English: Nail galls on lime The nail galls shown here were on one of the many lime trees that have been planted alongside a 1906324. As usual, to correctly identify the gall-causer, the host plant needs to be accurately identified; in this case, the trees are small-leaved lime, Tilia cordata. The galls were therefore produced by mites of the species Aceria lateannulatus.

These mites browse on the lower surface of the leaf; chemicals in their saliva cause cells in the leaf to proliferate, creating these hollow galls. The mites later enter the galls, and feed and breed there in relative safety.

The upper surface of some leaves was densely and completely covered by these galls, the blade of the leaf being greatly reduced in size. Such leaves probably contain around the same amount of material as other leaves, but with the bulk of it diverted into gall formation.

For a different sort of gall that was very common on the leaves of the same lime trees, see 1906472.

[The remainder of the present item deals with technicalities related to the identification of the galls, and is intended only for those who are interested in such matters.]

The identification of these galls relied upon the identification of the host tree as small-leaved lime, Tilia cordata, based, in turn, on the disposition of the cymes, the number of flowers per cyme, and the tufts of rust-coloured hairs visible in the vein-axils on the underside of the leaves. Another species of mite, Eriophyes tiliae, produces nail galls on large-leaved lime, Tilia platyphyllos. Those galls are larger, on average, and have pointed rather than blunt tips (see below). Both kinds of nail gall can be found on common lime (Tilia × europaea), which is the hybrid of small-leaved and large-leaved lime.

Since there otherwise appear to be few pictures of Aceria lateannulatus galls online, it is probably worthwhile adding some further observations of my own here. The standard reference work, "British Plant Galls" (2002), states that these galls are about 5mm tall, with rounded tips, while those of Eriophyes tiliae are usually more than 8mm tall, with pointed tips. While this is undoubtedly true in general, exceptions will be seen. I observed a great deal of variety in the Aceria lateannulatus galls alongside the road where this photograph was taken, although all the lime trees there are of the same species: some of the galls had tips much more pointed than those shown in this photograph, while their colours ranged from entirely red to pale green. In addition, some atypical specimens could be observed growing on the midrib of leaves, where, although slender, they reached an abnormal height (1cm or more). Since both kinds of nail gall occur on the hybrid tree, common lime, I can only assume that great care is needed in differentiating the two kinds of gall on that tree. These observations also drive home the point that, as noted above, the vital first step in correctly identifying a gall is to identify the host plant to species.