Giant Ichneumon Wasp – “Stump Stabber”
Sandy spotted this huge, sprawling ichneumon wasp on the inside of our garage window on June 27, 2012. The wingspan was about two inches, and the whole body length (including the abdomen) was about the length of my hand from fingertip to wrist (roughly 6 inches!)
This is certainly one of the Giant Ichneumon Wasps in the genus Megarhyssa, and I think it is Megarhyssa macrurus based on the pattern of dark marks on the wings.
It was very clumsy, and quite easy to catch, although it was so big that I had a hard time keeping it in my “white box” made out of a folded sheet of paper.
Sam was making an insect collection at the time [1], so we froze this specimen to kill it, and then posed it for mounting. The next pictures may have a somewhat unnatural pose because of this, but I think this would be pretty close to the pose it would have taken up just before laying eggs.
The common name for these is “Stump Stabbers”, because they are frequently found perched on rotting stumps and logs, driving that long ovipositor into the wood. They must have some sort of drilling mechanism at the ovipositor tip, because the whole ovipositor is so flimsy that simply driving it into the wood like a needle doesn’t seem practical. There are some bits at the end of the abdomen that look like they might act as part of a drilling head, but I’d probably have to get it under a scanning electron microscope to really see how it works. It appears the ovipositor has two parts, though: one part with a point on it that acts as a partial sleeve, and a second part that fits inside it, and has a comblike structure that looks like it might be an anchor. One way it might work is for the comb to dig into the wood, and then the sleeve to be driven past it. Then the comb could retract and the inside part could follow the sleeve deeper and then re-anchor. The whole process could then repeat until it gets to the desired depth.
And why do they stab stumps? Because their grubs parasitize other insects that are burrowing around in there. They somehow locate the burrowing grubs (probably by vibration?), and then drill into the stump until they find a tunnel, and then lay their eggs in it. It sounds like their primary hosts are the larvae of Pigeon Tremex, a very large species of “Horntail”, or wood-boring wasps. Since we clearly have the parasite, that probably means that we also have the host, so we need to look out for some horntails to photograph in the future.
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[1] The insect collection consisted of a number of large insects that had mostly been found dead or dying. Among the various specimens, it included a giant water bug, a big hermit beetle, a scorpionfly, and several butterflies and moths, among others. This ichneumon wasp was the only one that we actually killed specifically to collect it. At the end of the summer, the collection was given to the public library so that other kids could see the kinds of large, interesting insects that we have living around here. I always feel bad about intentionally killing insects to collect them, although the general position of professional entomologists is that insects are so prolific that even “rare” insects will still have hundreds of individuals per square mile within their range. And since they are also heavily preyed-upon by birds and the like, collecting a specimen here and there has essentially zero effect on the population compared to the number killed by predators. And when I do kill an insect, freezing seems to be one of the more humane ways to do it. They just slow way down and go comatose, with no fuss.
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The first time I saw one of these in the wild (hiking Yosemite valley learning about fire ecology) an NPS friend casually called it something a LITTLE less PG than stump stabber. I was a little shocked, but got the idea. =) Years later I got to see a lady stump stabber doing her work. It was pretty darn cool. Thanks for sharing!
Very impressive