Spider Parasitoids

When I was in Vermont recently, I collected a couple of round, white egg sacs with distinctive dark gray tufts.  My hope was to raise the spiderlings to adulthood and find out what species they are.  Something emerged from one of them the other day, but it wasn’t quite what I had in mind.

Spider egg sac (13 mm across) with an exit hole...

...of a female ichneumon wasp (Gelis; 3.8 mm).

It was a wingless adult ichneumon wasp in the genus Gelis, which had developed inside the egg sac, feeding on the eggs.  Many species of Gelis are known to parasitize spider egg sacs, while others are hyperparasitoids, attacking the cocoons of braconid wasps or other ichneumons.  There is much yet to be learned about this genus; Bob Carlson tells me: “I count 80 Nearctic species that were recognized at the time of the 1979 catalog [of Hymenoptera, for which Dr. Carlson wrote the Ichneumonidae section]. There may be many more than that that haven’t been described, and the only work that attempted to treat the Nearctic species was published in 1912 by a Canadian named Strickland, and it is of little practical use.”  So this wasp is unlikely to get a species-level ID any time soon, nor does it get me any closer to identifying the egg sac, which I think may be related to this one from Pennsylvania and possibly this one from New Brunswick.

While I’m on the topic of ichneumons that parasitize spiders, I’ll take this opportunity to share a photo I took back on April 1 of something I found crawling around in a bag containing galls I had collected a few days earlier.  I thought it was a mite at first, but when I put it under my microscope I saw that it was a beautiful spiderling… with a very tiny (about 0.2 mm) larva of an ichneumon wasp on its back.

Baby cobweb spider (1 mm; probably Theridion) with an ectoparasitoid ichneumon larva in the Polysphincta genus group.

I tried to raise the spiderling so I would get to see the wasp develop, but it didn’t seem willing or able to eat snow fleas, which was all I could find to offer it that early in the season.  To see what this development would have looked like, take a look at this series by John Maxwell, who is much better at raising spiders than I am.  There is apparently a lot of variation in the cocoons of spider-parasitizing ichneumons in the Polysphincta genus group; Noah Charney and I often find loose, woolly ones like this in spider webs when scouring the walls of buildings for invertebrate signs.

About Charley Eiseman

I am a freelance naturalist, endlessly fascinated by the interconnections of all the living and nonliving things around me. I am the lead author of Tracks & Sign of Insects and Other Invertebrates (Stackpole Books, 2010), and continue to collect photographs and information on this subject. These days I am especially drawn to galls, leaf mines, and other plant-insect interactions.
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4 Responses to Spider Parasitoids

  1. Pingback: Campoplegines, Part 1 | BugTracks

  2. Ika says:

    (Seems I just stumbled upon a blog to follow!) The egg sack looks like one made by a spider belonging to Tetragnathidae. In Northern Europe, you’d frequently rear Gelis melanocephalus (among other Gelis spp.) from these. Cheers, Ika.

  3. Pingback: Life in an Old Phoebe Nest | BugTracks

  4. Pingback: Spider Parasitoid | BugTracks

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