What causes these strange horny growths on tree leaves (see Photo)?
鈥� These 鈥渁ntlers鈥� are almost certainly galls. Many plants, both herbaceous and woody, react to irritants by producing a tumour-like growth around that irritant. The irritant may be a virus or bacterium, a fungus, another plant, a nematode worm or one of several groups of arthropods 鈥� insects and mites 鈥� which suck the sap or lay eggs on or in the plant. The larvae of insects that cause galls live in its centre, eating away happily. They will usually pupate when the gall falls to the ground into the autumn leaf litter and the adult insect emerges the following spring, in time to attack the following year鈥檚 spring growth.
Usually, the gall has a characteristic form related both to the plant species and to the irritant species. The same irritant may induce different shaped galls in different plant species and also at different stages of its life-cycle. The excellent British Plant Galls by Margaret Redfern and Peter Shirley, published by the (ISBN 9781851532148), runs to more than 500 pages and illustrates many hundreds of galls.
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So which gall is in the photo? There is no scale on the picture but the leaf is from a tree. It appears to be roughly egg-shaped with an irregularly and coarsely toothed margin. These features suggest the leaf is possibly from a poplar or lime tree. If that is so then the galls may be about 5 to 10 millimetres tall. British Plant Galls lists no galls of this general shape for poplars, but there are two likely candidates under , both are mite-induced. In the broad-leafed lime (Tilia platyphyllos) the mite is Eriophyes tiliae, producing a gall greater than 8 millimetres tall; in the small-leafed lime (Tilia cordata) the irritant is the mite Aceria lateannulatus, whose gall is about 5 millimetres tall. Both galls may also be found on the common or European lime (Tilia x europaea).
Mike Snow, Crymych, Pembrokeshire, UK
鈥� These are lime nail galls caused by the lime nail gall mite (E. tiliae) and the galls are widespread in the UK. The hollow structures are produced in response to the mites feeding on the underside of the leaf. The mites subsequently enter the galls to shelter, feed and breed. This particular mite is restricted to lime (or linden) trees. Similar galls are often produced on alder leaves by a related mite 鈥� E. laevis.
Myles O鈥橰eilly, Scottish Environment Protection Agency, Glasgow, UK