Pardon Me, Is That a Parasitoid on Your Pupa?


It’s pollen season, and that means everything outside has a lemon-lime colored coating and you may be sneezing more than usual. It also means butterflies and other pollinators are busy doing their thing.



On Earth Day at the Science Mill we will be talking about butterflies and the role they play in ecological health. We recently talked with a friend of the Science Mill, Dr. Carl Stenoien, who is a post-doctoral associate with the Department of Entomology at the University of Minnesota, where they have a Monarch Lab dedicated to better understanding the biology and natural history of monarchs. 

Dr. Stenoien’s specific focus is not on monarchs themselves, but on parasitoid wasps that attack monarch butterflies. You’re not alone if you’re thinking, “Para-what now?” 

Parasitoids, like parasites, need a host to survive. Unlike parasites that invade a host without killing it, parasitoids are lethal - and plentiful.

“There are way more species of parasitoid wasps than all of the mammals, fishes, birds, reptiles and amphibians combined,” said Dr. Stenoien.

During his graduate work, Dr. Stenoien studied a parasitoid called Pteromalus cassotis. Not a lot is known about these tiny wasps, but scientists like Dr. Stenoien are starting to discover just why parasitoids are so important on an ecological level.


This species seems to be a specialist on milkweed butterflies, like monarchs, queens and soldiers, and is known to attack monarchs across much of the US, including Texas
Dr. Stenoien

Scientists studying the monarch population consider parasitoids just one piece of the puzzle.

We don’t know how much it contributes to population declines, but it is probably a smaller factor than any of the following: breeding habitat loss, overwintering habitat loss, climate change, and insecticide use
Dr. Stenoien

He went on to explain that we might look at butterflies and moths very differently if it weren’t for parasitoids and other predators.

“Pretty much every species of insect is attacked by at least one or a few species of parasitic flies or wasps,” said Dr. Stenoien. “Butterflies and moths are wonderful, beautiful, and fascinating animals, but part of what makes them special to us is that most are not overpopulated pests. This is partially due to the control exerted by their natural enemies, including parasitoids.”

Visit Dr. Stenoien’s YouTube channel for videos of parasitoid wasps, or watch this webinar for a more detailed discussion.


Get to know Dr. Stenoien a little better! Read profiles from his graduate student days here and here. Dr. Stenoien is now a post doctoral researcher, and is still working on parasitoids, though not currently with butterflies. Instead, he’s working on a couple of species that may attack an agricultural pest, the soybean aphid.


Looking to learn more about monarch butterflies or start a classroom garden? Visit the University of Minnesota’s Monarch Lab for K-12 curriculum ideas.