HYMENOPTERA | Parasitic Wasps

Cynipidae (Gall Wasps)

HYMENOPTERA > APOCRITA | Parasitica > Cynipoidea > Cynipidae > Cynipini

Cynipidae is one of three closely related parasitic wasp families within the Cynipoidea superfamily. The other two families are Figitidae and Ibaliidae; the members of which are tiny endoparasitoids of insects, rather than gall-forming wasps, so will not feature here. While cynipid wasps are responsible for galls on various trees and plants, this page is purely for those species in the Cynipini tribe that are associated with oak trees (Quercus sp.).

Around 90 species of gall wasp have been recorded in Britain, with just about half classified as oak gall wasps. They are the better-known, and probably the most studied species, responsible for the more conspicuous galls. Most of the species have an alternation of generations with one being female only (agamic), more often referred to as the asexual generation, the other sexual with both males and females. And, if that wasn't complicated enough, the different generations very often produce different types of galls on different species of oak.

The adults can be quite variable, but are typically rather small, not much more than 4mm long, usually shiny brown or black in colour, with a somewhat humped thorax and, although most are winged, some are not. The general interest in the family though, is with the galls, as the adult wasps are so rarely seen. I guess, it's for this reason, that I haven't really spent any time searching out galls so, at present, this page only provides an overview of some of the more common species.

Andricus quercuscalicis

Knopper Gall Wasp

This now widespread species, which only became established in Britain about fifty years ago, infests two types of oak tree during their full two-year life cycle. It starts in early summer when the female lays her eggs in the developing acorns of the Pendunculate (English) Oak where they eventually form into ridged woody structures known as knoppers; the name being derived from the German word ‘knoppe’ meaning swelling or protuberance. The galls are initially green and sticky, but harden and enlarge as the encased larvae develop. In the following spring a second generation will emerge, which go on to infest the Turkey Oak where their eggs are injected into the catkin buds. There’s a bit more to it than that, but this explains the basic process.

Three Legged Cross, East Dorset | March 21

Andricus quercuscalicis - Oak Knopper Gall

Garston Wood, Cranborne Chase, East Dorset | Aug.21

Andricus foecundator

Oak Artichoke Gall Wasp

This species produces a gall known as either the Oak Artichoke Gall or the Oak Hop Gall, which develops as a chemically induced distortion of leaf buds on both Pendunculate (English) and Sessile Oak trees. The wasp larvae live inside a small hard casing that is inside the green ‘artichoke’, which will release and drop to the ground in the autumn. The asexual wasps that emerge in spring will lay a solitary egg in the male flowers of the oak, which will cause the formation of ‘hairy catkin galls’ that produce the next sexual generation.

Andricus foecundator - Oak Artichoke Gall

Slop Bog, East Dorset | Aug.21

Andricus lignicola

Cola Nut Gall Wasp

These round, rough-surfaced, purplish-brown galls that grow to around 15mm diameter, usually found in small clusters are caused by the asexual generation of Andricus lignicola, commonly known as the Cola Oak Gall Wasp. They typically start to appear in the early summer, at which time they're relatively soft with a greyish colour but, as they mature, they darken and become extremely hard. They are found on both Pendunculate (English) and Sessile Oak trees, whereas the resultant rarely recorded, tiny, egg-shaped galls that are produced from the larvae of the second generation adults, that emerge in the autumn, occur on Turkey Oak.

Kitt's Grave, Hampshire | Nov.21

Biorhiza pallida

Oak Apple Gall Wasp

As their common name suggests, the galls produced by Biorhiza pallida can resemble small apples. They develop on oak twigs and can be found during May or June after they've burst from a bud. They have a rough texture, but are soft and spongy, and can grow to around 4cm diameter. Each gall contains numerous chambers, each being home to a single larva that will slowly eat its way out, emerging as a winged adult in June or July. This generation can be males or females.

Once mated, the females burrow into the ground to lay their eggs in the oak's roots, thereby producing further galls from which the asexual generation will emerge about 18 months later at the end of the second winter. They are wingless females that, despite the cold, will slowly climb up the trunk of the tree to deposit their eggs in the buds. Because the species has a two-year cycle, there may well be two separate populations existing on the same tree.

Moors Valley CP, Ashley Heath, East Dorset | May 22

Cynips quercusfolii

Oak Cherry Gall Wasp

The larvae of this species are responsible for the spherical pithy galls that form on the leaves of some oak trees in late summer and early autumn. The galls are usually between 15-25mm in diameter, initially yellowish green, often suffused with red, but later darkening. They are smooth when found on Pendunculate (English) Oak trees, but more warty on Sessile Oak.

Alners Gorse, Dorset | Aug.21

Neuroterus quercusbaccarum

Common Spangle Gall Wasp

This rather attractive, disc-shaped gall that can be found on the underside of oak leaves in early autumn, contain the larvae of the asexual generation of the tiny gall wasp Neuroterus quercusbaccarum. The galls, of which there may be dozens on a single leaf, typically measure around 4mm or so across, and are a pale yellowish-brown colour with a slightly raised central pimple and scattered tufts of red hair. Occasionally, they may also be found growing on the upper side, but in the most part they're found underneath.

They end up falling from the leaves to see out the winter in the leaf-litter, often swelling quite noticably as the encased larvae mature. The adult wasps usually start to emerge in early spring to lay their eggs in the new oak buds. Galls of the asexual generation will develop on the male catkins or on younger leaves. They are known as currant galls and are soft and round, about 6mm diameter, mostly green with variable red markings. Adults from this second generation will emerge during the warmer summer months, with mated females laying their eggs in the leaves to start the cycle all over again.

Neuroterus quercusbaccarum - Common Spangle Gall

Kitt's Grave, Hampshire | Nov.21

Oaks | Quercus

Oak trees

While there are a number of varieties of oak tree that could be found growing in Britain, only two are truly native species; the Pedunculate or English Oak [Quercus robur] and the Sessile Oak [Quercus petracea]. When we talk about oak trees in general, these are the two species that we are typically referring to. Other forms, such as the Turkey Oak [Quercus ferris] and the Holm Oak [Quercus ilex] have been introduced.

The two common species can be easily distinguished, not just by their acorns - hanging in groups of two or three in roughly scaled cups via a stalk or peduncle in respect of the English Oak, or in small clusters, where the stalkless cups are attached directly by their base to twigs as in the Sessile Oak - but also by their leaves; deeply lobed on very short (5mm) stalks in the case of the English Oak, whereas with the Sessile Oak they are flatter, lobed but with more rounded edges, and on longer stalks (up to 25mm).

The Woodland Trust website has a page that provides some mind-blowing statistics with the regard to the number of wildlife species that a typical oak tree may support - the total number, including animals, birds, insects, other arthropods, lichens, mosses etc., is an incredible 2300, with 326 solely dependent on oak for their survival and a further 229 rarely found on trees other than oak.

Obviously, oak gall wasps are a good example, but there are many other insects featured elsewhere on this website that utilise oak trees either as a source of food or shelter and, of course, for a number of species, such as centipedes, woodlice and springtails for example, that will include dead or decaying wood if, for whatever reason, the tree has fallen.