Week 24: Wool Sower Wasp, E.P. Felt, Francesco Redi of Arezzo

Cover Image. “Oak Galls,” from Millett Taylor Thompson, An Illustrated Catalogue of American Insect Galls, ed. E.P. Felt (Nassau, New York: Rensselaer Co., 1915), p. 71.*

Waxing Crescent Moon; 3 Days to Summer Solstice (I missed last weekend so will be double-posting this week).

Headnote

We thought they were fungal. Or maybe even artificial.

Crystallized dandelion seeds.

Speckled balls of styrofoam.

Pink and white lollipops.

The confusion was not ours alone. Since at least Aristotle, who held that insects could arise spontaneously from rotting vegetable matter, it was widely believed that galls and the insects that emerged from them were products of the tree itself—a kind of arboreal spontaneous generation. Even Francesco Redi of Arezzo, whose controlled experiments on rotting meat in 1668 are credited with first seriously challenging the doctrine of spontaneous generation, still believed that the oak galls were essentially eggs laid by the tree itself.

But no, they were the galls of the wool sower wasp (Callirhytis seminator), a harmless, parasitic wasp that chemically manipulates white oak trees into creating edible nests for its larvae. The hijacking happens in stages. Unlike the fig-friendly family of Agaonidae wasps, gall wasps do not pollinate the tree’s fruit to support its growth. Instead, a female wool sower wasp injects its eggs into a bud on the oak tree along with a maternal fluid that dissolves some of the plant cells, suppresses the tree’s defense systems, and seemingly halts the growth of the bud. When the larva hatch, they begin to secrete enzymes and other chemicals of their own, which hormonally alter the tree’s growth patterns and change nearly a third of its genetic expression. Now, instead of growing branches, buds, and leaves, the tree instead creates a novel structure specifically tailored to feed and protect the larva. In fact, the entire tree system is hijacked in service of the gall and redirects nutrients away from the leaves and fruit, funneling them into the gall to sustain the developing wasp.

The process is similar, but with many interesting variations, across the approximately 1400 members of the Cynipidae family, although some use the galls created by other gall wasps (as inquilines) rather than create their own. The cecidiologists who dedicate themselves to the study of plant galls are like kids in an alien candy shop filled with strange slimes, jellies, hunks, orbs, and pellets. And the gall wasps themselves are as surprising as their genetically pirated abodes. For most members of the Cynipidae family are heterogonic, alternating their reproductive characteristics each generation: while the first generation reproduces sexually, with male and female individuals, the second generation comprises all females, which reproduce asexually through parthenogenesis—a sort of cloning or virgin birth that produces a mix of female and male offspring, all without fertilization. In fact, some species have done away entirely with sexual reproduction and are made up only of females. For the wool sower wasp, its fuzzy lollipop gall incubates the asexual generation of larva; but no one yet knows what form of gall is created by its sexual generation.

This strange science also precipitated an unexpected cultural turn. For we owe much of European manuscript culture to the brown spheres produced by the asexual reproduction of the marble gall wasp (Andricus kollari) and other branches of the Cynipidae family. For approximately 1500 years, from the fall of the Western Roman Empire until the 20th century, these hard, nut-like galls became the primary media of textual culture, crushed and used to create an acidic ink that could permanently mark parchment, vellum, and paper. Notable documents from the text of Beowulf and a plethora of monastic manuscripts to the Magna Carta, Leonardo da Vinci’s notebooks, and The Declaration of Independence were written in oak gall ink. Ironically, though, the acid that made this ink so indelible is now eating away at these documents, threatening to erase the textual record it enabled in the first place. Instead of spontaneous generation, the archivists now are dealing with spontaneous corrosion.

Fig. 2. Wool Sower Wasp Galls, Tisbury, Massachusetts (May 25, 2026).

