Oak Apple Gall
We were all walking on the trails in the woods behind the Copper Country Humane Society in September when Sam saw this hanging from a leaf on a small oak tree:
It was about half the diameter of a ping-pong ball. While it looks like some sort of fruit, it isn’t. Especially considering that oak tree fruits are acorns, and this is no acorn. The surface was dry and papery, and the whole thing was very light. It is what is left of a gall, but we were too late to catch the inhabitant – you can see the exit hole here:
I cut it open with a razor blade to see what was inside, and it was full of loose fibers with a little cyst in the center where the inhabitant used to be.
You can get some sense of scale, here; that’s my thumb and forefinger in this next picture.
The fibers were supporting what looks like a single cyst. The insect in it was probably only a few millimeters across, so this is quite a lot of gall for a single tiny insect.
According to BugGuide, there are two North American species of gall wasp that could make this: Amphibolips confluenta, and Amphibolips quercusspongifica. They don’t show pictures of the wasps, and I don’t have an adult specimen in any case. It looks like the gall is actually solid while the wasp grub lives inside it, but then after the wasp emerges sometime in July, the gall dries out to make what we see here.
There are other species of oak apple gall wasp elsewhere in the world, some of which have multiple grubs per gall. Like, for example, the one in this 3-d animation that was made by David Mills and Graham Davis using X-Ray Microtomography. This gall is from a species found in Great Britain, which is a bit lumpier than mine here, and there are at least four wasp pupae visible in it.
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And in other news, I have just finished a long-overdue update of the reference guide for this site. It has thumbnail images of all the arthropods posted to date, to simplify browsing to find any given one. Instead of being a single page, it is now broken up into subpages so that it will load faster and be easier to search. Not to mention easier to maintain. Please let me know if there are any changes you would recommend to make it easier to use.
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There are no oaks native to Alberta, but bur oak (Quercus macrocarpa) made it as far west and north as Manitoba on its own and is fully hardy in Edmonton. We don’t have Dutch Elm disease yet, but it is only a matter of time, and most of the rest of the older deciduous street trees are ash which have the emerald borer to worry about and besides are not especially drought hardy. So, bur oak is becoming more prominent in town and its tap root makes it good in a drought.
One of my neighbours put in the first oak on the block about 15 years ago – and I counted four different types of apparent cynipid galls on it last summer. Could be different developmental stages I suppose (I know almost nothing of insect galls), but I still found it impressive that the wasps are already here and thriving. One would think that such obvious deformities would be pruned out in the nursery, so I wonder how they got here.
Bye-the-bye, I suggested an answer to your “Nymphs on cherry tree – some sort of treehopper?” 6 Nov post = The Privet Leafhopper (see link in comment). I’ve convinced myself that I’m right and added it to the Home Bug List, but I wondered what you thought.
Wow, that is pretty fast work for the gall wasps to move into a new area. I guess that’s what wings will do for you.
Regarding the 6 Nov. nymphs, I don’t know. The nymphs on Bug Guide for the Privet Leafhopper are all bright green, so I think that my black-and-white nymphs are probably something else (although maybe a close relative). Given how numerous they were, some sort of invasive species is a good probability. I’ll have to keep an eye out for them in the future, and if we see them again we’ll have to see if we can raise some up to adulthood to check.
Hi Tim, Hi Dave!
A few years ago I managed to pickle a few wasps that emerged from galls that I found in a bur oak here in Edmonton. I’m going to have to try to photograph them one of these days. I remember the abdomens had a particularly ‘boxy’ appearance.
Hi, Adrian
If you ever do get photographs of those wasps, I’d like to see them. Especially since BugGuide doesn’t seem to have a single picture of the adults.
Oak Apples are also well known for the sheer number of hyperparasites and kleptoparasites they attract, at least in the U