Plant Galls of Britain & Ireland's Boletim

I don't look at Twitter very much these days, so have been missing tweets from the British Plant Gall Society (BPGS). However, I saw this one the other day:

Gall of the week is that of the mite Aceria tenella on hornbeam Carpinus betulus. Can be looked for now.
Photo by Graham Calow.
A reminder that not all galls are spectacular and colourful! pic.twitter.com/uURpqTdpvq

— British Plant Galls (@britgalls) May 9, 2024

This gall appeared relatively recently in my local patch on Hornbeams planted around 25 years ago. The only other location I know around Nottingham is Martins Pond, Wollaton.

As with many galls caused by mites in leaves these appear more or less as soon as the leaves open. Mites overwinter under bud scales and move onto the leaves immediately. There are Lime leaves opening now which are distorted by galls of Eriophyes exilis in the leaf axils. Although the galls are often there, they can initially be hard to find: red galls such as the Sycamore Cherry mite often are green to start with; erinea, hairy patches, can be almost invisible initially. It doesn't help that fresh leaves often have hairs which soon disappear (especially in leaf vein axils.) One consequence is that it is often

Anyway I looked for Aceria tenella yesterday, and found it very quickly. Here are the upper and lower side of a galled leaf:

and an ungalled leaf for comparison:

This gall is widespread, but so far we only have 14 observations on iNaturalist. The easiest way to look for it is to push a branch of the tree upwards so that one can scan the underside of many leaves for the tufts of hairs in the leaf axils.