23-Aug The Intricate World of Plant Galls

This well known quotation originating from Jonathan Swift aptly describes what may be going on in some of the galls we encounter. For instance, in the rose bedeguar gall which is one of the most brightly coloured of all it was a gall wasp in the spring that first laid an egg in a rose bud. This “attack” resulted in a mass of thread-like filaments growing, giving rise to the gall’s other name, Robin’s pincushion. The egg inside then hatches into a larva which in its small chamber feeds on the plant’s sap surrounding it. Other gall wasps may then choose to lay their eggs in the same pincushion. These are usually totally harmless co-residents and are called inquilines. However, parasitic chalcid or ichneumon wasps may choose to do the same. In this case their eggs hatch into larvae which either kill the original residents, eat them or we use them as a food source for their own developing larvae.

Not only this but they in turn may also be parasitized by hyperparasites. This means that the gall we look at could have had several different occupants all with varied life cycles. Looking for any differently-shaped exit holes can sometimes give a clue as to what adults eventually emerged from the gall, although a particularly large hole can just be where a bird pecked it trying to devour its contents.

Galls can be found the whole year but September is a particularly good time to look for them as many are now fully grown and often at their most colourful. Galls often change in colour. They usually start being paler where the original “attack” took place, then get greener, perhaps more yellow, brown, orange, red or even purple before eventually darkening with age. They are particularly common on our native trees and some of our native shrubs. For this reason they should be looked for in parks and churchyards where there is usually a good variety of different trees. For instance you may see nail galls on limes and maples, bean galls on willows, spiral galls on poplars, pouch galls on alders, witches broom on birch trees, roll galls on elms, blister galls on pears or artichoke galls on yew trees.

Oak trees are particularly rich in galls and at this time of year large infestations of knopper, marble, spangle and silk button galls can hardly be missed in places such as Wimbledon common. Others to look out for on oaks include oak apple, cherry, currant, cola nut and red pea galls. Shrubs such as hawthorn and broom may also be worth investigating and cabbage galls on box trees are usually common enough. Most flowering plants and even ferns are occasionally galled.

Interesting ones to look out for are lighthouse galls on ground ivy, the little black pudding gall on bracken and tassel galls on rushes. Other plants worth examining for the same reason are thistles, knapweeds, clover, speedwell, violets, nettles, sowthistles, hollyhocks and meadowsweet. One last most unusual form of gall is called fasciation. The plants involved are often seriously deformed.

Their stems may be swollen or more strap-shaped and their leaves reduced in size and yellower or redder than usual. Their branches can also be twisted, compressed or fan-shaped. Many different garden plants can suffer from this condition and some even die of it. However, some such as monstrosa and cristota ferns are so attractive they are encouraged by gardeners. The cause of these strange malformations is still not totally clear but it is thought viruses, bacteria and mites may all play a part as well as genetic abnormalities.