Galls

All images on this website have been taken in Leicestershire and Rutland by NatureSpot members. We welcome new contributions - just register and use the Submit Records form to post your photos. Click on any image below to visit the species page. The RED / AMBER / GREEN dots indicate how easy it is to identify the species - see our Identification Difficulty page for more information. A coloured rating followed by an exclamation mark denotes that different ID difficulties apply to either males and females or to the larvae - see the species page for more detail.

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.

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: British Plant Galls. 

Photos can be found on the BPGS website or on these northern European websites:

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. 

Galls caused by Aphids

All gall-causing Hemiptera, or true bugs, are in the Sternorrhyncha sub-order (aphids and adelgids, psylloids and scale insects). 

True Aphids have complex life-cycles with alternating parthenogenetic and sexual egg-producing generation, sometimes involving two host species.  Fertilised eggs hatch in spring into a nymph which develops into a female 'fundatrix'; she  is viviparous, producing live young  parthenogenetically, and often wingless ('apterous').  She is the foundation of the population over summer.  Later in the year, males and females are produced, which mate; the female lays an egg which overwinters before hatching in spring to take advantage of new nutritious growth on the host plant.  Winged adults or 'alates' are produced in those species that have their alternate generation on two different host species, and other 'morphs' or different body-forms can also occur.

The fundatrix of some aphid species produces galls which protect the summer population - these are often leaf folds or rolls such as Eriosoma ulmi, on Elm, or bladders such as Eriosoma lanuginosum or Tetraneura ulmi, also on Elm. Pemphigus spp. cause pouch and spiral galls on poplars.  Aphids inside the galls are often covered in wax or wool.

Many aphids do not produce galls, but will cause distortion of leaves and shoots of their host, sometimes called a 'pseudogall'.  These may be mistaken for galls, but do not show the enlargement or proliferation of plant tissue caused by the gall-organism. 

Further information about aphid life-cycles can be found on https://influentialpoints.com/

 

Galls caused by Adelgids

Adelgids or woolly conifer aphids are related to true aphids.  Life-cycles can be very complicated, involving host alternations and several different winged and nymph forms.  Some cause pineapple galls on the terminal and axillary buds of spruce trees (Picea). Some species need another conifer host to complete their life-cycle; other live on the same host and have lost the sexual generation completely (as with the common Adelges abietis).

 

Galls caused by Psyllids and allies

Psylloidea includes the families Psyllidae, Triozidae and Liviidae, collectively known as Psyllids or jumping plant lice.  Galls include shoot-tips galls such as Psylla (Spanioneura) buxi on Box, pit galls such as Trioza remota on oak, where the nymph can be seen in a hollow underneath the leaf, and leaf folds and pouches such as Lauritrioza alacris on Bay and Trichochermes walkeri on Buckthorn.  Most are host-specific, but Livia junci causes phyllanthy on a variety of rushes.

Galls caused by scale insects - Coccoidea

Some scale insects (Coccoidea) produce galls on the twigs of their hosts.

Galls caused by wasps - Cynipidae

Nearly all wasp galls in the UK are caused by wasps in the family Cynipidae, and most are on Oak, with a few on Rose and a few on other herbaceous species.   Life-cycles increase in complexity from the galls on Bramble (Diastrophus) and herbaceous species (e.g. Aulacidea, Phanacis, Liposthenes), to more complex Diplolepis, which gall roses, to the Oak gall wasps (e.g. Andricus, Cynips, Neuroterus) with the most complex life-cycles.

Oak cynipid wasps have alternating sexual/asexual generations, or ‘cyclical parthenogenesis’.  Typically, the gall formed by the females of the sexual generation appears in spring/early summer, and is on a different part of the oak to the later asexual (or agamic) generation.  In many species, the sexual galls are recorded less often than the asexual galls, which are often larger and persist longer. (An example of an exception to this is the oak-apple, Biorhiza pallida, which is the sexual gall).   Often the 2 types of gall are a very different shape, and originally some were thought to be different species; in some species, the sexual gall has not been found.  Some introduced species form sexual galls on the non-native Turkey oak and asexual galls on native oaks – e.g. Andricus kollari.  

The first generation sexual gall is produced in spring by an asexual female that lays eggs parthenogenetically, without them being fertilised by a male.  The females are of two kinds; those that lay eggs which produce male wasps, and those that produce females. From this gall, the second generation of sexual wasps emerges, both male and female; these mate, and the females then produce the asexual galls later in the year. 

In the accounts below, asexual and sexual galls may have different 'Red-Amber-Green' (RAG) ratings - click on the species account to check.  The gall most often recorded, with the lowest RAG rating, is shown first.