It’s the science of cecidology. Now there’s a Scrabble word for you! Cecidology is the study of plant galls (cecidia).
What are galls? They are a kind of growth or swelling on the external tissues of plants, fungi, or animals. When leading nature walks I am often asked about growths on leaves and stems. Many of these galls are often characteristic in size, shape, and color and can be easily identified. It’s what causes them that is not always apparent. Viruses, bacteria, fungi, insects, mites, and more, can all produce galls.
Plant galls caused by insects are usually a reaction to the larvae of midges (a type of fly), wasps, or aphids that tunnel into the leaf or plant stem. After being disturbed by the insect, the plant’s cells rapidly divide and often provide food and protection for the developing insect. Scientists don’t know all the details of how this happens. On a walk along the Farmington Canal Heritage Greenway in Hamden, I recently found an Oak Apple Gall blown off an oak tree during a recent storm. Oak Apple Galls are found in North America and Europe and form most often on members of the red oak group.
Here in the East, this gall is caused by the wasp Amphibolips confluenta. The wasp’s life cycle begins when it lays eggs underground among the roots of the tree. The eggs hatch and the larvae feed on these roots. They develop into pupae. Wingless adult females hatch, or eclose, in spring of the second year. They emerge from the ground and climb up the oak tree to the leaves. There the female injects an egg into the veins of a newly growing leaf. Chemicals and hormones released by these eggs alter the leaf’s typical growth, causing galls to form. The round galls create homes and food for the tiny wasp larvae and usually give protection from predators. In midsummer the larvae pupate, then the adults hatch and tunnel their way out. Males and females mate and females lay eggs in the ground to begin the cycle again. A video by Sir David Attenborough shows the fascinating life cycle of a British gall wasp species.
Oak galls are full of tannin. In Europe they have been used to make ink since the Roman Empire. Iron gall ink was made by mixing tannins with iron sulfate and gum arabic. It was the main medium used for writing from the Middle Ages to the early twentieth century.
In British folklore, if a “worm” (insect larva) is found inside a gall on Michaelmas Day, then it will be a good year. If a spider is inside, then it will be a bad year with food shortages and ruined crops. I guess spiders were persecuted long ago too!
Oak Apple Day is a former British holiday that was celebrated on May 29. It commemorated the restoration of Charles ll in 1660. Charles hid in an oak tree during the English Civil War.
You will often find different galls on goldenrods in our area. The Goldenrod Bunch Gall is caused by the gall midge Rhopalomyia solidaginis. This fly is a specialist on Canada Goldenrod (Solidago canadensis). The female Goldenrod Gall Fly (Eurosta solidaginis) lays her eggs in the stems of several goldenrod species, which then form the characteristic ball-shaped Goldenrod Ball Gall. The larva first tunnels an exit hole and then travels back to the center of the ball to spend the winter there. If you ever find a ball gall that looks like the hole has been enlarged, it’s usually the work of Black-capped Chickadees (Poecile atricapillus) and Downy Woodpeckers (Picoides pubescens), which eat these larvae.
Other galls include the Witch Hazel Cone Gall, which looks like reddish witch’s hats on the leaves of Witch Hazel (Hamamelis virginiana). They are caused by the aphid Hormaphis hamamelidis.
The mite Eriophyes cerasicrumena causes the finger-like galls on the leaves of Black Cherry (Prunus seratina).
Pine Cone Willow Galls really look like a pine cone. They are are usually found on the terminal buds of some species of Willows (Salix sp.) and are caused by the midge Rhabdophaga strobiloides.
Most galls don’t affect a plant’s health and it’s not necessary to control them. Only a few, such as those triggered by fungi, can actually damage trees by causing branch dieback. Remember that galls are part of the plant and the life supported by them are also part of the ecosystem.