Thursday, May 28, 2009

Two for the price of one

Last week, while I was harvesting fresh maple flowers for my carpet beetles, I noticed a couple of silvery-brown lumps on the underside of a leaf, so I cut that, too. They turned out to be tiny fat beetle-like insects, decorated with four neat rows of studs, each with its own blond hair. They were stuck firmly to the middle vein of the leaf, and didn't respond to tickling with a paintbrush. Dead, dormant, or in the middle of some buggy process? I couldn't tell.

I put the piece of leaf into my little viewing tin, where I could watch and see what happened next. Later on, I had a better idea, and looked over the tree until I found two more leaves with the sleeping bugs; these leaves I harvested with stems, and put them in water in a small aquarium with a good lid.

These "beetles," like the first two, were also lined up on the central vein of their respective leaves.

I've been watching all week; nothing changed until tonight, when I noticed that one of the tinned bugs had a big hole in the rear abdomen:

Evacuated bug on a drying leaf.

A tiny fly was flitting around in the tin. I checked the aquarium; sure enough, both bugs had holes in more or less the same location. And a tiny fly raced up and down the walls.

And another on a still-fresh leaf.

The flies were very small, very fast. I managed to photograph one:

Black fly on a vintage cats-eye marble.

The other, the one in the tin, I trapped (it took me a while) and dumped in a container where a lame spider was resting; he woke up and caught it in less than a minute.

"Just what the doctor ordered!"

So what has been happening? Were these bugs parasitized by the flies before they settled on the leaves, or afterwards? Or are they a previous stage of the flies themselves? (This doesn't seem possible, but I keep running into "impossible" things.)

Help! I don't even know where to start looking for these. Any and all hints, suggestions, wild ideas are welcome.

*Update: see comments for answers, also the follow-up post, The Case of the Pregnant Mummies.



  1. The "flies" are some form of parasitoid wasp (I couldn't tell you what, but I'd start looking for matches among the Chalcidoidea first [which narrows the list down to only about 50,000 possible options]). Flies don't have that distinctive kink in the antennae that you can just see in the photo, that's a wasp thing (but not all wasps have it).

    The bugs you originally brought in where definitely not wasp larvae, and that hole's the wrong position and shape for a moulting break, so I'd say that the wasps were inside the bugs and the holes are where they broke out. The hosts weren't beetle larvae, they're some sort of Hemiptera (true bugs). Again, I couldn't tell you what one, unfortunately.

  2. What a neat find. I was going to suggest parasitoid wasp, but not because I could tell that the little black insect was a wasp or a fly. :) I've found a few husks left by parasitoid wasps, and they all have a round exit hole on the upper side of the host insect's back end; this looked like the same thing to me.

    The bugs themselves looked like aphids or scale bugs to me, but Christopher is probably right in that they're a hemipteran of some sort. I wonder if the lining up along and grasping the central vein is a behaviour stimulated by the parasite, just like some ants who are infected with a particular parasite run up to the top of grasses and grab on, to be eaten by a herbivore.

  3. Yep, to follow up on Christopher and Seabrook -- the tan guys are/were "mummies," probably aphids of some sort, infected by the the larva of a parasitiod wasp (possibly a chalcid or maybe a braconid? -- there are hundreds or thousands of species that engage in this behavior). As the wasps mature they burst out of the "mummy" leaving a circular exit hole. Googling "aphid mummy" will turn up quite a lot more information.

    Awesome that you were able to witness this behavior in captivity. If you check out the plants, you should be able to find aphid colonies with other infected individuals and possibly adult wasps ovipositing. I have some photos of this I'll try to track them down...

    Not sure if the parasitiods affect the behavior of the host, but aphids often line up along leaf veins so probably no need to invoke this kind of explanation.

  4. Thanks, all!

    So: down to 50,000 options. :-)

    I didn't know that about the kinky antennae. I'll be watching for it now.

    I wondered about parasite-stimulated behaviour, too. But they may be plant bugs of some sort, feeding normally on veins, as Neil said. (Still guessing.)

    And I'm off to Google "aphid mummy".


  5. After a bit more digging the fact that they are on Acer indicates that the aphids may be Periphyllus sp. or Chaitophorus (these genera may be synonymous). Unfortunately there don't appear to be many good photos of these genera online, but I have found a few that seem to show ornamentation patterns consistent with your photo (assuming the IDs are right).

    As far as the wasp goes, I'm fairly sure it is a braconid, possibly Aphidius, possibly not.

  6. Thanks, Neil.
    I've found a few photos that match, too. Your suggestion of "aphid mummy" worked. And I've checked out the tree some more, and found live aphids that match the pattern. I'll post photos in a bit.

  7. What a cool post and a very sharp eye to catch such an interesting bug and then to take it inside to study it! I learned something today!


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