Robin’s pincushions.
NATURE NOTESI EXPECT many people will be familiar with the sight of a so-called 'witch’s broom’ in a birch tree, and perhaps also be aware of the existence of oak apples or Robin’s Pincushions. But do you know what causes them?
These are all types of plant galls that can be found on hundreds of different plants. You may have seen trees and plants with little circular patches on their leaves, or oddly shaped and swollen leaves, and not realised that what you are looking at are likely to be varieties of these galls.
A gall is an abnormal growth that develops as a reaction to a parasitic attack of some sort or other. It may be caused by the presence of eggs themselves or by a fluid injected by the egg-laying creature.
Usually the parasite is some form of insect or mite and the gall forms the protective covering for its eggs but fungi can also be the cause of galls, and so can viruses and bacteria.
Galls can form on any part of the plant — roots, seeds, flowers, leaves, fruits and stems — and some plants are more susceptible to them than others.
Willows and oaks are particularly good hosts.
Many galls grow as a result of eggs which are laid in the spring, and are therefore more obvious in the late summer or autumn.
However, oak apple galls can be seen earlier in the year because of the very complicated life-cycle of the particular gall wasp that forms them. Each variety of wasp produces a different type of gall. Oak apple galls are good examples of the 'double’ life cycle which produces the growth shown in the photograph.
After mating in the summer, winged female gall wasps burrow down to the thread-like roots of the oak tree and lay their eggs on them. The eggs mature inside a root gall, and the wingless larvae then emerge between November and January.
These larvae are all the same sex and are able to climb the tree and lay eggs, without fertilisation, in the dormant tree buds.
In spring the affected buds swell rapidly to resemble a soft, pinkish, spongy mass which, if cut open, will reveal numerous tiny chambers.
The wasp larvae develop inside these chambers and emerge through small holes in late summer. These wasps have wings, and they mate, repeating the cycle.
One oak tree may be the host of a variety of different galls, for example: hard dark-coloured marble galls and hop (or artichoke) galls on the twigs; little bunches of currant galls on the catkins; spangled and cherry galls on the leaves.
All appear to have this same double life cycle pattern so that, for example, the wasp that produces the hop gall is also responsible for some of the galls that appear on catkins, and the wasp that results in spangled galls on the leaves, produces the currant galls in the spring.
Robin’s Pincushion gall, also pictured, can be found in late summer on field and dog roses and may hold as many as 60 larvae.
The life cycle is less complicated; the adult wasps emerging from the pin cushion in May, having overwintered in the gall, and then repeating the cycle by laying their eggs in unopened buds.
If you are interested in galls, there is a web-site devoted to them (www.british-galls.org.uk/index.htm) on which you can learn more.
For a simpler guide, try www.uksafari.com/index.htm where you can purchase a fold-out laminated identification chart to 69 of the most common galls found in Britain, together with more information about them.
Happy hunting!