TRANSACTIONS PLANT GALLS DR. G .
D.
HEATHCOTE
ALL animals are dependent upon plants in one way or another, but the animal-plant relationship is particularly close and complex in what we call 'galls'—structures produced specifically by plants in the presence of an animal (usually an insect, but sometimes a mite, an eelworm, or some other small creature). Some fungi and bacteria can also induce gall formation. Galls are seldom studied by the amateur naturalist, partly because we take them for granted ('marble galls' of oak are almost as common as acorns) but largely because the small creatures that cause them are difficult to identify. One other discouraging factor is the complex nature of the Community of insects that can be found in a single gall. Not only is the original gall-former present, but insects that parasitise it, or insects that simply use the gall as a home ('inquilines'). There may also be insects that parasitise the parasites ('hyperparasites'). Truly, 'greater fleas have little fleas upon their backs to bite 'em, little fleas have lesser fleas, and so ad infinitum!' The 'Robin's pincushion' galls on wild rose provide an excellent example of the 'complex' within a gall. These 'hairy' red growths, an inch or more across, are common in Suffolk. They are also known by the unusual name of 'bedeguars' (apparently from the Persian Ianguage). In each gall are several Chambers, with a single larva of a gall wasp (Rhodites rosae) in each. The galls also may contain certain inquiline gall wasps (Periclistus brandtii), or parasites (Orthopelma luteolator or Torymus bedeguaris) that attack the original inhabitants or the inquilines. A hyperparasite (Habrocytus bedeguaris) may be present, or other parasitic wasps (chalcids). About sixty per cent of all types of gall in Britain are found on oak, representing the homes of some 500 species of insects, and of fifty-four species of Cynipids alone. Recently I found three types of galls on oak within flve minutes at the roadside near Barton Mills. The best known oak galls are probably the hard brown marble galls previously mentioned. These contain the larvae of a gall wasp (Cynips kollari), one in each gall. Marble galls were unknown in Britain until about 1830, but then spread rapidly. They contain seventeen per cent tanic acid and were probably introduced for use in dyeing cloth. Cynips can reproduce without males ('parthenogenesis'). Virgin birth is also found in greenfly and some other insects.
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The softer, rosy-coloured 'oak apples' (not edible by the way) are produced by another gall wasp (Biorrhiza pallidä). Several larvae occur in each gall. The female adults are wingless but the males are winged. The females crawl down the trunk of the oak on which they grew and lay their eggs on the roots, producing hard, round, brownish galls. T h e adults that emerge from the root galls are all female and parthenogenetic, and they climb the trunk and lay eggs in a bud. An oak apple is produced and the complex life cycle is complete. Yet another gall wasp (Neuroterus baccarum) causes the formation of the common 'spangle' galls of oak which look rather like minute 'Aying saucers'. They appear in July and are reddish at first. Later, when hard and dry, they become detached from the oak leaf as it begins to wither in September, and fall to the ground. The wasp larva continues to develop inside the fallen gall and pupates in March, emerging as an adult in April. The adult wasp is less than 3 mm. long. Even smaller than the gall wasps are the mites which commonly produce pimple-like galls on the leaves of many plant species. Many are almost impossible to see with the naked eye. The galls they cause have an opening (unlike most wasp or gnat-produced galls before they mature) and characteristically are 'hairy', as the mites cause the epidermal cells of the affected leaf to grow outwards. T h e galls caused by the mite Eriophyes tnacrorrhynchus are exceptionally common and can always be found on sycamore leaves in my garden in Bury St. Edmunds. These are typical of miteinduced galls, only about 3 mm. long, pink at the tip, Standing up from the upper surface of the leaf. The 'hair'-lined entrance can be seen as a small depression on the lower leaf surface. Dozens of these galls can be found on a Single leaf. Similar galls can be found on alder beside the 'Nature Trail' in the King's Forest. Galls formed by aphids are almost as varied in shape as those produced by gall wasps, but