Gall mites feed by sucking sap from their host plants. While doing so, they secrete chemicals that alter the plant’s normal development. The gall mites live and feed within the gall structures that they have helped create. Gall mites in the Eriophyidae family have slender elongate bodies with only two pairs of legs.
Some gall mites induce a dense growth of short hairs, mainly on the underside of affected leaves. This type of growth is known as a felt gall or erineum. The hairs are initially creamy white but often become brownish towards the end of summer. However, if the felt gall is developing on a purple-leaved form of a tree, such as beech or sycamore, the hairs are usually pinkish red.
Dispersal to new host plants is probably brought about by the tiny mites being carried by the wind, or by hitching a ride on more mobile insects or other animals.
Gall mites can have many generations within their galls. For most gall mites, the breeding season is during the summer but some, such as blackcurrant big bud mite, breed during the winter. Those that form galls during the summer usually vacate the galls in autumn and overwinter underneath bud scales or in bark crevices.
Role of gall mites in gardens
Most gall mites are relatively harmless animals that have no adverse effect on their host plants beyond causing the growth of the gall structures and so are additional biodiversity in a garden A few do have a more severe impact. Buds affected by blackcurrant big bud mite fail to produce any foliage or flowers. Heavy infestations can result on poor growth and little fruit. Fuchsia gall mite has only been in Britain since 2007 but is now widespread along the south coast of England and is spreading northwards. Once plants are heavily infested, normal new growth stops, with a complete loss of flowers. There are no effective control measures for fuchsia gall mite, other than destruction of affected plants.
Other sources of information
Chinery, M. (2011) Britain’s Plant Galls – a photographic guide. WildGuides Ltd
Redfern, M. & Shirley, P. (2011) British Plant Galls. A Field Studies Council AIDGAP key
Page text drafted by Andrew Halstead, reviewed by Andrew Salisbury, compiled by Steve Head