny; very very small and if you weren't looking for them deliberately, you would not even notice that they were around.


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Mike McGrath: I have to admit, I was not surprised that you guys found a beneficial that can help reduce these stink bug numbers. I was actually counting on you because I know this is what the USDA beneficial insect people just do so well, and have been doing it for decades. But I was surprised at the type of insect that you came up with. Did you know that you were going to find something like this or did you expect it to be a different type of beneficial?


Dr. Hoelmer: Well, there has been years and years of research on different stink bug species and we already know from previous scientific work that most stink bugs have parasitic wasps that attack their eggs. We already had a pretty good idea that this would be a type of natural enemy that would likely have an impact on the Brown Marmorated Stink bug in Asia, so this was one of the things that we looked for when we did our foreign exploration in Asia. We were looking for egg masses of the Brown Marmorated Stink bug on the plants that they like to feed on and looking for signs that their egg masses were attacked by these parasitic wasps.


Mike McGrath: Yes, for instance, I've noted in some scientific articles that have talked about the lady bug and lace wing larva, an expert like you can look at a cluster of eggs and tell which ones have been attacked.


Dr. Hoelmer: It's pretty easy within a day or two of the attack the wasp. The female wasp wanders around the leaves looking for the stink bug eggs, and when they find a mass, the female will lay her own egg inside the stink bug egg. Within a space of a few hours, that egg hatches and a small wasp begins to grow. As that wasp offspring gets larger and larger, it consumes the stink bug egg from inside. In the end, it will produce another wasp instead of a healthy young stink bug.


Mike McGrath: People who had these things all over their tomatoes last summer are now standing up and cheering at their radios, Dr. Hoelmer.


Dr. Hoelmer: A lot of people are eager to find some justice applied toward the stink bug.


Wasp Diversity


Mike McGrath: In the genus of these wasps that are parasitic, how many different kinds are there?


Dr. Hoelmer: A lot of insect experts feel that parasitic wasps may be one of the most abundant kinds of insects. We know already that there are probably several million different species, at least - and possibly, many millions of species. They are extremely abundant.


Mike McGrath: I know that the most abundant insects on the planet are ants and termites, but they don't have that kind of diversity. That's an astonishing number of species.


Dr. Hoelmer: When you think that most different kinds of insects have natural enemies of their own that include these types of parasitic wasps, that's a tremendous diversity of insect types. They are very specific in many cases so that each type of insect will have its own natural enemies that will not attack other types of natural enemies. That contributes the amazing diversity in parasitic wasps that we find in nature.


Mike McGrath: Right, because in some other insect forms, it would be one wasp who parasitized millions of other species.


Dr. Hoelmer: That's right, and that's not the kind of natural enemy we want to introduce. We want to find natural enemies that will be very specific to the eggs of Brown Marmorated Stink bugs.


Mike McGrath: You're in Newark, Delaware, that's where the ARS Beneficial Insect Lab is, and I'm in Pennsylvania. In our Mid-Atlantic region, how many mini wasp species would you say that we have in our farms and gardens?


Dr. Hoelmer: The number is in the hundreds of thousands, I'm sure.


Mike McGrath: That's amazing. I know about the trichogramma, I know about the braconid wasps, but I'm not even scratching the surface, am I?


Dr. Hoelmer: No, and again, for any insect you can imagine, you can imagine a parasitic wasp that will attack that specific insect.


Mike McGrath: So these beneficials, by definition and by the way they live, they're entire culture, makes them one of the safest insects to release.


Dr. Hoelmer: That's right, one of the reasons that we are so focused on parasitic wasps as biological control agents is that they typically are focused on finding and developing in very specific groups of insects. The wasps that we're working with for the Brown Marmorated Stink bug we know will only attack stink bug eggs. This is in contrast with a predator, which will feed on, in many cases, anything it can catch.


Predator and Prey Connection


Mike McGrath: We have a lot of native stink bugs. We have a lot of native parasitic wasps. Has any action been going on during this time?


Dr. Hoelmer: One of the things that we've been studying the first years that the stink bug began to spread through Eastern Pennsylvania and into the other Mid-Atlantic states was to try to find out if the parasitic wasps and predators of our native stink bugs were learning to feed on the Brown Marmorated Stink bug because if they were, there would be no need to go to Asia to find natural enemies in Asia and go through the research to determine whether they were safe to introduce. We could just rely on the native natural enemies of other stink bugs, but our studies have shown that this is not happening and the native stink bug natural enemies don't seem to have a taste for the Brown Marmorated Stink bug. They are not switching over to it.


Mike McGrath: I cannot think of a single instance, and I am a nut for insects, of this specificity between predator and prey.


