“Have you seen the newest monarch chrysalis?” My colleague Mollie asked, poking her head through the door to my office. My eyes lit up. While I counted 32 monarch butterfly caterpillars outside the Museum’s entrance doors earlier this summer, we only discovered three pupae, the next stage in their life cycle.
Once the monarch caterpillars have grown, they move several meters away from the milkweed plants where they had gorged themselves and seek a good hiding place. Choosing a safe place to pupate is essential, and it’s a skill they’ve developed over millennia.
Caterpillars are motionless, soft, and vulnerable for about 24 hours when they attach their rear end to a leaf or other surface with a small silk cushion, then curl into a J-shape and lose their exoskeleton to reveal the pupa. Once the chrysalis has had time to harden, it is a bit more durable for the remainder of the 8 to 15 days it takes for an orange and black butterfly to develop and emerge.
Mollie led me to the front door of the museum, then immediately turned to her right. There, under a stone ledge on the foundation of the building, hung a delicate water-green chrysalis. Even though I’ve admired them all summer, I can’t get enough of the beauty and magic encapsulated in these Lilliputian wrappers. I leaned over to admire the glittering golden dots on the tiny crests of the chrysalis, and in doing so, something small and black appeared around the chrysalis shoulder.
The body had three segments, six legs and two antennae. Was it an ant? No, there were tiny translucent wings. This creature gave me a bad feeling, and with the help of a pen, I tried to chase the young from the chrysalis. It hasn’t budged. Then the docent at the front desk pulled our attention to a question, and Mollie and I went back inside.
The little visitor did laugh at my conscience, however, so I did some research on Google. As I feared, the creature looked like a tiny wasp that was recently revealed to be a parasite of monarch pupae.
Pteromalus cassotis is one of the many types of chalcid wasps that each specialize in the parasite with their own insect flavor. Entomologists hypothesize that virtually all insect species, as well as many plants, are parasitized by one or a few wasp species specific to their host. Logically, it follows that there are probably more species of wasps on Earth than any other animal. Most of them have not yet been described by scientists.
And in fact, it would be nice if they just parasitized the bugs. Parasites steal energy from their host, but do not always kill them. The insects that make plant galls are pests. They injure the plant, but not to death. In contrast, P. casspotis and other chalcid wasps are parasitoids, meaning they have a catastrophically comfortable relationship with their host which results in the death of the host.
With this sad knowledge, I took a closer look at the monarch chrysalis a week later. Strange dark spots appearing through the translucent green casing confirmed that it was not developing normally. I was still curious. Carl Stenoien, a PhD student at the University of Minnesota, only confirmed the wasp’s relationship with monarchs in 2015, while working with Karen Oberhauser at UMN’s Monarch Lab. I wanted to see this phenomenon for myself.
So I carefully scraped the silk button off the concrete and glued it to a small stick. Next, I placed the stick and the hanging chrysalis in a glass jar with a cloth in place of the lid, and I placed it in a back corner of my desk.
About a week later, I looked up my emails and noticed movement in the jar. Hundreds of tiny wasps had appeared. According to Stenoien’s research, anywhere from 1 to 425 wasps could emerge from a single chrysalis, all raised on energy stolen from the monarch chrysalis. Most of them are larger black females, and as few as 9% are smaller bronze colored males. I could see the variation in color and size of the wasps crawling around my pot. With their big red eyes, sleek teardrop-shaped abs (no sharp sting, just an ovipositor used for spawning), and tanned legs, these little ladies were almost pretty.
While the population of monarch butterflies has declined by an alarming 80% rate over the past decade, natural enemies like these tiny wasps are unlikely to be a major part of their demise. Human-caused issues, such as habitat loss, the use of pesticides and herbicides, and climate change are much bigger issues.
Still, it would be easy to portray these little parasitoid wasps as villains, as they attack a beautiful, charismatic butterfly – but nature isn’t that simple. These countless species of parasitoid wasps do us a favor by killing insects that cause crop damage. They also help maintain the balance of the wilderness. There is a world with too many caterpillars – at least in the opinion of the plants that those caterpillars eat, and other herbivores that might need those plants as well, and animals that need to eat those herbivores, etc. at the top of the food chain.
Phew! It was quite a journey. Just going out to look at something pretty, I came to remember that even the smallest creatures have an important role to play on Earth.
I am much bigger than those wasps. What does this say about the impact I could have?
For over 50 years, the Cable Natural History Museum has served as your connection to the Northwoods. The museum is now open with our exciting exhibition The Mysteries of the Night. Connect with us on Facebook, Instagram, YouTube, and cablemuseum.org to see what we’re up to.