A Most Interesting Encounter

July 18, 2014 | Greg Dodge

I was standing in the vending area of Catch the Wind talking to some of the Adventure On Summer Campers here at the Museum. One of the benches had tan and gray stains on the back rest of the bench and I was explaining how this residue was the result of Carpenter Bees digging holes in the lumber above our heads.

I also mentioned how the males, which have a yellow-white rectangular mark on their faces, do not possess stingers and can therefore not sting.

The waste product from burrowing carpenter bees.

The waste product from burrowing carpenter bees.

During spring and early summer the bees burrow into wood, it can be a tree limb, trunk, the soffit of your house, or the pergola above our heads in Catch the Wind. The bees make nice 1/2″ holes which look to be the product of an electric drill. Once below the surface, about an inch or so into the wood, they make a ninety degree turn, continuing on until they excavate from 6 – 8 nest chambers. Food is placed in the chambers in the form of regurgitated nectar and pollen. Eggs are then laid and the chambers sealed.

As we stood there, I saw tiny bits of saw dust slowly drift towards earth. I looked up and could see a black object projecting from the hole directly above us. It was slowly wriggling from right to left, as if trying to get out of, or into, a tight pair of jeans. Was there a carpenter bee digging a hole right in front of us as we watched? Although a bit late is the season, it sure looked as if that’s what was happening.

I decided, for some reason, to touch the bee. I couldn’t see the insect’s face since only about a quarter of the insect was sticking out of the hole, the rear end, I assumed. Was it a male or female? I would take the risk and pat its rear end to see if it would react. I did, it didn’t.

I touched the insect several times. It continued on with its work paying little or no attention to my silliness. There was something odd going on here but I didn’t yet know what it was.

The insect began to back out of the hole. I saw that it was much thinner and longer than I had expected. I saw what looked like very small wings. I began to think that perhaps this was not a bee digging a hole in the lumber but a bee emerging from pupation. It wasn’t backing out, it was coming out of the hole head first.

The insect separated itself from what I now knew was its pupal skin, or exuvia, and crawled away from the hole and up the 2 x 12 to dry.

The "bee" and it cast of pupal skin at the entrance hole.

The “bee” and its cast off pupal skin at the entrance hole.

Had my eyesight been as keen as it was twenty or thirty years ago I would have noticed sooner that this was not a bee at all but some other creature crawling out of the lumber.

Once it spread its wings I recognized it immediately as a bee fly. But what was it doing coming out of a carpenter bee’s nest hole?

I’d been seeing many bee flies around the vending areas in Catch the Wind and Explore the Wild for the past month or more. The bee flies were all Tiger Bee Flies (Xenox tigrinus). After a bit of research I discovered that these bee flies parasitize carpenter bees. The bee flies lay their eggs at the entrance to the carpenter bee’s holes. The larvae of the bee flies then consume the carpenter bee’s larvae or pupae within the nest chambers, it alone emerging from the hole.

Tiger bee fly pumping fluid into it wings, its exoskeleton harding. getting ready to take on the world.

Tiger bee fly pumping fluid into it wings, its exoskeleton hardening, getting ready to take on the world.

This was a great learning moment for me and the happy campers assembled in the partial shade of the pergola in Catch the Wind. I didn’t realize that the bee flies that I’d been seeing with much frequency led the lives that they apparently lead, that of a parasite. I’d only seen one or two carpenter bees in the past several weeks but the bee flies have been increasing in numbers. They’ve obviously been doing their job in infiltrating the bee’s nest holes.

I should have suspected this long ago. Seeing as many bee flies hanging around the same areas where just weeks before carpenter bees were busy digging into the lumber at both vending areas here at the Museum, it should have come to me sooner that they were parasites. Many bees and wasps are parasitized by other wasps, bees, or flies. It seems to be the norm.

Happy insect watching!

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