feware of economic importance. The lettuce root aphid (Pemphigus bursarius) a pest to the gardener, also attacks poplar leaves, producing 'purse galls', usually on the midrib or the petiole. These galls are tinged with red and very variable in shape but from a swollen gall a 'spout' eventually develops from which the adult aphids emerge. The gall protects the developing aphids from predators such as ladybirds, but provide the aphids with a problem, for they cannot expel the sticky 'honeydew' they excrete. In fact this golden-yellow sugary waste is stored within the gall in droplets surrounded by wax, between which the aphids can move without getting stuck. Wax is also used by these aphids to give them protection against predators when they attack the roots of lettuce. They are found in a white fluffy mass. A relatively uncommon member of the Pemphigus
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PLANT GALLS
PLANT
GALLS
Marblc G i l l Robin's Pincushion —Rose
Pimple Galls - Sycamore
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group (P. spirothecä) occurs near Cambridge, producing twisted purse galls on black poplar. Perhaps better known are the less specialized red 'blister' galls on currants, caused by the aphid Cryptomyzus ribis. T h e upper surface of an affected leaf shows bright red blisters, and if the leaf is turned over aphids will be found feeding in the pocket formed on the lower surface. The blisters remain long after the aphids have gone (usually into the stomachs of ladybirds or larval hoverflies), but the white shrivelled shed skins of aphids can still be seen. The damage done to an otherwise healthy currant bush is probably not as serious as the appearance of the leaves suggests. Equally easy to find in Suffolk are galls formed on spruce by Adelges, close relatives of aphids. These galls consist of groups of flask-shaped Chambers which can be mistaken for cones. Two winged flies (Diptera) can cause plant galls. A gall of germander speedwell (Veronica chamaedrys) caused by Dasyneura veronicae was very common in Suffolk in 1966 (pers. comm. Mr. Derrick Martin). The terminal leaves are united at their margins by a mass of white 'hairs', and in the pouch so formed is an orangeyellow larva of this fly. Even a few beetles cause galls. A rather insignificant brownish weevil (Ceuthorrhynchus pleurostigma) causes roundish galls on the roots of cruciferous plants, a legless larva being found in each. These galls resemble 'club-root' disease of brassicas. 'Club root', or 'Finger and Toe disease' is of economic importance. It is a fungus that attacks turnips, swedes and brassicas growing in acid soil. The roots develop roundish swellings, totally unacceptable on a root vegetable, and root yield is reduced. Liming the soil will reduce the damage caused by this fungus (Plasmodiophora brassicae). Another fungus gall that troubles me in my garden is 'peach leaf curl', caused by Exoascus deformans. T h e young leaves develop thickened edges and become reddish. The disease can be controlled by using copper sprays at the right time. T h e list of plant galls is almost infinite and their shapes are often fascinating, but probably it is the actual formation of the gall that is its most interesting feature. No one has yet been able to make a gall experimentally, and it is certain that no simple injury can cause gall formation. It is thought that an auxin is produced by the gall-former, a 'plant hormone' which affects cell formation, slowing down or speeding up the growth of certain parts of the plant. These chemicals and the plant's reaction to them must be extremely specific, for quite closely related insects (often within the same genus) may produce very different galls on the same type of plant.
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Gall midges were the lifetime study of H. F. Barnes of Rothamsted Experimental Station, and his publications are readily obtainable; it is much more difficult to find reference books for galls produced by other insect groups and it is often necessary to consult foreign works, many of which are rather intimidating. E. W. Swanton's (1912) 'British Plant Galls' is most useful, and E. T . Connold's (1908) 'British Oak Galls' is pleasant reading. For the serious Student H. Buhr (1964 and 1965) has published tables for the identification of galls in Germany (listing the galls of central and northern Europe), but his books are expensive. There is a great deal we do not know about galls and they are well worth studying.