The Carlisle Mosquito Online

Friday, March 9, 2007


Biodiversity Corner: Oak Galls

Oak Galls
These are oak-bullet galls. The two outside galls show the holes where the wasp emerged.

An oak tree creates a gall when a tiny wasp in the family Cynipidae lays its egg in developing leaf or twig tissue. There are over 600 species of gall-making wasps in the U.S. and about 80% of them attack oaks. The type of gall differs according to the species of wasp. One of the easiest galls to find is the oak-apple which forms on the leaf and is usually over an inch in diameter, sometimes almost two inches. Another is the bullet gall which forms on the twig and is about two thirds of an inch in diameter.

When and where found. The bullet galls in the photo are beside the trail at Foss Farm near the entrance to the Great Meadows Wildlife Refuge. They are very hard and woody and have been there for at least two years. I also found some on March 4 in the Greenough Land and picked them to make iron-gall ink. The larger oak-apple galls seem to be more common. At this time of year they are pale brown and brittle. There are six on a small oak tree by the side of the road just a little south of the Maple Street bridge. The new season's oak-apples in May will be green and soft and by August they will have "ripened" to a reddish color.

Oak Bullet Gall

The interior of an oak-apple gall shows the capsule at the center where the larva lives. (Photos by Kay Fairweather)

Life cycle. Oak gall wasps have complex life cycles. Like last week's Shining Clubmoss, some species have alternate sexual and asexual generations. In a few species, the life cycle is independent of the existence of males. In many species, the female lays her eggs in the leaf buds or twig cells of next spring's growth. Some combination of plant hormones and chemicals produced by the developing larva cause the oak to form a gall. If you find a green oak-apple in May and cut it in half, you will see the wasp larva (a white grub) inside a little capsule suspended by threads in the center of the gall. The grub feeds on gall tissue and pupates in the protected world inside the gall. When it reaches adult stage, it chews an exit hole in the gall and emerges. Old galls that you find in winter all have a hole.

Iron-gall ink. Oak galls have been used for centuries to make ink. The black ink on the Dead Sea scrolls has been verified as iron-gall ink. Leonardo Da Vinci wrote some of his notes with it, and Bach too wrote many of his "notes" with it. It was very widely used for manuscripts and official documents right into the mid-20th century. Its advantage over the older carbon-based inks is that it binds with collagen in parchment and with cellulose in paper, making it indelible. Also, the ingredients were easy to come by and inexpensive. The downside is that it can be corrosive and eat away the document. Conservators around the world are trying to save important historic and artistic works.

Recipes for iron-gall ink seemed simple, so I decided to make some. You need only four ingredients: first some gallo-tannic acid which you get from the oak galls, some water, some iron, and finally some Gum Arabic as a thickening agent. I used a combination of bullet galls and oak-apples which I pulverized in the coffee grinder. I extracted some iron from a couple of old bolts by soaking them in a jar of vinegar and I substituted the Gum Arabic with sodium alginate. I also made a quill pen from a Canada Goose feather picked up at the Cranberry Bog. It worked reasonably well, so I wrote out the ink recipe using the ink and the feather I made (see photo). I would never have made it as a scribe. The most intriguing aspect of the resulting document is that I could dunk it in a basin of water — after the ink was dry — and it did not run. Nonetheless, I have a new appreciation for ball-point pens.

References. Borror and DeLong's Introduction to the Study of Insects, 7th edition; Ink Corrosion web site at

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2007 The Carlisle Mosquito