Cynipidae

Oak galls

HYMENOPTERA > APOCRITA | Parasitica > Cynipoidea > Cynipidae > Cynipini

Whilst the associated family account provides a broad general introduction to the Cynipidae, together with a few representative photos of the tiny and elusive adult wasps, this extended page purely focuses on the galls or, more specifically, some of the more common galls that certain members of the Cynipini tribe induce on the leaves, buds and twigs of oak trees.

There are currently 53 recognised species in this particular tribe, which are separated into nine genera as detailed on the 'intro' page, but in practice there are probably no more than thirty that are likely to be found by the casual observer:

Andricus aries, corruptrix, curvator, foecundatrix, grossulariae, inflator, kollari, lignicolus, lucidus, malpighii, quadrilineatus, quercuscalicis, quercusramuli, seminationis, sieboldi, and solitarius

Aphelonyx cerricola

Biorhiza pallida

Cynips diversa, longiventris and quercusfolii

Neuroterus albipes, anthracinus, numismalis, politus, quercusbaccarum, saliens and tricolor

Trigonaspis megaptera

It doesn't stop there though, as gall wasps have alternation of generations with one being female only (agamic) more often referred to as the asexual generation, the other sexual; complicated further by the different generations producing different types of galls that are sometimes on different species of oak. Typically, the galls formed by the females of the sexual generation appear during the spring or early summer, and are usually on a different part of the tree to the later asexual (agamic) generation. The later galls are generally larger, more evident and around for a longer period of time, which is why they tend to be recorded more frequently than the earlier sexual galls. As with all things, there are exceptions, and perhaps the most notable in this respect is Biorhiza pallida, the species responsible for the familiar Oak Apple, as that gall is induced by the larvae of the sexual generation.

The most frequently observed oak galls have accepted common names. In most cases these names relate to the asexual agamic form, but the previously noted Oak Apple and the Currant Gall which is induced by the sexual generation of Neuroterus quercusbaccurum are two good examples where the name applies to the other form. On the subject of names, the asexual and sexual generation have similar taxonomic history as found in some other species, such as anamorphic and teleomorphic fungi discussed in my article - 'the confusing world of asexual fungi ...', where the different forms were originally given different scientific names. Some of the older, but still very useful, guides to British Oak Galls use the names that were valid at the time and, obviously, that can lead to confusion, so for personal reference I've added an appropriate [footnote] at the end of each entry to confirm the previous name as well as the current accepted name and author.

Before taking a look at some of the species I've managed to find, it's worth noting that the variation in a gall's form and structure, as well as its location on the tree, helps to reduce the likelihood of attack by parasitoids, which are predominately chalcid wasps from the Eulophidae, Ormyridae, Pteromalidae and Torymidae families, but also various ichneumons and braconids. For example, a hairy covering makes it more difficult for a parasitoid to locate a suitable place on the gall surface where they can insert their ovipositor, while hard galls with thick walls would need more effort and time to drill through. Another interesting factor is that most of the larger parasitoids, such as Ormyrus nitidulus (featured earlier), only have one generation a year and, consequently, would only be able to attack galls induced later in the year.

The various gall inducers, inquilines and parasitoids are all part of a complicated community where one species is always trying to outdo another adopting different strategy of defence or attack.

Finally, I have to admit that I've never really given galls much attention but, from what I've read whilst compiling and subsequently updating this page, I can now appreciate why they receive the interest they do.

Updated, May 26 (v.3)

Andricus quercuscalicis

Knopper Gall

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.

[In older literature, notably 'British Oak Galls' (1908) by Edward Connold, this particular gall was only included as an add-on due to its occurrence in Europe and a few records from Jersey. At that time it was known as Cynips calicis. The current accepted species name is Andricus quercuscalicis (Burgsdorff, 1783) but, as with the majority of species, an official option to extend as either f.agamic as featured here, or f.sexual for the earlier form].

