This evening I went for a short walk to collect some mosquitoes to feed the spiderlings I’m trying to raise. On the way back I found these two insects standing perfectly still at the edge of a leaf:
The wasp seemed fixated on the beetle, but she wasn’t moving toward it. I watched for a minute or so and nothing happened, so I started taking pictures. Neither of them moved as I took seven shots, even though I had to move leaves around each time to get a clear view. When I tried for a top view, the wasp flew away. I may have saved the beetle’s life, for the moment: the wasp was a braconid in the subfamily Euphorinae, which as far as I know is the only group of wasps with larvae that are internal parasitoids of adult beetles (or of any other adult insects, for that matter; most parasitoid wasps emerge from the eggs, larvae, or pupae of their hosts). Probably the most commonly encountered euphorine is Dinocampus coccinellae, which parasitizes ladybugs. When the mature larva emerges from its ladybug host, it spins its cocoon under the beetle, taking advantage of the beetle’s warning coloration to protect itself.
The beetle pictured above is Isomira sericea, a comb-clawed beetle (Tenebrionidae: Alleculinae). No wasp in the 1979 Catalog of Hymenoptera is listed as a parasitoid of Isomira, so this may well have been an undocumented parasitoid-host relationship. If I hadn’t interrupted, I might have seen the wasp climb onto the beetle and insert her ovipositor to lay an egg. And I still might have seen this if I had waited around–I don’t think she went far. Or maybe the beetle had already been parasitized–it’s odd for a beetle to sit perfectly still like that while I’m harassing it with my camera. Whatever was going on here, it was a neat little scene to encounter.
Thanks to Bob Carlson for identifying the wasp, and to Vassili Belov for identifying the beetle.