Aceria campestricola

The mites pass the winter hidden in bark crevices and in the spring attack the leaves. The gall is a small pimple or pouch, 1–2 mm across, on the blade or the veins. They are rounded above and have a hairy opening below, with or without a conical projection. Galls are greenish-yellow and redden when exposed to sun, they can be numerous and contain mites.^[2][3]

There is some disagreement as to which species is involved in forming the galls. Mite pimples on elms with small leaves, vary in size and some of the large galls have stalked dimples The small galls have been described as Aceria ulmicola (Napela) and the stalked galls as Aceria brevipunctata.^[3][4]

Galls have been found on the leaves of the following trees; wych elm (Ulmus glabra), Dutch elm Ulmus x hollandica, European white elm (Ulmus laevis), field elm (Ulmus minor).^[4]

Found in most of Europe and North America.^[1]