If butterflies and dragonflies can be considered bird-watchers’ insects, then plant gallers are the botanist’s insects (although many galls are caused by mites and fungi), as identification of the host plant is essential to their identification. Although you are unlikely to see the small creatures responsible for the gall unless you open one up, the study of galls has many advantages: they don’t move; many can be easily seen; and they often are present on their host for a long time. Over the summer I will try and have a look at some of the galls on campus.
Although spotting galls is often easy, identification of some can be harder. The standard work is Redfern, Shirley & Bloxham (British Plant Galls – identification of galls on plants and fungi. Field Studies 10, 207-531, 2002). This was also published as a book by the Field Studies Council. It has been out of print for a while now, and is hard to get hold of. A revised edition has been promised for several years.
Although this book has excellent keys, and many black and white diagrams, its few colour illustrations are not that useful: Margaret Redfern’s latest book (Plant Galls, Collins, 2011) is excellent on the biology of gall-formers, but won’t help you identify them, and has rather fewer colour illustrations of galls and their formers than I would have hoped for. This takes me to two very useful and readily available books: Redfern & Askew (Plant Galls, Naturalist’s Handbooks 17, Richmond, 1992), which has keys to galls on many trees and some herbs, and some colour illustrations, and Darlington (The Pocket Encyclopedia of Plant Galls in Colour, Blandford, 1968), which with its combination of colour photos and coloured drawings is still the best photographic guide to galls we have. It also has useful tips on how to rear galling insects.
Cynips divisa
Now to today’s gall. The photo is of the Red Pea Gall, formed (like all galls, the gall is made from plant tissues, induced by the galler) by the oak pea gall wasp Cynips divisa on Quercus robur, taken in the Harris Garden last week. This gall is about 5mm diameter, a flattened sphere with a short stalk to the underside of the leaf. This gall is formed by the asexual generation of this wasp (many gall wasps have hideously complicated life histories) and contains one developing larva. In the autumn the larva will pupate inside the gall and an adult female will be formed. This matures in the gall and then bites its way out. It does not need to mate to become fertile, and seeks out oak leaf buds in which to lay its eggs. This induces the formation of another gall, the red wart gall, in the bud. Males and females emerge from these galls in the spring, mate, and eggs are laid in the veins of expanding leaves, which induces the formation of the red pea gall shown above.