Oak Galls and Gall Wasps


A gall after the emergence of the adult insect through the excised hole.

Common Name: Gall, Oak gall, Oak apple – the word gall has several applications with different etymologies including the yellow, bitter fluid of the gall bladder, a sore or wound, and, as a verb, to wear away by friction. In the botanical application, it refers to the swelling of tissue of a plant caused by fungal, bacterial and particularly insect parasites; it derives from the Sanskrit glau, a round lump.
Scientific Name: None – As a growth on an extant plant species it is not separately classified

Potpourri: A gall is a generic term for any abnormal outgrowth of plant tissue that is usually although not exclusively spherical. It is caused by the reaction by the plant at the cellular level to a fungal or bacterial infection or to insect incision irritants. The most common types are crown galls caused by the bacterium Agrobacterium tumefaciens, black knot caused by the fungus Apiosporina morbosa, Cedar-apple rust caused by the fungus Gymnosporangium juniperi-virginianae, and oak galls caused by gall wasps; the latter is the subject here.[1]

Gall wasps are in the family Cynipidae, insect arthropods of the order Hymenoptera which consists of bees, ants, wasps and sawflies. The hymenopterans are among the most ubiquitous of the animal kingdom, having evolved through flight and ecological dispersion to occupy every available niche; one tenth of the estimated one million species of insects are in the order, about 100,000. The eusocial colonies of ants and bees have long been the subject of fascination among naturalists and philosophers alike seeking to understand how an organism having a pollen-sized brain could erect, maintain, and defend a nest while feeding a population of thousands with coordinated food forays. While there are also some communal wasps, the yellow-jackets for example, the vast majority are independent operators with a wide range of adaptive attributes. Some have become necromancers, employing secret potions to chemically take control; the life cycle innovations of ichneumon wasps border on diabolical. [2]

Eat and reproduce before being eaten are necessary but not sufficient conditions for a species to survive and evolve. Sufficiency mandates that at least one offspring live to maturity in order to complete the cycle. Most wasps are carnivorous predators, employing their reusable sting to kill or immobilize their largely arthropod prey to sustain themselves and their progeny. A plump caterpillar carcass would seem the ideal place to lay eggs so that the emergent larvae would have ample food to grow and pupate to adults. True except for the reality of the food chain; a plump caterpillar carcass filled with plumping larvae left unattended is a tempting target for anything big enough to eat both the carcass and its resident brood. The ichneumon wasps evolved a work-around in the employment of an insidious subterfuge – the mobile meal. Their success is evident – there are 3,300 ichneumon species in North America and twenty times that many globally, each targeting a single or small group of insect host species. [3] The wasp eggs that are deposited externally hatch as larvae that burrow to seek the warm nourishing recesses of the host’s interior organs. With mindless yet cunning cruelty, the larvae consume the vital organs last to prolong the meal in a drawn out death by a thousand cuts. The evil implicit in this act troubled Darwin, who wrote “I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidae with the express intention with their feeding within the living bodies of caterpillars.” [4]

The cynipid wasps employ a different but equally subversive method to protect and feed their offspring, producing chemical irritants that induce plants to form anomalous growths called galls that range from rounded humps on leaves to globular attachments to twigs. The eggs deposited hatch as larvae that survive inside the growing gall by eating its interior but preserving the outer, papery skin as a protective shield. After metamorphosing as pupae to adults, they depart by chewing an escape hole to find a mate and eventually lay more eggs to make more galls on more plants to complete the life-cycle. It is not quite as simple as just gall wasps making gall apple nursery rooms; the Class Insecta is the epitome of random mutation and evolution. Using galls as palladia is a successful survival skill; other gall forming insects include flies (682 species known as gall gnats) that form galls on wheat and legumes, especially clover; beetles that form galls on maples, poplar and Virginia creeper; and moths of the genus Gnorimoschema that form galls on goldenrods and asters. And not all wasps in the family Cynipidae do – some parasitize other insects and there is at least one that is an internal parasite of one species of braconid wasp that is itself a parasite of plant lice; biological Matryoshka Dolls. [5] However, nature’s turf wars for habitat dominance produce winners and losers; oak trees are the climax species in many deciduous forests of the northern hemisphere and there are about a thousand species of cynipid wasps. The result is a plentitude of oak galls, a matter of profound importance to human civilization. Almost all of the ink used in Europe from the Middle Ages through the late 19th century was made using oak galls. [6]

