During yesterday’s walk, when checking for leaf mines on the leaves of Holm oak (large numbers of Ectoedemia heringella present – you can see one in the photo below), I noticed a huge abundance of small bumps covering many of the leaves.

These are the galls caused by the miniscule mite Aceria ilicis. On the underside (though sometimes also on top), the depression in the leaf is filled with tiny brown hairs (an erineum) where the mites make their home. The galls can be seen throughout the year, so next time you spot a Holm oak, check out its leaves.