Dr. Hoelmer: Again, a lot of the parasitic wasps have a very high degree of specificity and some are more specific than others. The ones that we want to consider introducing we want to be as specific as possible.


Mike McGrath: So right now, unfortunately, there is little to no benefit to doing the natural things in your garden that bring in a wide variety of these beneficial wasps.


Dr. Hoelmer: That's right, our native beneficial wasps just don't have a taste for the Brown Marmorated Stink bug. Every now and then they may sting an egg or two, but they're just not switching over to it. There are native predators that will feed on the stink bugs and they will have some impact, but it's not enough to keep the stink bug numbers down to levels that we're happy with. Ants, for example, ear wigs - we've seen these feeding on stink bug eggs. Other large predators like Assassin bugs and predatory stink bugs will also feed on adult and nymphal brown marmorated stink bugs. There are a variety of natural enemies that do feed on them from time to time but they're not doing the job.


Mike McGrath: And this Asian mini wasp is the terminator, it's the killer.


Dr. Hoelmer: We think it's the best chance to having an effective biological control agent for the Brown Marmorated.


Mike McGrath: In the articles I saw about your work, I saw control of up to 80%?


Dr. Hoelmer: Based on what we see in Asia, the egg parasites are capable of taking out very high proportions of the stink bug eggs in Asia.


Mike McGrath: As you and I know, there are insectaries all around the United States that breed beneficial insects for release to organic farmers and gardeners. They put them in the fields to control these creatures. I would imagine that your end goal is to be able to breed enough of these wasps to release them commercially, or at least experimentally. How far away are we from you being able to put together a critical mass of these beneficial insects?


Dr. Hoelmer: Actually, in this case, because we are dealing with a pest which is widely distributed in the environment, our goal is that when we're ready to release one of these Asian wasps, we hope that it will establish in the environment itself permanently and form large populations that follow the stink bug populations in nature so that there will be no further need for us to artificially manipulate their populations.


Mike McGrath: Because the more stink bugs we have, the more stink bug eggs, the more food there is for these wasps.


Dr. Hoelmer: We're hoping that these wasps will do exactly what they do in Asia - search for the stink bug eggs by themselves, wherever they can find them, and keep their numbers low.


Mike McGrath: All of these pest preachers will never be eradicated but in most cases where there is a balance, there is a predator to keep their numbers at a tolerable level.


Dr. Hoelmer: That's what we're looking for - to reestablish a reasonable balance between stink bug numbers in nature and what we would like to see.


Mike McGrath: I know the typical procedure for this point is being careful not to let any of these wasps out and into the environment. When do you think you'll begin any kind of release in the outdoors?


Dr. Hoelmer: The kind of research that we need to do in order to demonstrate that these wasps are safe enough and specific enough to release typically takes several years to conduct under quarantine laboratory conditions. All of our studies must be done in a containment laboratory until we receive permission to release an exotic species. So we've already begun this work, we're about a year into it and we believe that if we can find an agent that is safe enough to release this could happen within perhaps two years of laboratory work.


Mike McGrath: You have people on the edge of their seats, Doc.


Dr. Hoelmer: I know everybody would like to see it sooner.


Mike McGrath: They're dying to let Rocky into the ring to get some revenge here. So in the meantime, there is always hope that some native wasp will adapt and pick up a taste for these creatures. One of the things that I love about your work and the work of beneficial insects in general is it's another reason not to use pesticides in the garden. My line, and you're welcome to agree or disagree, is that today's modern pesticides are much more effective at killing beneficial insects than they are pest insects.


Dr. Hoelmer: Well of course most pesticides are designed to be used against pests but when they're used in an open environment where natural enemies also play a role, they often do have a major impact on beneficials and certainly we want to do what we can to minimize that.


Mike McGrath: These wasps have been a boon to everybody who grows crops, as you know, that are vulnerable to caterpillar attacks. I am so thrilled that we have found something that attacks these stink bugs. I'm sorry we have to wait a couple more years but we are looking forward to this research being released into the environment and give these pests a little something to take home with them.


Dr. Hoelmer: We're very hopeful, as well and we're looking forward to the day when we can release these and put them to work.


Mike McGrath: Alright, well we're going to follow your research very carefully and we'll be happy to tell our listeners when the bugs are in the air. Dr. Kim Hoelmer from the Beneficial Insect Lab at the ARS USDA Research Center in Newark, Delaware, working on a way to keep the stink bugs under control. Dr. Hoelmer, thank you for being on the show and thank you for your good work, sir.


Dr. Hoelmer: Thank you very much, Mike, my pleasure.


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