While IDing plants, I’ve noticed that sometimes when looking back at photos I’ve taken, the leaves have little bumps on them. I’ve always just assumed it’s a fungus growing or a deformity of the leaf. That is, until now, where I was informed those bumps actually have a name, and they can be caused by a wide variety of etiologies. They’re called “galls” or “cecidia” and are essentially growths/tumors that can form on any part of the plant, sometimes due to irritation and sometimes due to the organism of cause living inside/promoting the formation of it.
To the right I included an ID photo I took this summer out on the Little Grand Canyon Trail (yes, I know, I am months behind on any sort of for-fun writing). When I first saw it, I thought this plant had some sort of semi-translucent, gooseberry-esque fruit growing. Yet, when trying to identify the species, the plant was easily seen to be Wood Nettle (Laportea canadensis), yet I could not find anything about their fruit looking this way. Thankfully due to the fact plenty of other people must have had the same confusion as me, the almighty Google recommended I look at Wood Nettle Gall Midge (Dasineura investita). Sure enough, the “berry” I found was actually a gall formed by a midge/small fly!
In this case, I regret to say that I did not dissect one of the galls, as I have a tendency to be overly wary of touching (and especially consuming!) things found in the forests (although I am getting better with it as fungi ID typically requires slicing into them to look for bleeding, coloration changes, and gill types). Yet, now knowing a fly was responsible, I was curious to learn more about the relationship between these species. As I mentioned earlier, sometimes galls are just a product of irritation, and in other cases something, whether that be bacteria or entire physical insects, can be living inside.
I could not find much good info on this exact species, but I did see that generally midge galls are made so that the larvae have a place to live over the winter until they are ready to emerge in spring.
Since I wanted to check a cross section, I found a few posts on iNaturalist (shown above) with that info. Unfortunately, I cannot make out any detail that would definitively tell me a midge lives there. It is possible they have already emerged, or contrarily, are still far too small. In general, it might not be something observable with the naked eye. I am not sure!
In terms of the papers out there on this, I did find a paper from 2016 (and looks like a paper from the 80’s) with mention of the life cycle. Link is below in my sources, but if you want to check me on this, it was page 92 of the PDF (yes, I skimmed through that long just to find the right species).
“There are two generations a year, the full grown larva from the autumn generation overwinters in the central chamber of the gall.”
So it seems that indeed, the midge is living in the center of that gall, despite not seeing it from the photos myself, and let me tell you, I am beyond fascinated by this. I am not hiking as much as of late due to both the weather and lack of time, but I know I will be on the lookout for more galls to not only attempt to identify, but to cut open and see what’s inside!
Now, to break away from the topic at hand, galls are actually surprisingly positive for what sounds like such a detrimental or one-sided symbiotic relationship. In the same way plants don’t really get “cancer,” the plant galls rarely have highly negative effects on plants. As some gall-forming species rely on galls for their life cycles, the plants usually act as a somewhat neutral home for their parasite, of course having effects on the ecosystem as many of these species are important in pollination and their food chains (think birds eating insects from trees, etc.). Looking from a more societal point-of-view, galls actually have had some use for humans historically. Oak galls caused by the Cynips genus of wasps could be processed to make tannic acid, which then could be mixed with iron and other substances to produce an ink. This ink was then used to write major historical documents, such as the Declaration of Independence! Other uses for galls sometimes includes eating them, in the case of sage galls and corn smut. It seems a number of sage species can form galls which, dating back before even the 16th century, were eaten, made into tea, or preserved in sugar and have a sagey, apple-like flavor. Plus, as a mycology student and Spanish-language learner, corn smut (Ustilago maydis) immediately comes to mind as it forms galls and is used traditionally in Mexican cooking.
If I find more, I will surely upload my photos to my iNaturalist account as usual, and maybe I will add a part two if I find out anything else about galls that piques my interest.
Happy reading,
-Beppa
Sources:
The Morton Arboretum – Plant Galls
The Gall Midges (Diptera: Cecidomyiidae: Cecidomyiinae) From Allegheny County Pennsylvania