Oak Gall White Oak Canyon 150530
When first formed, oak galls look like small apples and are called oak apples

Oak galls have high levels of tannin, a key component of early indelible writing fluids. Tannins comprise between 5 and 10 percent of the dry weight of leaves, more than any other plant organic compound excepting the primary metabolites like cellulose that are necessary for plant growth and reproduction; tannin is a secondary metabolite. It is generally believed that the primary function of tannins is to provide some deterrence against insect herbivores. In nature’s hard-wired reality, all animals are heterotrophs and must eat either each other or the autotrophic plants, which must defend themselves to survive; since they can’t move, thorns, thick bark, tannins and poisons must suffice. Experimentally, it has been shown that tannin’s complex polyphenol compounds oxidize in insect digestive tracts resulting in the production of harmful free radicals. While this certainly deters some insects, others developed tolerance like the cynipid larvae that thrive in spite of it. [7] This posits a reasonable hypothesis for gall formation – a wasp injects a chemical signal that incites the plant to pump protective tannin to the affected area which swells into a fruit -like spherical growth. The appellation gall for an abnormal plant growth was certainly the result of the bitter taste that tannin imparted to those who bit into what literally looked like an oak apple; the bitter bile of the equally descriptive gall bladder.

The historical record of technological milestones like the invention of the wheel is nonexistent; this is particularly true of inks with which that record would have been written. Carbon compounds were the original fluidized writing compounds dating to 2500 BCE, a logical extension of using the charcoal of burnt wood to draw on any light colored surface. The first record of a tannin based ink is in the early Roman Empire; Gaius Plinius Secundus described a technique for soaking papyrus in a tannin solution and applying iron salt to produce a consistent black dye. The first account of using oak galls as a source of tannin for inks was documented in the Encyclopedia of Seven Free Arts by Martianus Capella in fifth century Carthage. By the Renaissance oak gall ink prevailed and was the primary ink up until the early 20th century when chemical dyes became available. The Dutch East India Company had a strict specification for the inks used in trade documents; the German government required oak gall ink until 1974. [8] The dark color of the ink is from the precipitate iron tannate formed by the combination of iron sulfate to gallic acid, a derivative also called gall tannin as the name suggests. Iron or ferrous sulfate (FeSO4•7H2O) was a byproduct of mine operations and was used historically in leather tanning and textile treatments. One of the byproducts of the chemical reaction is sulfuric acid, a corrosion chemical. This has been an historical issue because oak gall ink was used in pen and ink drawings by artists including Rembrandt and Van Gogh whose works therefore pose serious preservation problems. [9]

Interest in oak gall wasps was sparked by speculation as to the origins of the oak galls used in ink manufacture. Initially, oak galls were thought to be a function solely of plant physiology, an innate oak tree process with a practical purpose; a seedless fruit perhaps.[10] Even when insects became implicated, there was still doubt as to the specific mechanisms employed that resulted in gall incipience. [11] The debate among entomologists was whether adult female gall wasps injected a chemical in the process of egg laying that incited gall formation or whether it was the hatched larvae whose chewing induced them. In the late 19th century, a German researcher named Herman Adler conducted a series of detailed experiments with gall flies and demonstrated that it was the larvae or grubs of the insect that activated the plant to action noting that “if a grub dies before the gall is fully formed its growth is arrested.” He also discovered an unusual biennial life cycle of two alternating adult forms with one generation of all females and an alternating generation of both sexes. Further, the daughter generation looked like and made the same kinds of galls as the grandparent, but not the parent. [12] The complexity of gall wasp society became even more complicated when it was determined that “each different species of Cynipidae causes a different but entirely characteristic gall to form on the same host plant.” [13] Alfred Kinsey achieved international repute in his studies of human sexuality that culminated in the publication of the “seminal” book Sexual Behavior in the Human Male (and later female). He began his career at Indiana University after receiving a PhD from Harvard studying the unusual sexuality of gall wasps and was among the first to link taxonomy with genetics. He started his studies on human sexuality due to questions posed by his students that he could not answer, opening the Institute for Sexual Research in 1938. [14] While it is not clear that his students were asking sexual questions about wasps, it is certainly true that his curiosity was aroused by the peculiarities of both.

