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The cycle of nature Oak gall ecosystem includes tales from the crypt
O
ak apples, sometimes called oak potatoes, are actually galls. Galls are abnormal outgrowths of plant tissue — brown, smooth-skinned balls about the size of a small apple — that are caused by parasitic insects, mites or fungi. In the Napa Valley most of the visible galls are found on or around California’s Valley Oaks, which are grand trees that might live for up to 600 years or more. Since I was a child I have been fascinated by oak galls. Back then I would often use my fingernail to carve out a face on the smooth globes, perhaps placing a small stick into a perfectly round hole that always seemed present. I had no idea what these strange airy spheres were, but I assumed they might be oak seed pods. In fact, what they are is a fascinating and complex ecosystem for the benefit of a small wasp. Oak 30 | DISTINCTIVE PROPERTIES
TIM CARL
galls are also an important source of tannin. So before we get to the wasps, first a word about tannins. Tannins, also called tannic acid, are hydrolyzable chemicals (those that break down in water) that are pale yellow to light brown amorphous substances in the form of powder, flakes or a spongy mass. They are widely distributed in plants and used chiefly in tanning leather, dyeing fabric, making ink and various medical applications. Tannin solutions are acidic and have an astringent taste, making them responsible for the astringency, color, and some of the flavor and texture in tea and wine. Tannins occur normally in the roots, wood, bark, leaves and fruit of many plants, particularly in oak species. Now on to the wasps. In North America there are more than 800 species of gall wasps (often called gallflies), each of which can cause different types of
galls to form on trees and plants. Worldwide there are over 1,600 different gall-wasp species. A female gall wasp injects her egg into the base of a developing oak leaf in early spring. The injection brings with it a hormone that causes a blister to form and then grow into a visible oak gall. Early on these are green and elastic, but within one to two years they dry into the hard, surprisingly lightweight brown balls with which we are most familiar. Galls created by other insects or different plants and trees can come in a vast array of shapes and sizes — from spiky to fuzzy, with some even appearing as tiny red cones on the undersides of leaves. Inside each oak gall is a single egg that will develop into a wasp. The embryo pupates into a juvenile wasp, eating the nutritious pulp from inside and carving out a center as it grows. Eventually the young wasp will bore a tunnel out of the gall so that it might
start the process all over again. Oak gall wasps are characterized by complex cyclically parthenogenetic life cycles. That is, there are times when these insects reproduce asexually (without a male). At other times or in subsequent generations the gall wasps might reproduce sexually (male/ female). Partheno comes from the Greek parthénos, meaning “maiden” or “virgin.” The rarely used adjective parthenic, meaning “virginal” or “pure,” also derives from this root. The Parthenon, located at the Acropolis in Athens, Greece, is the temple of Athena — the virgin goddess who is associated with wisdom, handicraft and warfare. Gall wasps are parasitic in that they manipulate the oak tree into raising, protecting and nurturing their young; however, oak galls are not believed to injure the oak trees in any meaningful way. The word “gall” can mean MAY 2021
