Hi folks,
I have a super-cool study to tell you about today, that comes from the lab of yours-truly. It was just published in the journal, Insects, which is fully online and readable, linked here. Feel free to check out the paper itself, or otherwise just read about it here. Full disclosure - I'll be talking about spiders eating butterflies in this post, which is something that happens in nature all the time - if this turns you off, then you should probably stop reading.
This is a bit of a worlds-colliding situation for me, since for the first time on this blog I get to talk about my latest "research side-gig", which is my other work on the infamous "jorō spider", Trichonephila clavata, which is a non-native spider from East Asia that has become established here in the southeast. That's right, I don't just study monarch butterflies in my lab (though I have studied them the longest). Anyway, for the past few years I and my students have been studying this new critter, ever since they basically arrived in my backyard. Each fall, my backyard in northern Georgia has hundreds of these things! For folks in other states who have not had the pleasure of seeing these things yet, hold on to your butts - they are heading your way. In fact, prior research from my lab has already shown how these spiders have a physiology that is well-suited for living across most of the U.S., and even southern Canada. You can easily google this, and in fact, that research project went viral a couple years ago, and it was featured on just about every major news outlet around the world. Since then, I've done more work examining the behavior and physiology of these spiders, including how they can exist so well around humans!
OK that's the backstory on these new spiders in town, so let's get to this new study. The title of the paper sort of tells you the gist, or the main question: "Do invasive jorō spiders (Trichonephila clavata) from Asia avoid eating unpalatable monarch butterflies (Danaus plexippus) in North America?" Being orb-weaving spiders, they make a living catching flying insects and eating them. However, most folks may not realize that spiders can choose not to eat certain insect that fly into their webs, and some spiders will refuse to eat anything that tastes bad - like monarchs for example! As we all know, monarch butterflies grow up eating toxic milkweeds, which makes them unpalatable to birds. When birds get their first taste of a monarch, they either spit it out or throw up later, and then they learn not to eat a monarch again. Here is the interesting bit for these spiders - since they are new to this continent, they've never seen a monarch before. So, we wondered if they would be able to recognize if monarchs were toxic at all?
To answer this question, I had a small team of students, who conducted some "field trials" where they observed what happens when jorō spiders "encounter" monarchs in their web. In other words, they tossed monarchs into the spider webs, and recorded what the spiders did!
Let me pause for a second and point something out here, because I know there will be some shock over this part. Remember, spiders are not the enemy in the world of mother nature - they eat bugs for a living, including butterflies. People need to get over this part. And just to clarify, these were captive-reared monarchs that had been used for other research purposes, and were no longer needed for research. And before you say it (why didn't you just let them go free?), the research permit does not allow that. I know, it's weird, but it's a government thing. So in reality, these monarchs were slated for euthanasia anyway.
OK, so here is what happened - the majority of jorō spiders (80%) did nothing! They did not attack the trapped monarch, or try to wrap it up, or anything like that. Most of the spiders basically took one look at the monarch and waited for it to free itself (50% of the monarchs did free themselves). A couple of spiders even helped the monarchs do that, by cutting them free! Crazy. To be fair, there were a small number of spiders that did attack and wrap up the monarchs, though we rarely did see the actual eating part. But, by and large, there was a clear pattern of "avoidance" here.
I'm pasting below a youtube video we made during the project that demonstrates this "field trial" bit. Note how the spider does nothing when the monarch became trapped in her web. Notice how long my students watched and waited too - at least 2 minutes in most cases, but in some, my poor students sat around and waited for hours to see if the spider changed its mind or if the monarch eventually escaped.
Here's the other neat part - for comparison, we also tested the spiders' reactions to two other locally-common species, tiger swallowtails and gulf fritillaries. Tiger swallowtails are actually bigger than monarchs, but in our trials, there was no avoidance at all (60% attack rate). The spiders didn't really have a problem with the large butterflies. Nor was there any avoidance of gulf fritillaries (80% attack rate), which are also an aposematic (orange) species. This species is not really toxic, but they do employ a form of chemical defense when attacked. The jorō spiders didn't seem to care. Below is a collage of images that show the appearances of the butterflies in question, and, some shots of the jorō spiders eating them! The picture of the jorō spider eating the monarch was the only one I got, since it was such a rare thing.
Here is another surprise from the study - for comparison, we also tested if a native spider would also avoid monarchs. We used the locally-common species, Argiope aurantia (the common garden spider), and we performed similar "exposure" trials. Our sample size here was not large, but we noted there was NO avoidance by these spiders - the garden spiders attacked the monarchs about 80% of the time. And for confirmation of this, we also surveyed the internet for images of "spiders eating butterflies", and we came up with lots of cases of native North American spiders eating monarchs, including many garden spider pictures. So in other words, the garden spiders were ok with eating monarchs. Below is a picture from our study.
So it turns out that our native North American spiders will gladly eat monarchs, but yet the invasive spider from east Asia will not? Hmm...This is certainly a head-scratcher. It's almost as if the native spiders have somehow learned "how" to eat the toxic monarchs, perhaps in the same way that some birds have - i.e. by only eating the body parts that don't have the cardenolides. Just guessing here.
One other cool thing we learned is that the jorō spiders seem to have the ability to detect the monarchs' distastefulness even without physically tasting them! In most of the cases in the field trials, the spiders took one look at the monarch and backed off. They didn't even touch them or get near them. This indicates that these spiders (perhaps all spiders?) have the ability to sense the cardenolides from a distance, almost as if it is a chemical pheromone being given off by the monarchs into the air around them. We do know that spiders can "taste" chemicals using specialized hairs in their feet, but these results suggest they don't even need to touch their prey.
So, what does this mean for the future spread of this invasive spider? Recall that this spider is expected to continue spreading throughout most of the eastern seaboard at least. Well, it looks like monarchs probably won't be greatly affected (whew!). In fact, despite what we found in the trials, it looks like even other butterflies won't be greatly affected either. I haven't mentioned yet that for another part of the project, we examined existing jorō spider webs for their prey items, and we determined that butterflies in general make up less than 5% of their natural diet. Furthermore, research by another lab has shown that the majority of their prey is flies and mosquitos. In fact, we have records of them eating spotted lanternflies, which is a really nasty invasive species in the northeast. Keep all of this in mind if you are all concerned about the spread of this new spider.
As you can see, we learned a lot about the biology of these jorō spiders with this project, and, we were left with many new questions that deserve further study. That is the way research is supposed to work, really, so this was a success all around.
Thanks for reading! I may do some more worlds-colliding research in the future!
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