The galls I have described from campus so far are all recognisably gall like, but not all are. I’m having quite a bit of trouble at the moment trying to distinguish some types of gall, which involve leaf rolling and folding, from this type of activity undertaken by non-galling species, which may also fold and curl leaves to provide shelter, but do not feed there. Really, a modified plant structure needs to provide both food and shelter before it can be called a gall, but there is considerable confusion here.
One type of inconspicuous structure on plants, usually their leaves, does provide both food and shelter. This is the filzgall or erineum. These fancy names conceal their real nature, a patch of hairs which provide both nutrition and protection. These are not normal plant hairs, but are specially induced by the feeding creature; eriophyoid mites are the only creatures which can induce this – apparently something that insects cannot do.
The production of these hairs are induced by mite feeding, first by the fundatrix female in the spring, and then by her offspring as they develop and multiply through the summer. The mites feed on the individual hairs (which may be uni- or multicellular), a scar develops on the feeding site which prevents further feeding, and also as the hairs mature their walls thicken and mites can no longer feed upon them, and these hairs subsequently senesce.
These galls can look much more like fungal infections, and indeed some were classified as such until the mites that caused them were found. Some, such as the erineum caused by Eriophyes leisosoma on common lime (Fig. 1) are simple, with hairs that are cylindrical with rounded tips, rather similar to the hairs on young leaves.
These galls cause no swelling to the leaf, and are invisible from above – I only found these when I examined a leaf that had one of the other mite galls I mentioned in an earlier post.
The erineum on sycamore, caused by Aceria pseudoplatani (Fig. 2) is also not visible from above, at least early in the season, but a yellowish bulge may develop later. These bulges are caused by the induction of the upper epidermal cells to enlarge. Some erinea are accompanied by significant bulges, and one can envisage a possible evolutionary sequence here from the simple erinea, through the bulging ones to the fully enclosed nail galls on limes I mentioned earlier. An example of a more visible erineum is that caused by Aceria ilicis on holm oak (Quercus ilicis) (Figs 3,4).
It was interesting to find this on campus, on a holm oak outside Cedars, as it is not that common. This oak, introduced around 1500 to the UK, is unusual in having a mite gall, as most galls on oaks in the UK are caused by gall wasps. Holm oak has gradually become galled in the UK by several species of introduced gall wasps during the last 7 years, but I have yet to find them on campus.
All the erinea described above develop on the underside of leaves in no particular places, but some are more particular. Aculus leionotus forms erinea in the angles of veins of birches (found on silver birch, Betula pendula on campus), giving a quite regular appearance (Figs 5,6), and if they were not accompanied by a swelling above (which becomes red-brown by the end of the season), they could be mistaken for a rather hairy form of the plant.