Readings

1. Before going further, let us glance for a moment at the different types of insects possessing this gall-making habit.
   
   The Hymenoptera, best known because of the industrious honeybee, has two important families, the Cynipidae and the Tenthredinidae, members of which live in this questionable manner. The first named are minute, four-winged gall flies with legless white maggots. They are moderately numerous in species and remarkable for an alternation of generations: the structural variations between the adults in different generations being so marked, that before the relationship was suspected, they were referred to separate genera. Certain Cynipids or gall wasps are believed to reproduce only by parthenogenesis. These little insects display a marked partiality for oaks and roses and produce striking types of galls, such as the cortical swellings of the gouty oak gall, a species occasionally becoming so abundant that five hundred thousand individuals may be reared from one tree and its conspicuous galls form giant, bead-like swellings on almost all the smaller branches of a large oak. Occasionally the peculiar bud-like swellings of Andricus gemmarius Ashm. are very abundant on pin oaks and the sweet exudation issuing therefrom attracts hosts of bees, flies and similar insects. Another oak gall occasionally numerous is the oak leaf stalk gall.? The gall of the wool sower? is another striking type and results from the female depositing eggs in a ring of buds around white oak stems, and from the series of wounds inflicted, there develops a seemingly delicate, globose, white, pink-spotted mass which on examination is found to consist of numerous cells, each supported and guarded by a thick fungoid, hairy growth. A more ordinary type may be seen in the familiar banded bullet gall,* a representative of a considerable series generally known as “bullet galls.”

E.P. Felt, “Gall Insects and Their Relations to Plants.” [Gaul Insect Papers]. Np, n.d. Rpt. The Scientific Monthly (June 1918: 509-525). pp. 510-511.

2. [I]f I must disclose my real feeling in the matter, I would state my belief that fruits, vegetables, trees and leaves become wormy in two ways. One way is that the worms come in from outside, and, seeking food, gnaw a path to the very heart of the fruit and the wood. The other way, which I esteem worthy of credence, is to be found in the peculiar potency of that soul or principle which creates the flowers and fruits of living plants, and is the same that produces the worms of these plants. Who knows? Perhaps many of the fruits of trees are produced with a secondary, rather than a primary purpose, not as pre-eminent in themselves, but as objects of utility, destined as a matrix for the generation of these worms, which remain in them for a determined length of time, and thence come forth to enjoy the sunshine.

   I think that my idea will not seem paradoxical to you, if you consider the great variety of growths, such as glands, galls, knobs, warts, etc., produced by oaks, holm oaks, live oaks, and other acorn-bearing trees. In the hairy tufts of the oak, and in the woody tufts of the ilex, and in the galls of the holm oak leaf, it is manifest that the first and principal intention of Nature is to create therein a winged animal, for there is an tgg in the interior of the gall, and this egg enlarges and matures in proportion to the development of the gall, and finally gives birth to the worm, which, when the gall has reached maturity, becomes a fly, that breaking the egg and commencing to gnaw the gall, makes a narrow and always round road from the center to the circumference, and abandoning its native prison, escapes and flies boldly away in search of food.
   
   I confess to you frankly that before making these experiments in the generation of insects I believed, or rather suspected, that galls were originated by the fly[…] Hence, I have changed my opinion, and I think it probable that the generation of worms in trees does not occur fortuitously, nor does it proceed from the eggs deposited by flies, especially as every gall or growth has its own peculiar kind of worm, gnat, or fly, which never varies. It is wonderful with what consummate skill Nature forms the egg and prepares a place for it ; admirable is the industry and patience with which she surrounds it in a network of fibres and filaments, connecting it with the gall, like so many veins or arteries, which furnish the necessary supply for the formation of the egg and the worm, and the indispensable nourishment of both.

Francesco Redi of Arezzo, Experiments on the Generation of Insects (1688), trans. Mab Bigelow (Chicago, Open Court, 1909), pp. 91-92, 94.

Supplementum

• Adam Kranz, et al. Gallformers.org.

• Joe Boggs, “Cotton Balls on Oak Trees” (May 28, 2024) and “Oak ‘Apples’ and the Gall-Making Process“ (May 12, 2017). Buckeye Yard and Garden Online, The Ohio State University.

• “Making Manuscripts: Oak Gall Ink,” The British Library (November 22, 2018).

• Nathalie Roy, “Make Ancient Ink With The Help Of Oak Tree Parasites,” Science Friday (August 21, 2024).

• Usha Lee McFarling, “Making Ink from Oak Galls,” The Huntington Library and Museum (May 1, 2019).

Fig. 3. “Wool Sower Gall,” from Carl F. Gronemann, Fifty Common Plant Galls of the Chicago Area, Botany Leaflet 16 (Chicago: Chicago Field Museum, 1930), p. 13.