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  • Andy Davis

The wasps eating monarch caterpillars in my yard are also saving my trees!

Hello everyone,

Today's post is not about a new scientific paper on monarchs. I'm going to briefly share with you some anecdotal observations I've made this summer in my backyard. I thought this would be a good time to do this because of all of the hoopla lately over the recent PNAS study on the effects of indoor-rearing of monarchs.

Let me explain why this fits in here. The backlash generated by the recent paper revolved around the practice of bringing monarch caterpillars indoors to raise them. It seems that a lot of well-meaning people do this in order to "protect them from predators", or to "give them a fighting chance", or whatever you wish to call it. The fact that over 95% of monarch larvae never survive in the wild seems to be the main motivation behind this practice. I'd like to use this blog entry today to set the record straight on this. Not that the 95% thing is wrong, it's totally correct. But what most folks don't realize is that this is the way mother nature designed these creatures. In other words, those 95% were never supposed to survive. Monarchs are at the bottom of the food chain (or close to it), which means their eggs and caterpillars are designed to be nature's tasty snacks! And this is totally ok - that's why mother nature designed the female monarch to be able to lay over 500 eggs - because she only needs ONE to survive to adulthood in order to replace herself in the population. ONE...OUT...OF...500.

So, consider this post as a bit of a lesson on why caterpillar predation is not a bad thing.

Before going further, readers should know that there are graphic images (video) ahead. So if you are turned off by the statements above, you definitely should not read further, because you'll definitely be turned off by what's ahead.

So let me set the stage for you. In my backyard I have lots of milkweed plants, as you might expect. And each spring as the monarchs pass through the yard on their journey north, some of them leave some eggs. Last year there were lots and lots of eggs on the plants, since it was such a humungous year for monarchs. I recall that there were a fair number of caterpillars too. This year, I've noticed what seems to be a lot of larval predation. Despite seeing lots of eggs, I've only seen a few caterpillars (that's a sure sign of increased predation). So I strongly suspected something was eating the caterpillars this year, but I wasn't sure what it was...

One day I happened upon a scene on one of the milkweeds that was absolutely fascinating. It was a paper wasp that was in the middle of eating a 5th instar monarch caterpillar! Aha! So, as any good scientist would do, I of course filmed this and put it on youtube! See it for yourself here:

If you look close at the video you can see the wasp is neck-deep into the guts of this caterpillar! I'm not sure what tissue the wasp was going for, but it was somewhere in the middle of the larva! By the way, the black goo is the caterpillar blood - it's green when it first comes out, but turns black after about 20 minutes. The fact that there's blood tells me that the wasp actually killed the larva on the spot, as opposed to the wasp eating an already-dead larva.

From a biological perspective, this is absolutely fascinating, and disgusting, all at the same time! It's kind of like the weird desire to gawk at a crash scene on the highway, even though we know something bad happened. But, disgusting as it is, keep in mind that this, this right here, is nature, in all it's harshness. This is a learning moment. This is a monarch caterpillar fulfilling it's biological purpose in life - to be food for something. Remember, only the lucky few ever get to become butterflies. The rest are designed to be eaten.

I'll admit that I don't know very much about these wasps, other than the fact that they are predators of caterpillars. If you look this up you'll learn that most of the time the wasps actually pick up the caterpillars and carry them off. I guess this larva was too big for that wasp so he just decided to chow down right there. I've also read that they sometimes will take hunks of caterpillar meat back to their nest.

That same week I actually saw a couple more incidents just like this. And when I checked all of our milkweeds, I'd see lots of indirect evidence of predation too (chewed milkweed leaves, but no sign of a caterpillar). So it looks like the wasps are having a great year this year - it's their turn!

Now, let's get to the other side of this story. Also in the yard are a variety of nice deciduous trees, including some maples. They're probably about 10 years old, and maybe 30-40 feet tall. They provide lots of shade for us who have to suffer the Georgia summers!

Each summer, these poor maple trees become food for another Lepidopteran, the rosy maple moth. This is an absolutely beautiful creature, as you can see from the picture below.

These moths are aptly named - they are rosy, and their host plant is maple trees! Last year, we saw thousands and thousands of these larvae (pictured below) on the maple trees. Apparently, last year was a good year for this Lep too.

If you look close at this picture, you can see what these larvae do to the tree leaves. They basically strip down the leaves until the leaves are nothing but stems. And last summer, these trees had been stripped of about 50% of their leaves! I thought I was going to lose them at one point. I remember once bumping the trunk of one tree with the mower, and all these caterpillars rained down on me!

But the trees came back this year, and so did the maple moth larvae, at least early in the summer. But now, things are different. The trees never did become loaded with larvae, and I'm almost positive it's because of the high abundance of wasps. There's just a few pockets of leaves here and there with larvae now. And, I can stand under these trees and see paper wasps buzzing around the leaves, like they're searching. And as final proof, I've seen many instances where there is clear caterpillar damage on a patch of leaves, but no caterpillars to be found.

So, as the blog title stated, the same wasps that eat the monarch caterpillars on our milkweed are saving our maple trees.

Here's a neat way to end this post. As I was writing this over the weekend, I happened to witness some more predation at our house - this time on some paper wasps! I had noticed some barn swallows were swooping down really low to one of our house gutters, which seemed odd. I went out and checked and found a big paper wasp nest under that gutter! (sorry, the shot is blurry because I didn't want to get too close!)

I realized that the swallows were swooping in and swallowing (ha!) these wasps as they flew off their nest! So I guess that's the way it goes in nature - eat or be eaten, it's a bug-eat-bug world, or whatever cliché you want.

And remember, it's not our responsibility to change the way nature works.



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The science of monarch butterflies

A blog about monarchs, written by a monarch scientist, for people who love monarchs

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