Many mites produce galls on plants. Most gall mites occur in the superfamily Eriophyoidea; these mites don’t resemble the globular spider mites or predatory mites that you may be familiar with; rather they are cone or worm shaped with only two pairs of legs, all at their widest end. They are very small, adults usually 0.1-0.3mm long.
Sycamore (Acer pseudoplatanus) and field maple (A. campestre) leaves are galled by several species of eriophyoid mite, but in all cases the development is the same. In the spring adult mites emerge from overwintering sites in bark crevices etc and move over the undersides of the developing leaves.
They feed on individual cells, causing them to collapse, and stimulating the cells around to divide. This leads to the leaf blade bulging upwards and later a layer of nutritive tissue develops on the underside. Thus a pouch gall is formed – this is not an enclosed gall, it is open to the underside of the leaf (with hairs at its mouth probably for protection and to prevent water loss, Fig. 7). This gall takes a while to form, and the mite comes back later, by mid-May, to lay eggs inside the developing gall. This raises several questions: does the mite return to its own galls, if so, how and if not? Is there competition between mites for galls (each mite can produce about 20 galls, and a leaf may contain several hundred galls)?
Continued gall development is stimulated by the feeding of the mites inside; if a colony dies out then the gall reverts to normal leaf structure, and fills up with hairs inside. Several generations of mites may develop during the summer in a gall, and the adults leave the gall in the autumn before the leaf is shed to overwinter.
The photos illustrate three species of eriophyoid gall on acers on campus at the moment. The names of these species (and even how many species there are) changes regularly, I’m using those in Redfern et al. (2002).
Sycamore has Aceria cephaloneus (Figs 1-2) a fleshy bright red elongated gall with a round end, c. 3mm tall. I shall be looking out for another very similar species on sycamore, A. macrorhyncha, which produces slightly longer (to 6mm) galls with a pointed end, but Redfern et al. (2002) states this may be the same species as A. cephaloneus. On field maple there is another very similar gall, A. aceriscampestris (Figs 3-4) – is it really a different species to A. cephaloneus?
Also on field maple is A. macrochelus (Figs 5-7). This is a 2-4mm diameter round, hairy gall that becomes rather woody as it matures. It usually forms in the angle between a primary and secondary vein. It does not occur in such profusion as the other Acer galls and is rather less commonly distributed.
The other, introduced, Acer species in the UK are much less frequently galled, but if anyone knows of any other acers on campus can they let me know so I can check them out.