Gall Flies and Midges

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 Flies and Midges

Most fly galls are caused by gall-midges in the family Cecidomyiidae.  The adult are tiny (< 5mm), rarely seen, and very hard to identify.  They have distinctive antennae like a string of beads.  Galls caused by Cecidomyiidae take many forms - leaf-rolls, folds and  distortions; swellings in stems and petioles; pouches or blisters on leaves; swellings in fruits, catkins or buds; rosettes or artichoke galls in buds that cause the new leaves to bunch together. 

Cecidomyiid larvae are small maggots - white, yellow, orange or pink in colour.  They usually have a ‘sternal spatula’ on the lower surface of the thorax; a tiny structure that (if present) is diagnostic of the family, although lost in a few species. Some larvae (e.g. Contarinia) can jump when disturbed.

A special type of Cecidomyiid gall is the Ambrosia gall, such as those formed by Lasioptera rubi on bramble and Asphondylia sarothamni on Broom.  A fungus is also involved in the gall formation, and is transferred into the gall by the adult midge during oviposition.  It appears to be a mutualistic relationship, where both the larva and the fungus benefit - the larva from a nutritious food supply and the fungus from transference into its habitat within the host plant - but the exact nature of the relationship between host, fungus and midge is unclear. The fungal hyphae can be seen inside the gall chamber when it is cut through.  The exact identity of the fungus is unclear because the larva, in some way, prevents the fungus developing sexual spores.

Some Tephritidae cause closed galls in the achenes and receptacles of flower heads of Asteraceae (Knapweeds, Burdock, Thistles, etc.); a rare exception this is the common stem-gall on Creeping Thistle caused by Urophora cardui.  The adults of these species are larger and frequently recorded; they usually have patterned wings.  The flower-head galls are not obvious and can be hard to find - the best way is to search dead flower heads in winter; by gently squeezing the head you may feel the hard woody lump that shelters the over-wintering larva. 

Galls are rare in other Diptera families.  Some species of Anthomyiidae (Chirosia) species cause galls in ferns and bracken, as well as causing leaf-mines.  One species of flat-footed fly (Platypezidae – Agathomiya wankowiczii) galls Ganoderma applanata - a rare occurrence of a fungus as host.  'Cigar' galls on reeds and other grasses have been recorded in VC55 and are caused by a species of Chloropidae.