While there has been some progress in resolving the confusing taxonomy of the cynipid wasps, there are many unresolved details at the species level. In general, a two-year life cycle starts with eggs deposited at a selected location on a specific plant from which a gall arises due to either the insertion of the ovipositor or from the saliva of the hatched larva varies according to wasp species. Once a gall is formed, the larvae feed entirely on its contents and there is no additional damage to the tree; gall wasps are not pernicious parasites and the host tree for the most part will remain healthy [15] However, there have been instances of severe gall infestation from invasive wasp species; Hawaii’s native wiliwili tree has been nearly extirpated by a species of gall wasp that originated in Taiwan. [16] The gall wasps hit upon what seemed the perfect solution to predation in having their host plant produce both food from and shelter within the confines of the protective gall. Wasps being wasps, however, there is no safe sanctuary. The euphemistically named crypt-keeper wasp (Euderus set) lays its eggs on an established oak gall with its embedded feeding gall wasp larvae. The hatched E. set larvae penetrate the gall and take control or the growth and development of its occupants in a manner not well understood. With waspish treachery, the victimized gall wasp larvae become too large to escape through the hole drilled in the papery gall envelope for that purpose, becoming lodged with their fattened heads stuck in the exit hole At this point, the crypt having been established, the crypt -keepers consume the body of the host gall wasp larva and emerge by eating through their heads to escape. [17] One wonders what Darwin might have thought of wasp cannibals had he known of them. Nature has no morality beyond survival.

1. Benton, H. ed. The New Encyclopedia Britannica, 15th Edition, Micropǣdia Volume IV, London, 1973, p.388
2. Milne, L. National Audubon Field Guide to North American Insects and Spiders, Alfred A. Knopf, New York, 1980, p. 339, 801, 813.
3. Marshall, S. Insects, Their Natural History and Diversity, Firefly Books, Ontario, Canada, 2006. pp 518-521, 532-533.
4. Haskell, D, The Forest Unseen, A Year’s Watch in Nature, Penguin Books, New York, 2013, pp 143-145.
5. Milne, L. op. cit.
6. Dicks, L. “Small wonders” Science, Volume 365, Issue 6451, 26 July 2019, p. 329
7. Barbehenn, R. and Constabel, C. “Tannins in Plant-Herbivore interactions” Phytochemistry, Volume 72, Issue 13, September 2011, Pages 1551-1565 at https://www.sciencedirect.com/science/article/abs/pii/S0031942211000690
8. Eusman, E. “Iron Gall Ink – History” at https://irongallink.org/igi_index8a92.html
9. Lemay, M. Yale Traveling Scriptorium, 21 March 2013 available at https://travelingscriptorium.library.yale.edu/2013/03/21/iron-gall-ink/
10. Britton, W. “Our Insect Friends and Enemies” W. E. Science 27 Aug 1909: Vol. 30, Issue 765, p. 283. Quoting Professor of Entomology John Smith at Rutgers University.
11. Cook, M. “The Origin and Structure of Plant Galls” Science 5 January 1923: Vol. 57, Issue 1462, pp. 6-14
12. “Garden Destroyers – Gall Flies of the Oak “Cynipidae.” Scientific American, 18 November 1882. The article is signed G. S. S. in the Garden.
13. Britton, W. op. cit.
14. Payne, F. “A. C. Kinsey, Zoologist” Science 2 November 1956: Volume 124, Issue 3227, p. 881 One of Kinsey’s obituaries.
15. https://ento.psu.edu/extension/factsheets/galls-oak
16. Gramling, C. “Hawaii’s Coral Trees Feel the Sting of Foreign Wasps” Science 16 December 2005, Volume 310, Issue 5755, pp. 1759-1760.
17. Frederick, E. “‘Crypt keeper’ wasp brainwashes far more victims than thought” Science 24 September 2019.