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

Ringwood Forest, Verwood, East Dorset | July 25

Andricus foecundatrix

Artichoke Gall

This species, formerly known as Andricus foecundator, produces a distinctive gall commonly called the Artichoke Gall or, less often, the Oak Hop Gall, which develops as a chemically induced distortion of leaf buds on both Pendunculate (English) and Sessile Oak trees. These galls appear in mid-summer with the wasp larvae developing inside a small hard casing that is inside the enlarged 'artichoke' bud. Once mature, the inner casing will be forced out to drop to the ground leaving the dried scales of the bud on the twigs. The parthenogenetic female wasp that emerges from the casing in the spring will lay a solitary egg in the male flowers of the oak, which will cause the formation of the Hairy Catkin Gall that produce the next sexual generation where mated females will continue the cycle by injecting their egg in an oak bud.

[In older literature, the asexual form of this gall was historically known as Aphilothrix fecundatrix and the alternate sexual form as Andricus pilosus. The current, accepted species name in either state is Andricus foecundatrix (Hartig, 1840), which can be extended as either f.agamic in relation to the Artichoke Gall featured here, or f.sexual for the Hairy Catkin Gall].

Slop Bog, Ferndown, East Dorset | Aug.21

Garston Wood, Cranborne Chase, East Dorset | Sept.25

Andricus lignicola

Cola-Nut Gall

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

[In some older literature this species isn't even mentioned as it was only discovered in the UK in the early 1970's at which time, I believe, it was known as Andricus venheurni and then more recently Andrius lignicolus. The current accepted name is Andricus lignicola (Hartig, 1840)].

Kitt's Grave, Woodyates, East Dorset | Nov.21

Biorhiza pallida

Oak Apple

As the common name suggests, these galls can resemble small apples. They develop on oak twigs, and may 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 of which is home to a larva that will slowly eat its way out with winged adults emerging in June or July. They can be either males or females.

Mated females from this first generation crawl into the ground to lay their eggs in the oak's roots, producing further galls from which the asexual generation emerge about 18 months later at the end of the second winter. They are wingless females that, despite the cold, climb up the trunk of the tree to lay their eggs in the buds. This two-year life cycle means that two separate populations may well exist on the same tree.

[In older literature, the sexual form of this gall was known as Teras terminalis and the alternate sexual form as Biorhiza aptera. The current, accepted species name in either state is Biorhiza pallida (Olivier, 1791), but with the option to extend to f.sexual for the Oak Apple featured here, or f.agamic for the Root Gall].

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

Cynips quercusfolii

Cherry Gall

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 15-22mm in diameter, initially yellowish green, often suffused with red, but later darkening. They are smooth when found on Pendunculate (English) Oak trees, but a little warty on Sessile Oak. There are normally between 3-5 on a leaf, occasionally a few more, but very rarely more than 10. They usually fall to the ground in the autumn when the tree starts to lose its leaves, but sometimes they remain attached. The larva pupates in the gall and, when the time is right, the adult wasp gnaws a passage from the central core and, somewhat dependent on the weather, will emerge at any time from late autumn through to early spring. They lay their eggs in dormant buds where the sexual generation will develop in little purple egg-shaped galls that surprisingly are rarely recorded.

[In some older literature, such as 'British Oak Galls', the asexual form of this gall was known as Dryophanta scutellaris and the alternate later sexual form, with the common name of the Purple Velvet-Bud Gall, as Spathegaster taschenbergi. The current, accepted species name in either state is Cynips quercusfolii (Linnaeus, 1758), with optional extensions of f.agamic for the common Cherry Gall featured here, or f.sexual for the rarely seen purple form].

Alners Gorse, Hazelbury Bryan, Dorset | Aug.21

Neuroterus anthracinus

Oyster Gall

These tiny, often colourful, egg-shaped galls, which are typically formed on, or between, the mid-rib and the lateral veins of oak leaves, are induced by the asexual generation of the cynipid wasp Neuroterus anthacinus. They're initially a creamy to yellowish, pale green colour, later developing reddish to reddish-brown spots or blotches that become slightly elevated before maturing and falling from the leaves in autumn. I've found quite a few on the ground, occasionally still attached to a leaf, but generally laying or partially buried in litter. Interestingly I've never spotted any that are still on the tree. The attachment point leaves two small flaps of tissue, which sometimes come away when the gall becomes detached, but are usually still evident if you inspect a fallen gall. It's an important distinguishing feature although, generally speaking, they're so distinctive that it's unlikely they'd be misidentified.

