Gall mites, also known as eriophyid mites, are minute animals usually less than 0.2mm long when fully grown. They have elongate bodies and two pairs of legs, unlike other mites which have four pairs. Their size means they can usually only be seen with the aid of a microscope. Their presence is readily detected by the distinctive abnormal plant growths induced by their feeding activities.

There are many species of eriophyid mite which are usually host specific this means that each species of mite will only feed on a single, or several closely related, plant species.

Eriophyid mites feed by sucking sap but while doing so secrete chemicals into the plant tissue that converts some of the normal (parenchyma) plant cells to meristem tissues which can give rise to a range of growth forms. These then grow to produce the gall. The mites can then suck sap from plant cells lining the gall structure, which are invariably more nutritious than unaffected tissue, but often do not cause serious damage to the plant.

Each species of gall mite causes a recognisable set of symptoms and so these can be used to identify the mite causing the damage. Some typical gall mite symptoms include:

• Excessive growth of hairs on the foliage
• Thickening of leaf margins
• Enlargement of buds
• Stunting of shoots
• Proliferation of growth to produce witches' brooms
• Blistering of the foliage
• Development of hard raised structures on the leaf surface

Similar types of galling, however, can also be caused by other organisms such as certain insects, fungi and bacteria.

Most species of gall mite overwinter on their host plants as non-feeding females in crevices in the bark, especially near buds, or underneath the bud scales. In the spring, when the host plant comes into growth, the mites begin feeding and laying eggs. Up to about 80 eggs may be produced at a rate of two to three a day. They hatch into miniature versions of the adult mite and there are two nymphal stages before the mites become adult. There may be two or three generations during the summer with both male and female adults being present. In late summer overwintering females are produced which will not lay eggs until the following year.

Gardens can support a wide range of gall mites, most are additions to the biodiversity of a garden and do not adversely affect the health of host plants. Below is a list of some of the gall mites commonly found in gardens.

Apple felt gall mite (Phyllocoptes malinus): Creamy-white or reddish-purple patches of hairs on the undersides of edible and crab apple leaves.
Ash inflorescence gall mite (Aceria fraxinivora): Feeds on flowers and prevents them from developing into fruits. The flowers become clubbed woody structures that persist after leaf fall.
Beech leaf roll mite (Acalitus stenaspis): Leaf margins curl upwards to form a tight marginal roll which can  lead to more extensive leaf distortion.
Beech felt gall mite (Aceria nervisequa): Dense creamy-white hairs (reddish on copper beech) along the veins on the upper leaf surface or on the undersides of leaves.
Birch big bud mite (Acalitus calycophthirus): Enlarged buds that fail to develop; frequently found in witches' broom growths; initiated by a fungs.
Birch felt gall mites (Acalitus longisetosus and A. rudis): Dense creamy-white hairs to develop on the leaves, mainly on the underside. Leaves are often distorted.
Blackcurrant big bud mite (Cecidophyopsis ribis): Buds become abnormally swollen and rounded.
Broom gall mite (Aceria genistae): Enlarged and proliferated buds on the stems of Cytisus. The galls are initially whitish green and soft but later in the summer become greyish-brown and woody. Can develop to the point where there is little new growth or flowering.
Elm pimple gall mite (Aceria campestricola):Small whitish-green hard raised structures on the upper leaf surface.
Fuchsia gall mite (Aculops fuchsiae): Severe stunting and distortion of growth at shoot tips.
Grape blister mite (Colomerus vitis): Dense creamy-white or occasionally pink hairs on the underside of leaves, with upward bulging of the affected areas.
Hawthorn leaf margin mite (Phyllocoptes goniothorax): Pale thickening and downward curling of the leaf margins.
Hazel big bud mite (Phytoptus avellanae): Rounded enlarged buds that fail to develop. Rarely sufficiently abundant to affect growth or fruiting.
Holm oak felt gall mite (Aceria ilicis): Patches of rusty brown hairs to develop on the underside of leaves of Quercus ilex.
Large maple pimple gall (Vasates quadripedes): Globular galls up to 5mm high on the leaf surface of maples which can be very numerous. They are wrinkled and a glossy yellowish green or red, becoming dark red or black.
Lime nail gall mite (Eriophyes tiliae and E. lateannulatus ): Pale yellow or red pointed tubular structures up to 5 - 8mm long on the upper leaf surface of limes (Tilia species).
Lime felt gall mite (Eriophyes leiosoma and Phytoptus erinotes): Patches of creamy-white hairs on the undersides of leaves and upward bulges on the foliage.
Mountain ash blister mite (Eriophyes pyri): Whitish or yellowish-green blistered patches 2-4mm in diameter on the foliage of Sorbus aucuparia in late spring. Later in the summer the galled areas become brownish-black. Other gall mites cause similar symptoms on other Sorbus spp., hawthorn and apple. Also occurs on Pear when it is known as pear blister mite.
Plum leaf gall mite (Eriophyes similis): Swollen whitish or yellowish-green growths on the leaves, especially around the margins and on the veins. May also cause some distortion to the fruit in heavy infestations.
Raspberry leaf and bud mite (Phyllocoptes gracilis ): Leaves develop pale yellow blotches on the upper surface.
Sycamore/maple pimple gall mites (Aceria spp.): Several species of gall mites cause small reddish swellings on the upper leaf surface.
Sycamore/maple felt gall mites (Aceria pseudoplatini and A. eriobia): Dense growth of cream or pinkish-purple hairs on the undersides of leaves.
Walnut blister mite (Aceria erinea): Upward bulging of the leaf surface. The upper surface of the galled areas are yellowish-purple, the underside is densely covered in creamy-white hairs. No effect on the nut crop or the tree's vigour.
Willow witches' broom mite (Stenacris triradiatus): Catkins or buds are converted into a dense tuft of short shoots bearing many scale-like leaves. Especially on Salix fragilis, S. alba and S. amygdalina. Virus particles have been found in the galled tissues and it is possible that this growth may be virus-induced, rather than being caused by the gall mite.
Willow catkin gall mite (causal species uncertain): Converts catkins on goat willow, Salix caprea, into enlarged knobbly structures covered in greyish-green hairs which eventually persist as dried up woody structures. This type of willow gall may also be virus-induced.
Yew big bud mite (Cecidophyopsis psilaspis): Enlarged rounded buds that fail to open. Can also induce distorted and abnormally long shoot extension.

Most gall mites do not cause damage to the health of plants, control is not necessary and they are part of the biodiversity a healthy garden supports. 

Raspberry leaf and bud mite, fuchsia gall mite and blackcurrant big bud mite can however, affect the vigour of plants. Please see the relevant profiles for further information on preventing damage from these mites.