Have you ever seen round extrusions on oak trees that seem to hang like fruit? Oak galls, also called oak apples, are a common phenomenon produced by the oak trees' reaction to wasps that lay their eggs inside of the oak bark. Oak galls are high in tannic acid and have been used traditionally as medicine by many cultures around the world; they are also a source of dye and tanning material. Oak galls contain unique and potent properties that are just beginning to be studied by mainstream scientists for a host of useful applications.
Oak galls come in many sizes, shapes and colours but are all products of the oak trees' reaction to the larvae of certain wasps known as gall wasps. These larvae cause the oak tree to manufacture cells and substances that produce the gall and in turn the wasp larvae use the gall as both food and shelter. The galls usually do not harm the oak; however, the gall formation is a defensive measure by the oak tree and therefore contains strong natural astringent compounds such as tannic acid. In fact, according to Botanical.com, oak galls are the most astringent vegetable compound in the world.
Oak galls are used in Chinese medicine as a bitter warm remedy called moshizi, used for dysentery, ulcers and haemorrhoids among other things, according to Subhuti Dharmananda, PhD in a paper entitled "Gallnuts and the Uses of Tannins in Chinese Medicine." American Indians used poultices of ground gall nuts on sores, cuts and burns, according to Christopher Hobbs in "A Field Guide to Western Medicinal Plants and Herbs." Botanical.com recommends oak galls used as a tincture in cases of diarrhoea, cholera and gonorrhoea.
The high tannic acid content of oak galls also makes them a good source of tanning and dyeing material. Many tribal groups employed oak galls for a variety of decorative and curing uses including pottery, leatherworking and basketry.
Recently there has been an interest in using oak galls as a source of natural pesticide, as that is its original function and intent by the oak tree itself. In a 2009 article in the journal Parasitology Research, researchers at the University of Mysore in India found that an extract from oak galls had larvicidal activity against Anopheles stephensi, a type of mosquito that is responsible for the spread of malaria in urban areas.
Oak galls are easy to identify and are found worldwide in many different varieties. Collecting oak galls neither harms the oak tree nor hurts the gall wasp larvae once they have hatched. There are many possible uses for oak galls as an easily accessible source of tannic acid while out in the wild, and the possibilities for using oak galls in creative ways have yet to be exhausted.