The parthenogenetic females of the agamic generation emerge from these fallen galls in late winter or early spring to lay their unfertilised eggs in developing oak buds. The resultant swelling, known as the April-bud gall, is usually inconspicuous and of short duration as new male and female wasps emerge within a week or so of the eggs being injected. The fertilised eggs from this generation produce the Oyster gall.

[In older literature, the asexual form of this gall was historically known as Neuroterus ostreus and the alternate sexual form as Spathgaster aprilinus. The current, accepted species name in either state is Neuroterus anthracinus (Curtis, 1838), but with the option to extend as either f.agamic as featured here, or f.sexual for the unnamed green 'blister gall'].

Woods Corner, Burley, New Forest, Hampshire | Sept.25

Barrow Moor, Bolderwood, New Forest, Hampshire | Sept.25

Neuroterus numismalis

Silk-Button Spangle Gall

As with some of the other members of the genus, this wasp produces two distinct types of gall; the bright ochre to golden brown coloured cup-like structure featured here, known as the Silk-Button Spangle Gall which can often be found during the autumn, and a green 'blister gall' which, although harder to spot, occurs in the spring. The asexually induced cups that are formed on the underside of oak leaves, have a raised rim which, on close inspection, appears to have been embroidered with silky golden hairs, hence the common name. They're typically seen in large numbers, often with hundreds grouped together on a single leaf.

The small, roughly circular 'blister galls' that are formed earlier in the year on young leaves are produced by the sexual generation of the species. They are bright green, blending well with the colour of the oak leaves, and have pale lines that radiate towards the edge from a central barely raised pimple. A single larvae will live and pupate within each gall, emerging as an adult female or male wasp from late spring to early summer.

[In older literature, the asexual form of this gall was historically known as Neuroterus numismatis and the alternate sexual form as Spathgaster vesicatrix. The current, species name in either state is Neuroterus numismalis (Geoffroy, 1785), but with the option to extend as either f.agamic as featured here, or f.sexual for the unnamed green 'blister gall'].

Neuroterus numismalis - Silk-Button Spangle Gall

Garston Wood, Cranborne Chase, East Dorset | Sept.25

Neuroterus quercusbaccarum

Common Spangle Gall

This attractive, disc-shaped gall that can be frequently seen 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 many dozens on a single leaf, typically measure around 4-5mm or so across, and are a pale yellowish-brown colour with a slightly raised central pimple and scattered tufts of red hair. Occasionally, you may also find them growing on the upper-surface of the leaf, but in the most part they're found on the underside.

They'll become detached and fall to the ground once mature and will probably end up being buried in leaf litter during the winter months. As the encased larvae develop the gall will swell quite noticeably. The adult wasps usually emerge in early spring to lay their eggs in new oak buds. Galls of the sexual generation will develop on the male catkins or on younger leaves. They are known as Currant Galls and are round and soft, about 6mm in diameter, mostly green, but 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.

[In older literature, the asexual form of this gall was historically known as Neuroterus lenticularis and the alternate sexual form as Spathgaster baccarum. The current, accepted species name in either state is Neuroterus quercusbaccarum (Linnaeus, 1758), but with the option to extend as either f.agamic as featured here, or f.sexual for the Current Gall].

Neuroterus quercusbaccarum - Common Spangle Gall

Kitt's Grave, Woodyates, East Dorset | Nov.21

Garston Wood, Cranborne Chase, East Dorset | Sept.25

Shave Wood, New Forest, Hampshire | Oct.25

Neuroterus albipes

Smooth Spangle Gall

Shave Wood, New Forest, Hampshire | Oct.25

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.