Gall Mites

Galls

The British Plant Gall Society define a gall as ‘an abnormal growth produced by a plant or other host under the influence of another organism. It involves enlargement and/or proliferation of host cells, and provides both shelter and food or nutrients for the invading organism'. (https://www.britishplantgallsociety.org/)

Organisms that cause galls to form on plants encompass almost the entire spectrum of life – from viruses and bacteria, through Protozoa, fungi and allies, slime-moulds, nematodes, mites, aphids and psyllids, flies, beetles, moths, sawflies and wasps, and even a few algae and vascular plants.    The organism that causes a gall is usually very difficult to identify, but often the gall can be identified.   However, some galls are hard or impossible to identify, and it may be necessary to find the gall-causer and get expert help.  The presence of parasitoids, inquilines or other organisms sheltering or feeding inside an occupied or vacated gall can be misleading, and it is common to find atypical specimens or chimaera formed by two or more organisms in close proximity - usually these can't be identified.  

When recording galls, always start by identifying the host plant accurately and make sure you have included information on the host in the notes attached to your record.  Most species of galls are specific to a host, and this is always the first step in identification.

Facebook Group: British Plant Galls

The Field Studies' Council's  Aidgap guide by Redfern, M. & Shirley, P. (2023) ‘British Plant Galls’ (3rd edition). FSC is recommended. The BPGS Facebook group can help with identifying galls.  Photos can be found on the BPGS website or on these northern European websites:

Pflanzengallen - Comprehensive guide to galls of Germany.

Plantengallen.com - English-language version of Dutch site.

Volkers Pflanzengallen - Volker Fäßler’s gall website (in German).

Leafminers and plant galls of Europe - Dr. Willem N. Ellis' website (‘Bladmineerders’).

An excellent account of the ecology and biology of galls is in Redfern, M. (2011) Plant Galls. Collins New Naturalist.

In the accounts below, there are sometimes two or more Red-Amber-Green or 'RAG' ratings - one or two referring the gall, and one to the gall-causer.  Galls usually have a lower 'RAG' rating then the causer.  Click on the species to find out more. 

Gall Mites

Most gall-forming mites are in the Eriophyoidea super-family and are tiny, less than 0.5mm long, with an elongated cylindrical body.  Unlike other species of mite, they have only two pairs of microscopic legs, at the front.  A few species are in the Phytoptidae family, but most are in Eriophyidae.  Leaf-folds, erinea, pustules and pouch-galls, blisters and enlarged buds are some of the gall-types caused by mites.  A few mites that cause galls on plants are in the family Tarsonemidae, with ovoid bodies and four pairs of legs.

Gall mites cannot be seen clearly with the naked eye or hand lens, and high magnification microscopy is needed to see details of the organism.  Identification of mites is therefore very difficult, and the taxonomy of gall-mites is often tentative pending further research.  Mites that cause erinea (felt-like or woolly patch of hairs, usually on the leaf underside, in which the mites live) are particularly difficult, and microscopic examination of the 'hairs' of the erineum might be needed. 

Life cycles can be complicated. There is more information in this paper by Sam Buckton as the BPGS. https://www.ynu.org.uk/images/insects/Acer_erinea_key.pdf