Wednesday, August 23, 2017

The Wasps and the Fly

You literally don't have to do anything but step out the door to observe fascinating phenomena in the world of insects. I did just that on August 9th, and found myself watching peculiar behavior between a nest of European Paper Wasps, Polistes dominula, and an unidentified fly.

The paper wasp nest has been on the back of our fence gate for months, now. The occupants are amazingly tolerant, even as we come and go. They may get a bit disoriented if we leave the gate open for a lengthy period, but otherwise they are innocuous and we let them be. Imagine my surprise when the thing that has agitated them most was what appeared to be a house fly.

Upon closer examination, the fly had slightly different markings than a house fly, was spinier on the abdomen, and had perhaps longer legs. It walked a lot, nervously, and it was difficult to even get an image of it. Finally, it stopped walking and started, well, stalking towards the wasp nest. One wasp took notice and adopted a very alert posture. Suddenly, the fly flew off its perch and looped around the wasp nest at dizzying speed. It was so quick I was not sure what I was seeing.

The aftermath of the fly's reconnaissance mission was even more dramatic. The one wasp that had seen the fly in the first place suddenly began running frantically and erratically all over the comb, for probably at least ninety seconds. The fly was long gone, but I was suspecting that maybe it had laid an egg and that was the object of the wasp's energetic searching behavior.

Still puzzled by the fly's identity, I took to the internet for the most likely suspects: a tachinid fly, family Tachinidae; or a parasitic sarcophagid (Sarcophagidae). I found a tachinid that looked promising, but viewing images of the species it became readily apparent this was not the right one. Sarcophagids came up empty as well. Furthermore, I have witnessed paper wasps eating sarcophagids, not falling victim to them. Next I looked to my library and found a potential match in a European field guide, of all things. Eustalomyia festiva, a member of the diverse but obscure family Anthomyiidae, looked good. According to the text, it "Breeds in [the] bodies of flies stored by solitary wasps." Ok, but paper wasps are social....

I posted the images shown here on the help group "Hymenopterists Forum" on Facebook, and got this reply from Rui Andrade:

"It looks like Eustalomyia (Anthomyiidae). The larvae are kleptoparasites of wasps."

This makes sense to a degree, but social wasps do not store their prey for later consumption by their larval offspring. They feed masticated prey directly to the larvae. There is therefore no opportunity for a fly larva to develop in a social wasp nest under those circumstances, as a kleptoparasite feeding on prey intended for the host's offspring. The only alternative I can fathom is that the fly is parasitic on the wasp larvae themselves. Cursory review of the literature does not inform my opinion, as they all stubbornly state that solitary wasps are the victims of Eustalomyia. The wasps are not going to let me peer into each cell to see if there is something other than a wasp egg or larva inside, either.

Perhaps this one individual fly was just confused as to the proper host. Maybe it was initially attracted to the bee block ("bee condo") hanging up on another part of the fence, where we have indeed had solitary wasps nesting. This year the bee condo has been devoid of activity for the most part, so maybe scaring the paper wasps was an amusement borne of frustration for this fly. We may never know.

Sources: Chinery, Michael. 2012. Insects of Britain and Western Europe. A Domino Guide. London: Bloomsbury Publishing. 320 pp.
McGavin, George C. 1992. The Pocket Guide to Insects of the Northern Hemisphere. London: Parkgate Books, Ltd. 208 pp.


  1. Very interesting. There is always so much going on even just outside our doorstep. Its like an onion with layers.
    Our European paper wasps are also very tolerant. I should use this to keep an eye on them too to maybe see parasitic/kleptoparasitic intruders.

  2. I'm searching for a wasp ID, and came across this article. Very cool, I think I need to observe wasp nests more!

  3. I saw a similar thing today and took to the internet to find out what was going on? A solitary wasp was making a nest in an outdoor closet on my balcony. I first saw it going under the door frame inside the closet and I panicked because I didn’t want to get stung. But then I noticed a fly hovering around the bottom of the door watching the wasp, almost stalking it. It was the wildest thing I ever saw! A tiny fly following a much bigger wasp. The wasp seemed scared of the fly. I figured the fly was trying to lay eggs or something. Thanks for confirming my suspension! Nature is so awesome!

    1. Yes I have observed the same. A singular wasp in open air, often flanked but a single fly, seemingly stalking him at a distance. Oddly they can scare the wasp off while it will come right at me 😂😂

  4. I watched a similar wasp and fly interaction this afternoon on a paper nest that is hanging outside my window. A common fly landed on the nest several times. When discovered, the two red paper wasps chased it around and around the nest until the fly left. I watched the same pattern at least a dozen times. The wasps did not seem to care if the fly was on the wall an inch away but only if the fly was on the nest. I would love to know more. Thanks for your information.

  5. I put a potter's wasp nest in a jar to watch it hatch earlier this month. Four flies came out came out today. My guesses are the wasp took a fly pupae in the chrysalis and it hatched before the wasp. Or a fly laid her egg after the wasp built her nest. Very curious, thank you for this post it has been the closest info I could find similar to my situation.


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