The Parklands

Silo Lookout Wasps – Take II

If you visited Brown-Forman Silo Center this month, or read Olivia’s informative blog post, then you know about the Silo Lookout wasps. They first showed up about four weeks ago (a large swarm of paper wasps gathered around the top of the silo) and at least some have been back each day since. On a recent chilly day, there were hardly any at all, but on the busiest day, I saw what I’d guess was well over a thousand. They aren’t associating exclusively with silo, and they only show up in the afternoon and leave as the temperature and light fall in the evening. If you visit in the morning when it’s chilly out, all you’ll see are a few scattered carcasses.

So what’s going on? It frustrates me to say that we can’t be certain. I’ve heard some good ideas, and I’ll share my own gut feeling later, but we haven’t been able to find enough definitive descriptions of wasp behavior to match what we’re seeing on the silo and leave no doubt.

This isn’t how I imagined this would turn out when I first saw the wasp swarm. Here was a group of common, well-studied insects behaving in a very specific way at a specific time around a specific kind of object. In today’s world when you see something so distinct it usually isn’t hard to find a description of someone else who saw the exact same thing. I googled “paper wasps swarm autumn” expecting an immediate and quick explanation. As you may have guessed, I didn’t get one. I switched to the scientific genus name but failed with “Polistes silo swarm” and “Polistes mating swarm” and a dozen other variations as well.

In total I found two articles (“Paper wasp swarming around structures”, and “Paper wasps swarm in the fall”) describing wasp swarms around towers but neither provided any explanation for why they happen. A few scientific papers described autumn paper wasp pre-hibernation clusters but said nothing about tall objects or daily free-flying swarms. Some commenters in online forums mentioned fall mating swarms around tall objects but didn’t share their credentials. I have some trusty old insect reference books at home. I found nothing in them. After three days of searching I was, to put it lightly, frustrated.

At a certain point, the frustration gave way to real, genuine curiosity. I started asking everyone I saw at the park about the wasps, co-workers and visitors alike. It’s been my go-to conversation starter for a week. I’ve pointed them out from the ground, climbed the tower with people, shared pictures and videos, swapped theories, and reached out to a former professor. At a certain point it dawned on me that, though I hadn’t answered my original question, I had learned quite a bit about Polistes wasps in general.

This is all to say that observing wildlife doesn’t always go as planned. It’s easy to be discouraged when you come up with a great question about some fascinating thing you noticed outdoors, and the only answer you get is messy, non-definitive, and spilling over with additional questions. But more often than not that’s the norm. Life, generally speaking, resists being categorized into little answer boxes. The most astute observer and active thinker almost always comes back with more questions than they left with.

So what have I learned about the Polistes wasps? As Olivia described they construct their nests in the spring and live, for most of the year, in hierarchical colonies composed entirely of females. Oftentimes the inhabitants of one nest, from the queen to the workers, are all closely related to each other, but not always. Males are only produced in the fall and only exist to mate with the female wasps who will found next year’s colonies. In what may be an important detail for us, males leave their nests to mate with females from other colonies, a practice which prevents inbreeding.

At a dead end, I spoke with Professor of entomology and Tom Wallace Endowed Chair of Conservation Steve Yanoviak from the University of Louisville. He was able to confirm that our wasps belong to the species Polistes carolina (of the family Vespidae in the Hymenoptera order) and reiterated that this would be an extremely unlikely time of year for any nest construction to be occurring. Though a full explanation remains elusive, speaking with Dr. Yanoviak reminded me that animal behavior can sometimes be explained by the strangest of quirks. Perhaps, he suggested, the “shiny surface of the dome is serving as an artificial attractor to them – like moths to a porch light.”

I like this explanation. Perhaps the swarming behavior is ultimately about mating, but the swarm was attracted arbitrarily to the biggest shiny object around. As I mentioned before I am only left with more questions. Can we find other tall structures with wasp swarms that aren’t shiny? How about wasp swarms near shiny objects that are close to the ground? I’d like to observe the behavior of the wasps on top of the silo, which are mostly obscured from our view. Are they pairing up as mating wasps do? If we sampled some could we determine their sex and, if so, would we find more females or males?

As my frustration faded I was able to see the wasp swarm in another way, as a strange, beautiful, and fleeting phenomena. I went back up for another look late last week. Inside the dome the pinging of wasps on the metal played like a giant, never-ending rain stick. The ever-shifting individuals, all taken together, were like watching grass blow in the wind. The hum and buzz of the wasps was evident the second I climbed the silo, but after a few quiet minutes, I became aware of a second noise. At the top deck a stiff breeze blew and the silo, like a giant glass bottle, murmured, rattled, and sang in the wind.

While I calmly leaned against the railing, a wasp landed on my hand and then left, then another landed on my shirt sleeve. I watched it use its front legs to groom its smooth, rust-colored face. At the same time an inscrutable and entirely relatable being. The details on this inch-long insect came into focus before a backdrop of miles and miles of unimpeded view. In a moment like that the ever shifting mysteries of the natural world don’t seem so hard to live with.

As I turned to head back down the stairs I noticed, on the outer rim of the silo, a solitary stink bug (order Hemiptera, family Pentatomidae). It sat unmoving, pointed out towards the Pignic Barn and the expanse of Turkey Run’s forest beyond. “What,” I wondered, “were the eyes of this stink bug processing from that view?” I felt a terrible pang of inter-species kinship. “Pondering life’s mysteries!” I thought to it jovially. “You and me both.” And I headed down the curving stairs, the swarm of Polistes carolina receding above.

Final Note:

Out of an abundance of caution, because paper wasps are capable of delivering a sting, Parklands staff may choose to close the Silo Lookout when wasps are present. We have elected not to attempt eradication. Paper wasps are tremendously beneficial insects. I can attest, from my time on top of the silo and from a lifetime of working with these guys in the backyard, that paper wasps are incredibly docile. You need to really make one feel threatened before it thinks about stinging. We fully expect the wasps to disappear from the silo area as the weather cools, a prediction seconded by Dr. Yanoviak.


Chris Erickson

As a Parklands Zone Gardener, Chris Erickson supports the planning and maintenance of gardens throughout Turkey Run and Broad Run Parks. Chris joined The Parklands team in the spring of 2016 after spending time as a teacher in the northeast.

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