The story behind unknown stem galls on Common Mugwort plants

This is a story about an unknown gall former, and the story is still only partly understood.

Last summer (August 2020) in Central Park and on Randall's Island, I found six stem galls on the very common and extremely invasive weed Common Mugwort, Artemisia vulgaris. The galls look like this one:

@steven-cyclist was with me several times when I found them, and he probably also photographed one or two of them.

I had never noticed these galls on mugwort before 2020, so I photographed them all, but I had no luck working out what the gall-making organism was that had caused the galls to form.

Fortunately iNatter @jeffmollenhauer is a lot more of a thorough and persistent naturalist than I am. Jeff collected some of these galls in New Jersey. Jeff then carefully cut one gall open, and photographed the larva that was inside the gall. It appeared to be a beetle larva:

Jeff also carefully saved a few of the intact galls to see what hatched out in the next spring or summer.

Sure enough, this spring, 2021, beetles started hatching out, like this one:

It is a Tumbling Flower Beetle, in the genus Mordellistena.

However, it seems that the beetle is almost certainly not the original gall former, but instead is an inquiline parasite. Mordellistena is a stem borer and a gall borer, but it does not create galls.

The Mordellistena larva apparently takes over the gall, a gall which is probably initially occupied by, and created by, a Eurosta fly larva. Eurosta are gall flies, and they are the causal agent of some similar galls on Golden Rod species. For example this large gall, which was caused by the Goldenrod Gall Fly, Eurosta solidaginis:

After an initial gall is formed on Common Mugwort, then presumably a mother Mordellistena beetle lays an egg on the outer surface of the gall. That egg hatches and the young beetle larva chews its way into the gall. The beetle larva starts out by eating the soft inner tissues of the gall, and then probably it often goes on to eat the little Eurosta larva too! Talk about eating you out of house and home!

For more info on this type of interaction, read the results section in this paper (especially under the Differentiation of Mordellistina convicta section).

Perhaps the Mordellistena species in our case might also be Mordellistena convicta? On BugGuide that species has been reported from inside galls on Common Ragweed, Ambrosia artemisifolia, so maybe it could also bore into galls on Common Mugwort, a plant which is somewhat closely related?

I asked @borisb, a beetle expert, what he thought about this beetle, and Boris commented, "What you have resembles those WE [in Europe] have in ragwort (Mordellistena weisei-group), though ours do not produce galls." I am assuming that Boris means that this group of Mordellistena beetles in Europe bore into the stems of Ragwort plants (does Boris mean the plant Jacobaea vulgaris?), but that they do not cause galls to form. I explained to Boris that we assume that the Mordellistena beetles here do not actually cause the galls in the Common Mugwort, but instead they take advantage of galls already created by a gall-forming fly.

And incidentally, here is a 2008 paper about differentiating the larvae of that European group of Mordellistena beetles:

@megachile made an entry in the excellent database (a great new resource from @megachile and @jeffdc) for this still currently unknown stem gall:


Posted by susanhewitt susanhewitt, May 20, 2021 20:55


@susanhewitt Thanks for the great summary on what we have learned to date on the Common Mugwort galls! I would still love to figure out which Mordellistina species this is and it would be super interesting to discover which insect is the original gall former before the Mordellistina larvae bores into the gall. I'm happy to have helped uncover some of the story, but would love to learn the rest of it. The galls were abundant on Common Mugwort in southern New Jersey in the fall of 2020.

Posted by jmole 8 months ago (Flag)

I agree. This is a fascinating story and still very incompletely understood.

Maybe we can try to find some of these galls in a very early stage (perhaps now) and cut the galls open to try to find what we assume may be the fly larva that causes the gall to form in the first place.

Posted by susanhewitt 8 months ago (Flag)

@srall -- you might like to read about this.

Posted by susanhewitt 8 months ago (Flag)

I had the same thought. I'll check on the Mugworts tomorrow and see if there are any galls forming yet. :-)

Posted by jmole 8 months ago (Flag)

Great! Please do let me know if you find any.

Posted by susanhewitt 8 months ago (Flag)

Enjoying following along with this story! I do feel like these galls suddenly popped up all over last year, but maybe it is just an increasing number of eyes on the lookout for such things.

Posted by calconey 8 months ago (Flag)

That's a good point. I don't know if maybe I was being more carefully observant last year, on the lookout for galls, or whether there was indeed a sudden increase in the number of these galls last summer.

I must say that I suspect the latter.

Posted by susanhewitt 8 months ago (Flag)

Neat, thanks for tagging me. I've seen these many times.

Posted by srall 8 months ago (Flag)

@Thanks @srall -- have you. seen good numbers of them every year, or have they been increasing in recent years?

Posted by susanhewitt 8 months ago (Flag)

I've only been looking for them for two summers (before this one), so I can't really say if they are trending. Generally, in the fall, if I look at any large stand of mugwort I will find some.

Posted by srall 8 months ago (Flag)

I think, in general, with galls, mines, diseases, etc., people simply didn't photograph them, didn't notice them before, so they look explosive. For instance, I never knew I could ID oak shothole miners until @susanhewitt started posting them, but my oaks have had holes in them for a decade at least.

Posted by srall 8 months ago (Flag)

I imagine you are right in your opinion about these phenomena, although I do remember reading about the oak shot hole leaf miner that there was big outbreak a few years ago in the northeast.

Posted by susanhewitt 8 months ago (Flag)

Images of an adult Mordellista sp. reared from one of these galls:

Posted by susanhewitt 3 months ago (Flag)

I just became aware of these galls about a month ago, and I was wondering if you all had any luck finding any of them last spring. It seems that this late in the season, all that's in the galls are the mordellid larvae, but based on the lack of updates- were the galls absent from the mugwort in the spring? I wonder if they might be a later season gall. In any case, ever since I first learned of them I've been seeing these stem swellings everywhere, they seem to be quite abundant. (I've seen them here in PA, as well as in CT and NJ when I've traveled.)

Posted by cecildomyiidae 2 months ago (Flag)

Yes, I could not find any of these galls in the spring this year, although I looked. I don't know when they first started appearing this year. I suppose it must be in the early summer? Or late summer?

Posted by susanhewitt 2 months ago (Flag)

That might make sense. I'll keep an eye out for them more this next season, too. Hopefully I'll be able to catch when they start appearing then.

Posted by cecildomyiidae 2 months ago (Flag)

I will also try more carefully to search month by month in an attempt to find out when it is that they start appearing.

Posted by susanhewitt 2 months ago (Flag)

I assume the mugwort needs to get fairly tall before these galls start forming.

Posted by susanhewitt 2 months ago (Flag)

Unfortunately I was not able to search for the galls until late summer this year due vacation and work schedule in the spring and early summer. Hoping to do better next year. Planning to rear some more galls this winter to see if I get more Mordellistena though.

Posted by jmole 2 months ago (Flag)

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