New study shows monarch caterpillars don't like noise from predatory wasps
This is going to be a very cool post, about a very cool study that was just published, and I can't wait to tell you about it - this will be cool! Did I mention this was cool? I know, I get excited myself about studies like this because it is on a topic that is very similar to my own work on stress in animals and insects. In fact, after reading this paper, I find myself wondering if this is something that I could build upon here in my own lab... stay tuned!
As you can see from the title of this post, this study was all about monarch caterpillars, their hearing, and how they react when they hear a predator! The study was just published earlier this month in the scientific journal, Acta Oecologica (link here for the paper), and the authors were all researchers from the University of Rhode Island (Zachary A. Lee, Alex K. Baranowski, and Evan L. Preisser). The lab where they all come from has a research focus on how animals and populations deal with the threat of predation, so it looks like this project is along the same line. Here is a link to their lab website.
Let's start with the basics before getting into the study. It is well-established now that monarch caterpillars can hear sounds. A study from the lab of Jayne Yack a couple of years ago worked out exactly how they do this, and I blogged about that study when it came out (link). We also know that they seem to react very strongly when they hear loud noises. Readers here are probably even familiar with this behavior - they will twitch or bob their heads when startled, or when they hear loud voices. And, a few years back, I spearheaded a project that showed how monarch caterpillars become stressed when they hear loud highway noises (link to that blog)! Incidentally, Jayne Yack told me that she doesn't think that adult monarchs can hear, since they don't appear to have any known auditory sensory structures on their body. Weird, right?
So for this new study, the researchers knew that monarchs could hear sounds, and they also knew that monarchs have a number of predators that make noise, such as buzzing wasps. They wondered then, if the noise of wasps would impact the growth of monarch caterpillars. In other words, they wanted to know what happens when caterpillars hear lots of wasps buzzing around them - do they grow just as well? Imagine growing up right next to a whole bunch of hungry-sounding predators...
The researchers conducted a very nifty experiment to figure this out. They first obtained some monarch larvae from their local area to grow in their lab. They placed these larvae in plastic tubs with milkweed. They set up a series of speakers around the containers, and the speakers were connected to an ipod or laptop. The researchers kindly provided me with a photo of their setup, below. As you can see, the caterpillar containers are completely surrounded by speakers, which are all wired together.
Here's the cool part - they obtained a recording of wasps that are known predators of caterpillars, the paper wasp (Mischocyttarus sp.). A screenshot of one of these is below, taken from Brittanica.com. I'm sure that most gardeners and homeowners are familiar with these critters. They then played this buzzing recording through the speakers, on a loop, so that the buzzing could be heard every 6 seconds, from 10am to 10pm. They played this everyday during the entire larval development time of the caterpillars.
To the caterpillars in the containers, they would essentially be hearing the constant noise of hungry wasp predators, and thus, they would "think" they were in a threatening situation, all day, every day. To them, it would sound like maybe they were on a milkweed plant that was a few feet from a wasp nest!
I should point out that the researchers had a couple of control groups as well. In one, caterpillars were placed in similar containers, and exposed to the sounds of mosquitos through the speakers! Since mosquitos are not caterpillar predators, this should not have any effect, but, it is a sound, so this is a way to ensure that the caterpillars were not simply responding to any random sounds. We call this a sham control in the science business. They also had a set of caterpillars who simply were left alone and not exposed to any sounds. These were a true control group.
The researchers tracked how many caterpillars survived to pupation in each treatment, the length of time each caterpillar spent in the larval stage (i.e. from egg to pupation), and also, the weight of each pupae. These are all pretty straightforward metrics.
Here is what they found: Caterpillars that were exposed to the predator noise had a higher rate of mortality than those in the mosquito treatment, and the silent treatment. They also had a shortened development time, and their pupal weights were lighter. In other words, the predator treatment had a negative effect! High mortality is never a good thing, nor is low pupal weight. Below is a screenshot of the main figure from the paper, showing the differences between each treatment.
Biologically, the caterpillars exposed to the predator noise decided to "escape" from the threat by pupating sooner than they normally would, and try to metamorphose quickly, so they could "get outta dodge." There are actually a number of animals out there that will do this too, and I'm thinking specifically of frogs. Tadpoles will enter metamorphosis sooner if they sense a threat. In the paper, the researchers mentioned that mayflies, grasshoppers and spiders also do this. This is a risky strategy, because you end up becoming a small adult, and you're stuck like that for the rest of your life - small adults, whether frogs or butterflies, usually have fewer offspring, reduced life expectancy, and so on.
The fact that there was actually higher mortality in the predator treatment was fascinating. Remember, these caterpillars were being reared in containers, with as much food as needed, and with no "real" threats. Yet still, a number of larvae died. The researchers stated that this indicates that chronic predator stress can be fatal to monarchs, which is an apt way to say it. Interestingly, they never pointed out exactly how the caterpillars died, like whether they died during pupation, or what. But regardless, this finding means that wasps can have a significant impact on the caterpillars in your garden, even if they don't actually touch them - just the sound of the wasps can have an impact!
The other fascinating thing about these results is that they demonstrate that monarch caterpillars can "discern" different sounds. They did not react to the sounds of mosquitos (at least not as much). This means their tiny brains are hardwired to know which sounds are which out there in nature - they instinctively know which sounds are bad and which are not bad. That's amazing!
As you can tell, I thought this was a cool, cool study, and now my brain is buzzing (see what I did there?) with thoughts about how I could build on these findings in my own lab.
Before I go, let me finish here with some thoughts for all of the gardeners out there who are horrified after reading about this study! Keep in mind that wasps are not the enemy. I know, they technically are the enemy of caterpillars, but from a mother nature standpoint, wasps have a job to do too. Wasps actually keep some insect populations in check. I wrote about this a few years ago (link). And, wasps are important pollinators, much more so even than monarchs are. And, wasps can be food themselves for someone else in the web of life. So, please don't kill them because they are a threat to monarch caterpillars. Mother nature designed caterpillars to be eaten - that's why momma monarch lays hundreds of eggs. This is nature at work, and all of nature is beautiful.
That's all for now.
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