A Lot of Gall(s)
Coercion and Plants
Everybody knows that oaks produce acorns, at least I hope that’s true, although after 45 years of teaching I am not convinced that it is. But sometimes oaks produce what look like oranges. The ancients thought they were apples. We have no record of why the ancients thought that an oak could produce apples, but the name still sticks--- apple galls, and these are my Found Objects for this essay.
Of course, galls are neither apples nor oranges, but growths that the oak produces in response to some kind of coercion, some magic trick played on the oak by a gall wasp (Family Cynipidae). A large apple gall is an attractive Found Object, and I have made drawings of them more than once. Just how the little female gall wasp or her larva makes the oak do their bidding is uncertain--- perhaps some shamanic substance--- but however it comes about, the female lays an egg in an unfolding leaf or stem, and the oak responds by forming a protective gall around the larva, who then develops inside the protection of the gall, eating its tissue. Although cynipid gall wasps of various species use a range of plants, they are especially fond of oaks, and 70% of the 800 North American cynipid species reproduce on oaks.
Occasionally, gall wasps get really busy and induce dozens of galls on a single tree, but usually there is only one or a few on a tree.
Opening such an "apple gall" simply deepens the mystery of how a little wasp can induce a plant to make so intricate a structure on demand. What control panel, what buttons, what code does the gall wasp have access to? For inside the hard outer shell of the gall is another very much smaller, hard sphere suspended exactly in the center of the gall's volume by delicate filaments between the inner walls of the gall and the central sphere in which the larva lives. It looks like one of those electrostatic spheres on top of a Vander Waals generator, throwing of sparks in all directions. Is this a ploy to foil potential burglars that penetrate the outer skin, only to find themselves wandering aimlessly in a confusing forest?
You might think that living, as it does, inside a protective gall, the larval gall wasp is safe, but you would be wrong, for there is nothing in biology that is not an opportunity for exploitation by some other living thing. Thus, for apple galls on oaks, another species of wasp, one whose egg-laying tube is hardened (i.e. the right tool for the job), bores its way into the gall and lays an egg on the (defenseless) gall wasp larva whose fate is then to be rapidly consumed by the parasite. No doubt, a large fraction of a mother gall wasp’s offspring dies in this way, their substance moving to the next higher rung on the trophic ladder. Should the larva escape parasitism to survive and develop through the pupa, the adult (see image above) emerges and cuts its way out of the gall to start the cycle over.
Each species of gall wasps creates different but equally structured galls for their larvae (or more precisely, they induce the host plant to make this structure), and as a result galls range enormously in size, shape and elaborateness, depending on the wasp and host plant species. Cynipids are far from the only creatures that cause plant galls. In fact, considering that other gall wasps, mites, sawflies, midges, flies, aphids, scale insects, psyllids, nematodes, bacteria, and fungi all contain gall-inducing species suggests that plants have a serious computer security problem. A complete treatment of gall-forming creatures would require a thick book. The many species that have hacked into the plant system suggests that it probably isn't very hard. Or maybe it is just that there are so many insect species and so many plant species that code-breaking by some pairings is inevitable. After all, repeated hacking attempts are standard in the hackers' playbook.
Once you become aware of galls in general, you encounter them often. I drew these willow galls while camping on the Feather River in northern California. They were probably made by sawflies, not gall wasps, but I didn’t open one to check. In any case, there was a fair chance that had I opened one, I would have found a parasite rather than the gall-former.
Gall-forming can be a temporary phase of a life cycle. Every spring, the pecan tree in our back yard sprouts numerous red galls on its leaves (below). Careful inspection reveals that these galls have a tiny opening on the underside of the leaf. The galls were formed by the ballooning of leaf tissue upward to form a hollow ball. Breaking open such a gall shows them to contain a dense crowd of aphids, all of which have their proboscis inserted into the inner wall of the gall, sucking sap and growing and reproducing at an insane rate. Except during fall and winter, most aphids are all female and reproduce only females by parthenogenesis, with a mother aphid’s granddaughter already growing in the body of her daughter who is also still within the mother’s body--- a telescoping of generations that results in a very high rate of population increase, an increase that is driven by the fact that aphids are everybody’s lunch, a consequence of being very good at converting plant sap into animal tissue. For aphids to exist at all, their reproduction must exceed the appetites of the creatures that eat them. In the case of the pecan aphids, they get a head start on the reproductive season by producing the first generations inside the protective leaf gall, but then become free-living on other host plants later in the spring. To find their next host plants, some daughter aphids develop wings, circled in the image below. Keeping a step ahead of predators and parasites ain’t easy. Just ask an aphid.
Another Found Object gall has been in my collection for decades, that of the (aptly named) goldenrod gall fly (Eurosta solidaginis), related to fruit flies. Goldenrods are very abundant in old fields throughout the eastern USA, putting on a brilliant yellow pageant every fall. Gall flies lay their eggs in the buds of the growing goldenrods during spring and summer, and the hatched larvae induce the formation of the conspicuous galls, feeding on gall tissue inside their snug little homes. The mature larva burrows out to the “skin” of the gall in the fall but does not break through. Rather, it pupates in this location, ready to emerge as an adult fly in spring. Adult flies don’t have chewing mouth parts, so they cannot chew their way out. Rather, they evolved a ptilinum, an inflatable pouch on the head that allows them to pop off the end of their puparium, and presumably to rupture the gall skin.
Gallfly larvae and pupae in their galls gotten a lot of scientific attention because they have no way of escaping winter conditions and are exposed to the full fury and frigidity of this season. Whatever the temperature is outside the gall, it is inside too. Because they are ectotherms, there is nothing these little creatures can do to keep warm. As the fall temperatures drop, the fly secretes glycerol, sorbitol and other antifreezes into its blood. It also secretes cryoprotectants to coat the proteins in its cells and moves water from its cells into the intercellular space. In this way, the fly first lowers its freezing point as it gets colder, and when this is no longer possible, it simply freezes solid into an ice glass (non-crystalline ice) so that expanding ice crystals don’t damage its cells. After that, it doesn’t matter how cold it gets--- let winter do its worst! The fly seems to have taken its cue from the song,” My Logger Lover”,
The weather tried to freeze him, it tried its level best,
At a hundred degrees below zero, he buttoned up his vest,
It froze clear down to China, it froze to the skies above,
At a thousand degrees below zero, it froze my logger love.
A similar situation must confront the caterpillars that live inside a stem gall of fireweed in the San Juan Mountains of Colorado. Inside the gall, the caterpillars merrily chew and defecate away, and when they are ready to pupate, they chew an exit hole, but don’t exit. Instead, they close the exit with silk, then spin themselves a silk cocoon and pupate. In this way the adult, which is without chewing mouth parts, can still escape when its time is upon it. Whether the pupa also overwinters in this exposed gall and faces the same conditions as the goldenrod fly, I don’t know. Most likely.
If you like to make collections of interesting objects with some unifying theme, galls would be a good choice. Combine the thousands of species that are able to induce galls with the thousands of plant species that respond by making galls and you have a limitless pastime that can keep you occupied well into your old age. I have resisted making such an intensive, comprehensive, time-consuming collection, but so far, I have not been able to resist old age. Maybe I should have started that collection a long time ago….
Another wonderful essay, with clear explanations and photographs. Your drawings of course are beautiful (I am proud to have some of them on my walls at home). When I read these lovely essays that illustrate the complexity of life I am struck by wonder. (proof that one does not have to be religious to feel awe)
You've certainly got a lot of galls, for which I'm thankful. These relationships between/among species are intriguing, indeed.