wasp preying on aquatic insects?

Discussion in 'Fly Fishing Entomology' started by Richard Olmstead, Aug 20, 2013.

  1. Richard Olmstead BigDog

    Posts: 2,485
    Ratings: +786 / 0
    I spent a week on Rock Creek in MT a couple weeks ago (1st week of August). While there, I observed something new to me, although I acknowledge it may represent my limited powers of observation, rather than anything unusual.

    There were significant numbers of what looked like a small hymenopteran (though perhaps a mimic) moving quickly and occasionally hovering just over the water surface in the smoother stretches. They seemed to be preying on emerging aquatic insects and would often swoop in and hover over my fly while inspecting it (and inevitably rejecting it - good thing I wasn't trying to fool them). The yellow/black banded critters were ca. 1 cm in length and moved quickly, but rarely more than a few cm above the surface. I tried a few times to capture one with my net, but never succeeded. I did see a couple of them caught in the surface film, but didn't think to catch and photograph one.

    Does anyone know what these were? Are they common (and I just not tuned in previously), or unusual? Does anyone tie a pattern for them (and do they work!)?

    Inquiring minds...
    Dick
  2. Taxon Moderator

    Posts: 952
    Ratings: +111 / 0
    Hi Dick-
    Although I have not previously heard of the behavior you recently witnessed on Rock Creek, here is what WikiPedia has to say about Wasp paraisitism:
  3. Richard Olmstead BigDog

    Posts: 2,485
    Ratings: +786 / 0
    Thanks, Roger. That's interesting. I guess it could be that they would capture an emerging insect momentarily and inject an egg then fly off, leaving the parasitized insect to go about its life. But they seemed to be most interested in very small prey (midges and mayflies), and probably with a short adult life expectancy.
    Dick
  4. cabezon Sculpin Enterprises

    Posts: 1,713
    Ratings: +237 / 0
    Hi Richard,
    These wasps are more specifically parasitoids. They generally inject their eggs in the larvae or adult insect/spider. Sometimes the prey is paralyzed and sometimes it is not. The wasp larvae then feeds on the developing prey larva, avoiding vital organs until the very end. Then, they pupate and emerge from the now dead larva as an adult wasp. Think Alien. These wasps do not feed on adults (as far as I know).

    I wonder if what you were seeing weren't robber flies (see http://en.wikipedia.org/wiki/Asilidae). Some have banding patterns that either mimic or are similar to bees/wasps (see https://www.google.com/search?rlz=&...I&biw=1062&bih=607&sei=3NAUUq2dIq78yAGu7YHYDA). Your description of their predatory behavior is very similar to the Wiki description.

    Steve
    Richard Olmstead likes this.
  5. Richard Olmstead BigDog

    Posts: 2,485
    Ratings: +786 / 0
    Steve -

    Robber flies, huh? That could be it. I definitely had the impression that they were predatory on small emerging aquatic insects. The wikipedia entry suggests most are found in open habitats and doesn't mention water, but it is a big group and this may be one speciality.
    D