Acorn Plum Galls (and friends)

Hey, this blog now has over 1000 subscribers! Thanks everyone for your continued interest in my esoteric natural history investigations.

I’m still slowly working my way through the photos I took last summer, during which one of my several jobs involved exploring ridges and summits in the southwestern corner of Massachusetts. On August 9 I visited Jug End in Egremont, where the Appalachian Trail passes through a very nice example of a ridgetop pitch pine – scrub oak woodland, which is a rare thing in Massachusetts.

One of the nice things about scrub oak (Fagaceae: Quercus ilicifolia) is that all the acorns are down low, providing opportunities to see “plum galls” of Amphibolips quercusjuglans (Cynipidae) while they’re still attached. I normally only see these galls on the forest floor, after they’ve dropped from canopy red or black oaks, and I’d been noticing them for several years before I learned that they grow out of the sides of acorn caps. Here are some of the ones I saw at Jug End that day:

The acorn to the left has a single gall, the one at lower right has two, and the one in the background has three full-sized ones plus a fourth underdeveloped one—quite a load for one little acorn to carry. But back to the lower right, notice that there are a couple of bugs sitting on one of the galls. These are an ambush bug (Reduviidae: Phymata) eating a crabronid wasp that I’m told is in the genus Crossocerus. When I saw this I wished I’d lugged a better camera with me, but here’s the best I could do with the little point-and-shoot that fits in my pocket:

If you cut one of these “plum galls” open, you’ll find that it consists mostly of apple-like flesh, with a hard, spherical cell in the center where a single wasp larva is developing.

According to Weld (1959), “Tranformation takes place in the fall; emergence in the spring Feb. 17 – May 14. Mo. and is distributed over more than one season.” I think he meant by this that the larva pupates and becomes an adult in the fall, but overwinters before chewing its way out of the gall—and the “Mo.” means that those were the emergence dates recorded in Missouri. MJ Hatfield reported here that she collected galls in Iowa in September 2008; nothing had emerged by June 1, 2010, so she cut one open and found a live larva inside, prompting her to save the remaining galls, and an adult emerged from one of them in April 2011.

I collected some oak plum galls from the forest floor near Amethyst Brook in Amherst, MA in August 2011, and nothing emerged the following spring, but some time in the fall of 2012 (while Julia and I were traveling around the western US for two and a half months in search of leafminers), two cynipid wasps emerged and died. They were not Amphibolips quercusjuglans, though; they were inquilines (developing in galls induced by A. quercusjuglans), and Matt Buffington identified them as belonging to the genus Ceroptres. One was a female…

…and one a male.

The container with the galls also had a tiny, shriveled moth larva in it:

I think it must have been the same larva that I photographed on September 1, 2011, when it had just emerged from one of the galls:

I’m not sure what it would have needed to complete its development, but I guess I just left it in the container with the galls and hoped for the best. That dried larva is now at the Smithsonian along with the two wasps, so theoretically it could still be identified using DNA barcoding.

On September 21, 2019, I was leading a walk at the Fitzgerald Lake Conservation Area in Northampton, MA, when a little girl presented me with an acorn plum gall she had just found and demanded that I cut it open. I did as I was told, and somehow managed to cut right through the central cell without killing the larva inside. The larva, I was surprised to see, was not that of a gall wasp—it was another caterpillar! I have no photos of it, because I put the gall back together and kept it that way to let the larva finish feeding without being disturbed further. However, I just found this photo on BugGuide, taken by Tom Murray in Concord, MA on October 23, 2011, that shows the same thing:

It appears to be the same type of larva that I’d had emerge from a gall collected in Amherst just a few weeks earlier.

As for the more recent gall from Northampton, nothing emerged until the following May—and instead of the adult moth I was hoping for, it was an ichneumon wasp, which had developed as a larva feeding on the moth larva inside the wasp gall.

So the question remains: Who is this caterpillar that feeds inside of acorn plum galls, and does it do so exclusively? The fact that it lives inside the cell where the gall wasp is supposed to be suggests more than a casual association (and also suggests that it may eat the wasp larva). It would also be interesting to know whether this ichneumonid exclusively parasitizes this moth species that feeds inside of cynipid wasp galls, but based on my understanding of ichneumonids I think it more likely parasitizes a variety of moth larvae that feed in concealed situations (fruit and stem borers, leaftiers, etc.).

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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6 Responses to Acorn Plum Galls (and friends)

  1. dvaunhowe says:

    We had many of these in our yard last year; maybe because we had so many windstorms. I cut some open for the kids to look at. I should check to see if some were left behind. We might have some dried up wasps or maybe even a caterpillar to look at. (I’ll feel a little guilty about that).

  2. Barry Cottam says:

    Congratulations on the milestone, Charley, and thanks for this fascinating account. Nature never ceases to amaze!

  3. Michael Hawk says:

    Congrats on 1000 subscribers! And I love seeing the gall content. Galls offer so much fertile ground for discovery, even in our own yards.

  4. Erin Hilley says:

    A classic whodunit story. Fascinating.

  5. captomfo says:

    I have found similar thin skinned oak galls growing in spring and summer in Saluda County, SC. Not certain when the wasps emerge. One that I split open last week (July 15th) contained an immature wasp. Wings were in an immature state, but it moved head and legs when I touched it. So, adults will probably emerge in a few weeks. I’ll find out–I have some galls in Ziplocs. I have found the galls always on twigs of white oaks rather than acorns as you have. As I understand the cynipid generation cycle haploid wasps construct soft galls (parthenogenic generation), while fibrous woody galls are made by the diploid generation. The latter emerge in winter (Feb-March, in my experience in SC). Fibrous galls usually form on oak leaves and at this time are in a growth stage. I have also found lepidopteran caterpillars in fibrous galls in late winter (identical to the ones in your photos) and have had a chalcidid wasp (Hockeria sp. –ID made by BugGuide.net) emerge from a winter gall (April)–considered a parasitoid of lepidoptera. Perhaps, this wasp consumed a caterpillar before emerging. Do you think the caterpillar/chalcidid connection could be a normal part of cynipid oak gall cycles? Would surviving caterpillars eventually chew out of the gall and pupate in soil? I’m very interested in hearing your comments. Thanks.

    • Enough of these caterpillars have been found that it seems to be a regular occurrence. Based on what I’ve seen so far, my impression is that they exit the galls to pupate; whether in leaf litter or soil I couldn’t say.

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