Notes on Galls

Galls are highly specialized plant tissues whose development is induced by another organism.”

The relationship is essentially parasitic, says Britain’s Plant Galls: A Photographic Guide, but “few gall-causers seem to cause more than localised, short-term damage to their host plants.” This book continues: “Plant galls can be caused or induced by a very wide variety of organisms, including bacteria, nematodes or eelworms, and even other plants, but most of our galls are caused by insects, mites, or fungi. Although we do not know exactly how the galls are initiated, it seems clear that most gall-causers interfere with the development of the host plants’ cells, either chemically or mechanically, and cause them to develop into gall tissue.”

This Britain’s Plant Galls is good: “Many gall wasps induce no galls of their own but lay their eggs in the galls of other species. Their grubs benefit from the food and shelter found there and, although they do not harm the gall-causers directly, by competing for the food supply they can starve them to death. These ‘lodgers’ are known as inquilines. Quite a few flies have similar life styles. Parasitic species, which include many, often brightly chalcids, actually feed on and kill the rightful occupants of the galls. The female parasite tends to have a long ovipositor with which she penetrates the galls to lays her eggs in or on the host grubs. Strictly speaking, these insects should be called parasitoids because typical parasites do not kill their hosts. Parasitoids also attack the inquilines, and a large gall, such as an oak apple, can house a very large insect community, consisting of dozens of individuals of several different species.”
Here’s a good description of gall-makers in an article on the co-evolution of plants and gall-forcing herbivores like the Cynipid wasps:

“Gallers are specialized herbivores that induce the development of, and feed on, modified host plant tissues that, relative to surrounding nongall tissues, have elevated nutritive value but contain very low concentrations of toxic secondary plant metabolites.” ~ “Extreme Host Plant Conservatism during at Least 20 Million Years of Host Plant Pursuit by Oak Gallwasps” by Stone, et al. Evolution Vol. 63, No. 4 (Apr., 2009), pp. 854-869.

They somehow side-step the plant’s defenses and get better than average food out of the deal. Nice work if you can get it!

From the same article: there are about 1000 oak gall wasp species. They’re associated with oaks (Quercus) and other Fagaceae. Almost all alternate sexual and asexual generations, and each of these generations has a distinct gall — sometimes on other oak species.
You’re unlikely to ever see any of these small wasps unless you raise them yourself, so we know them by their work, the galls themselves.
I’ve logged 26 species of Cynipidae gallwasps in iNaturalist. Twenty-four of these have been in Brooklyn, all in Green-Wood Cemetery but one. (Years ago I also found an oak apple on a Cobble Hill sidewalk, but don’t seem to have preserved any memorial of it. Got my eye out for them now.)

Cecidologist: one who studies galls.

All the gall posts here at B & B.

1 Response to “Notes on Galls”


  1. 1 gaiainaction October 4, 2020 at 3:00 pm

    Very interesting. I have an oak tree in the garden where I see galls and I always wondered what they were.


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