Post written by Joe Ballenger
This post is going to be odd, because I’m going to start with a confession:
This question initially stumped me, and it actually took me a couple weeks to fully figure out what was going on here.
It stumped me for one main reason, and that’s because the answer turned out to be an insect identification request where the key characters had little to do with the pictures I was sent. The clues which allowed me to identify the insect, and explain the picture below, were actually given to me during the correspondence and I initially overlooked them.
So here’s the original message we received from a reader named Katharine:
Your Name: Katharine
Your Bug Question: So, I planted a monarch garden a while ago, and I’ve been lucky enough to get at least two monarch chrysalises. The thing is, they’re often wet and dripping, even when the grass around them is dry. What’s going on here?
She also sent us a picture of the chrysalises. One of them looked normal, but the other one was dripping:
The one which looked normal just so happened to have a small wasp hanging out on the side.
She later told us that the Chrysalis produced several small wasps, and managed to photograph two of them:
So what’s going on here? Why is this chrysalis perpetually dripping wet?
What are those wasps?
The first thing we need to do is to ID the wasp. Wasp ID can be particularly difficult, because they’re small and they have a lot of really complicated features. However, since we know the host, we can narrow the possibilities down by quite a bit.
Although there haven’t been a whole lot of studies on monarch parasitoids, there are three families of wasps which have been documented to parasitize monarchs:
Braconidae I admittedly couldn’t find a whole lot of information on which exact species parasitizes monarchs. The only reference I’ve seen given is a host-parasite catalog of Tachinid flies from 1978. I did a search in the document (which is massive-about 1,000 pages) for the scientific name*, Danaus plexxipus, and each time the name is mentioned in reference to a fly, which makes sense given that it’s a paper on flies. So I’m not sure this is accurate information because I couldn’t find a Braconid record in this paper even though it’s the one everybody cites. There are forums where people claim to have reared Braconids from monarchs, but I’ve been unable to find pictures which would help me confirm this and narrow down an ID.
Chalcididae The one which seems to be mentioned the most in reference to being reared out of monarchs is Brachymeria ovata, which is a black and yellow Chalcidid which is a generalist parasite of Nymphalid butterflies. They lay eggs in the larvae, and the adult emerges from the pupa. Although some Chalcidids produce many offspring from a single host, the ones which attack caterpillars only produce one individual from a host.
Pteromalidae The Pteromalid most commonly reared out of monarchs is Pteromalus cassotis, although this is usually misidentified as Pteromalus puparum. P. puparum will happily lay eggs in the Monarch, but its larvae die before emerging. These are small, metallic, black wasps which can produce dozens of offspring from a single host.
When looking at pictures of wasps, I don’t assume that any individual wasp is laying eggs on a host at that particular time unless I can see the ovipositor. It’s always possible that you can snap a picture of a bug taking a breather, and end up drawing the wrong conclusion about what’s going on. Plus there are wasps which accidentally lay eggs in the wrong host, and this is surprisingly common. There’s a lot of mistakes like this in the scientific literature, and I would be happy to go through a specific example if anyone’s interested.
So I tried IDing the insects from that second picture, but the problem is that the main physical difference between the two types of wasp I mentioned earlier are the legs. Take a look:
The Chalcidid, on the left has these really big, muscular thighs which the Pteromalid on the right doesn’t have. The picture Katharine sent us above doesn’t show the legs of the wasps in detail, so I initially thought it wasn’t possible to tell the difference between the two.
However…sometimes the ID isn’t all about the picture. In fact, the thing that tells us what these wasps are isn’t in the picture at all!
What did I initially overlook?
The number of parasites. Brachymeria is a solitary parasitoid, which means that they only produce one wasp per host. Pteromalus is gregarious, which means that they produce a lot of wasps per host.
This chrysalis produced a lot of wasps, so Pteromalus cassotis is the most likely culprit.
So why is this chrysalis always dripping?
This is where things get really cool.
When a wasp lays it’s egg in a host, it sometimes eats the blood that wells up from the wound. Insect blood is rich in a whole lot of nutrients, so it provides the resources the wasp needs to make more eggs. Although some wasps become adults with all the eggs they’ll ever lay, a lot of wasps make them on the fly. These guys need to eat to make eggs.
Pteromalids are incredibly violent host-feeders. A lot of them will hang out on the host for a long time, continually injecting venom, laying eggs, and eating the resulting blood which flows out of the wound. Some species will stay on their host for up to a week, although Pteromalus only stays on it’s host for a day or two. Some species will even use their stingers to rip out large chunks of bug flesh, and eat that to procure whatever resources they need. This feeding by the mother is what actually kills the host, and the larvae actually develop on the remains that the mother killed for them.
Many species are highly specialized for this lifestyle. Some Pteromalids parasitize hosts which hide inside things, and these species have specialized glands which produce a gluelike substance that makes a straw to connect the host to the outside. They drink the blood through the straw like we’d drink a milkshake. The illustration from Clausen, 1940 shows a wasp doing exactly this.
The Bottom Line
Even if you have a picture, insect ID can be really hard. However, if you know a little bit about the bug, you might not need one in some cases. These cases are exceptionally rare, but they do happen. This is one of them.
Although Katharine wanted to rear Monarchs she ended up getting wasps, which is a really common thing that happens when you’re dealing with a natural environment. In this case, she probably managed to get Pteromalus cassotis, a common parasitoid of monarchs. The dripping she saw was the remnants of the mother feeding on the host.
I think this also teaches us a really neat lesson about conservation. There are a lot of efforts underway to conserve monarchs, and monarch butterflies aren’t the only insects that benefit from these efforts. There are other animals, like wasps, which need the monarch to survive. Those conservation efforts will also help those bugs survive.
Clausen, Curtis Paul. “Entomophagous insects.” Entomophagous insects (1940).
*I actually searched for the abbreviation, D. plexippus. I also searched for the specific epithet, plexippus. The scientific name is frequently abbreviated in these sorts of documents, so searching for the abbreviation and epithet makes more sense than searching for both the genus and species.
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