A Wasp Whodunnit

Something crept in…

and compromised the wasp nest. My last post showed adult paper wasps (Polistes metricus) evicting some of their young. The workers were clearing the nest of sick and poorly-developing larvae and pupae. A deadly agent had entered the nest. What was it? I had filmed eighty hours of surveillance video. Maybe I could figure that out.

My first clue was a scene that screamed foul play.  

Larvae, and pieces of larvae, littered the door track below the nest. The dark, shriveled portions of otherwise plump little bodies seemed familiar. I’d seen similar damage in an article about the sooty-winged chalcoela moth. Its larvae feed on developing wasps.

A-ha! One of my videos had captured a moth lingering near the nest. Was she the perpetrator?

I noticed the moth only when reviewing that day’s video, so I couldn’t go back and zoom in on the suspect. I had no mug shot to confirm her ID, and the circumstantial evidence was weak. Sooty-winged moths lay their eggs at night. Yet I’d filmed my suspect during the day. Was it possible I’d caught a nocturnal moth executing a daytime reconnaissance mission? That seemed improbable.

The answer came in November when I took down the nest and searched for direct evidence connecting the moth to the crime.  I’d seen photos of empty moth cocoons wedged inside wasp pupal cells. There were none in this nest. I had to rule out the sooty-winged chalcoela moth.

In pursuit of additional clues, I turned to the other victims of whatever evil had lurked in the nest. A dozen cells held dead and damaged late-stage pupae.

Here’s a male, seemingly frozen as he tried to emerge from his cell.

His eyes were missing… which made me think of a zombie.

Wait!

I remembered reading an article about Apocephalus borealis. This fly is a parasitoid, injecting its eggs inside bees and wasps. The victims become disoriented, often abandoning their nests at night and wandering cluelessly about. The fly even has an apt alias, a common name that reflects its victims’ behavior.  Apocephalus borealis is also known as The Zombie Fly.

I’d filmed a horsefly poking in and out of the wasp nest’s empty cells. Could that have been my perp?

Nah. Five minutes of further reading told me that zombie flies are small like fruit flies, and they lay eggs only in adult victims. Zombie flies could not have been responsible for the wasp nest’s malformed larvae and damaged pupae.

But did that mean the horsefly was off the hook? Do these supersized flies ever parasitize paper wasps?

Not that I found. Indeed, horseflies have more to fear from wasps than vice versa. Horseflies are the favored prey of Stictia Carolina. Commonly known as horse guard wasps, they hover around horses’ rear ends, where they capture adult horseflies and feed them to their own larvae.

That means the horsefly, like the moth, was an innocent bystander.

What did cause the nest’s larval and pupal malformations? Something must have compromised the nest, prompting some larvae to be removed and some near-adult pupae to die in their cells.

My research continues. A kind bug expert is now asking around on my behalf, so in time, I hope to have an answer.

My wasp-watching experience reminds me that in Nature, one insect’s loss often becomes another insect’s win. As the queen was losing some of her offspring, winners arrived on the scene.

When I removed the nest, another winner surfaced from a cell:

Is the queen wasp a loser? She lost some of her colony to the mystery agent. By November, she’d abandoned the nest, destined to die with most of her workers when temperatures plummet. But through a few special daughters, the queen will yet score a win.  Called gynes, these daughters will survive our Michigan winter, sheltering in bark crevices or elsewhere. Next spring, they’ll become foundresses, initiating their own nests.

Resources and a Related Children’s Book
You’ll find lots of posts about wasps on Eric R. Eaton’s blog, BugEric.  He’s the author of a book I recommended in an earlier post: Wasps-The Astonishing Diversity of a Misunderstood Insect. Like the book, Eric’s posts are reader-friendly for non-entomologists like me. Here’s a link to Eric’s current collection of wasp-related posts, including one where he shows and talks about the horsefly-eating wasp.

For the kids, here’s a fun-tastic book: Who Would Win? – Ultimate Bug Rumble, by Jerry Pallotta, illustrated by Rob Bolster. The book’s premise is a sixteen-creature, bracketed tournament between fearsome insects, spiders, and other creatures commonly called bugs. There are no paper wasps, but the competitors include hornets and killer bees. Bracket by bracket, Pallotta pits one bug against another, filling the pages with facts about their anatomy and fight tactics. This is one of a series of twenty-six (and counting!) Who Would Win books that kids love.

 

 

Image Credits: Carol Doeringer.