The cycle of nature: Oak gall ecosystem includes tales from the crypt

Photos Tim Carl LLC

The cycle of nature

Oak gall ecosystem includes tales from the crypt

Oak 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

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

brazen or refer to boldness coupled with impudence and insolence. It can also mean bitter or suggest a bitterness of spirit or bile (e.g., the gallbladder’s fluid). As a verb, gall signifies irritation, vexation or being worn away by friction — as in, “The loose saddle galled the horse’s back.” As a noun it means an abnormal outgrowth of a plant induced by a parasite or a sore on the skin or exasperation. Its origins are from Middle English galle and Old English gealla, galla, which goes back to Germanic gallōn, Galla — each a derivation of Greek, chlōros (greenish yellow) and Sanskrit, hari (yellowish).

Bile is a yellow-green fluid made and released by the liver and stored in the gallbladder. Bile helps with digestion by breaking down fats into fatty acids, which can be taken into the body by the digestive tract. If you vomit for a long time you may see bitter bile coming up.

Euderus set is a parasite to the parasitic gall wasp. As you might know, scientists use the genus and then species for taxonomical ranking of organisms. Therefore, in this case, for this wasp, Euderus is the genus and ‘set’ is the species. As for some foreshadowing, Set was the Egyptian god of chaos and war and often infected those he touched with pain and death. His partner was the goddess Tawaret, a hippo-headed deity who presided over fertility and childbirth. In one story Set locked his brother Osiris in a crypt so that he might perish.

The female E. set uses a tubelike ovipositor to place her egg into the developing oak gall. There a tiny juvenile E. set excretes an unknown chemical that causes the juvenile gall wasp to start drilling a hole up and out from the center. The problem is that the hole this mind-controlled gall wasp creates has a taper, so that what was supposed to be an escape route ends up being a trap that is too small for the gall wasp to fit through and fly away. Just as the gall wasp sees light, the hole becomes too small. As it pushes farther, it only becomes more entrapped. Instead of building an escape tunnel, the gall wasp has been manipulated into building its own grave.

Deeper inside, in the heart of the oak gall, the little E. set takes its time. Eventually, after it has grown to its full size, the slightly smaller intruder begins to ascend the predrilled tunnel, eating its way up through the body of its host before emerging into daylight. These findings were first documented in a 2017 study published by the Royal Society titled, “Tales from the crypt: a parasitoid manipulates the behavior of its parasite host.”

Next time you see an oak gall lying on the ground, have a look and see if you might notice a small round hole in its exterior. If you peer more closely, you might just see the remnants of a gall wasp’s body ringing the diameter. At the very least, you will be observing a fascinating world that scientists are just beginning to understand.

When I drill down into nearly any subject I normally find many more topics of interest and unanswered questions. The very fact that the world remains full of mysteries fills me with an intense hunger to look more closely and mindfully at what is around me. When I do I experience equal parts of wonder and awe.