Insect Communities in oak galls

In addition to the gall-inducing wasp, oak galls are home to a wide diversity of other organisms, particularly insects. These other insects can be divided into two main groups. Inquilines occupy galls, but usually do not feed directly on the gallwasp or other insects, while parasitoids develop as parasites on or in the bodies of other insects.Ê These insects are fed on by a range of general predators. A striking feature of oak gall communities, and one that makes it easier to work on them, is that most of the inquilines and parasitoids are only found in oak galls. This makes it possible to look at galls as natural microcosms, separated ecologically from other sets of animals in the same environment.

Here is a more detailed introduction to the different groups of gall inhabitants.


Inquilines in oak cynipid galls belong to two groups. The inquiline gallwasps (members of the tribe Synergini) feed only on gall tissue, and though unable to induce their own gall, they have some ability to modify the plant tissue that immediately surrounds them. Cynipid inquilines either induce their larval chambers in peripheral gall tissue (non-lethal inquilines), or develop within the gall-inducer's own larval chamber, smothering it in the process (lethal inquilines). The second inquiline group includes a range of midges, moths and beetles whose larvae feed predominantly on gall tissue.

Inquiline midges. Several groups of Cecidomyiidae (commonly known as gall midges) feed obligately in or on oak cynipid galls. The larvae of Clinodiplosis cilicrus feed on decaying tissues within the spring sexual generation galls of Biorhiza pallida and among the scales of the asexual generation gall of Andricus foecundatrix. Larvae of Paralellodiplosis galliperda and Xenodiplosis laeviusculi live between the oak leaf and the asexual generation 'spangle' galls of Neuroterus quercusbaccarum and N. albipes, feeding on plant tissues from the undersurface of the gall and sometimes causing the death of the gallwasp.

You can see an image of a Pammene caterpillar and a pair of Curculio villosus weevils here.

Inquiline moths. A range of tortricid and noctuid moths live as inquilines in cynipid galls. Larvae of these species feed predominantly on the gall tissue, but some kill and eat the gall-inducing cynipid larva. The need for plant tissue on which to feed excludes these groups from the smallest galls. For example, Pammene amygdalana (Tortricidae) attacks the asexual generation galls of many different European oak cynipids (e.g., Andricus hungaricus, A quercustozae, A. kollari, A infectorius, etc.), but does not attack the much smaller sexual generation galls. Killing the larvae of the gall inducer and cynipid inquilines probably has two advantages for these moths. (1) These larvae probably represent an important extra source of protein, supplementing the low nutritive value of much of the outer gall tissue. (2) Death of the gall inducer prevents the lignification and increase in toxicity that occurs late in gall development, and thus preserves the gall tissues as a suitable food source for the moth larva.

Inquiline beetles. Larvae of the gall weevil Curculio villosus Fabr. (Coleoptera: Curculionidae) consume plant and cynipid tissues in the sexual generation galls of the oak cynipid Biorhiza pallida, sometimes inflicting high mortality. Curculio villosus is unusual among inquilines of cynipid galls in that it only attacks this one host. In non-oak gallwasp galls, the weevil Rhynchites bicolor feeds within the galls induced by the herb gallwasp Diastrophus kincaidii on thimbleberry (Rubus parviflorus), and an unknown species has been reared from Paraulax galls on southern beech (Nothofagus).

Parasitoid wasps

The true parasitoids all depend on an insect host as their main or only food source and most of those found in cynipid communities are found only in cynipid communities. Eupelmus urozonus and Macroneura vesicularis are exceptions, and attack a range of endophytic hosts from a number of orders. A few (Sycophila biguttata, S. flavicollis, Torymus nitens, Mesopolobus sericeus, Aulogymnus skianeuros) occasionally attack cynipid galls on plants other than oak (usually on roses). Very few parasitoids are restricted to just one type of oak gall. Most of the parasitoids in oak cynipid galls are solitary ectoparasitoids (living on the outside of their hosts), while endoparasitoids (those living inside their hosts), such as Sycophila biguttata, and gregarious species, such as Baryscapus berhidanus,Ê are rare. Some images of oak gall parasitoids can be found here.


In addition to the inquiline moths discussed above, other moths can cause high mortality in leaf galls by consumption of the host leaf and/or gall tissue. An example is the virtual extinction of three Neuroterus species at a site in Denmark due to consumption of their leaf galls during an outbreak of a defoliating caterpillar. A range of vertebrate predators extract cynipid larvae from their galls. Woodpeckers and rodents are able to open even large and heavily lignified galls, while smaller insectivorous birds can cause significant mortality in smaller, thin walled galls. In Edinburgh this year, almost all the Andricus kollari galls were opened by grey squirrels! Vertebrate predation has yet to be studied in detail in many oak cynipid galls, and it is possible that it has a more important role in the population dynamics of cynipids and the evolution of gall traits.


Fungi can kill a high proportion of gallwasps. Plant tissues are commonly occupied by a range of fungi (termed endophytic) whose hyphae grow through plant tissues without symptoms in the plant host. The endophytic fungus Discula quercina (Coelomycetes) has been shown to cause almost 100% mortality in artificial gall infection experiments and it has been proposed that cynipids and other endophytic insects minimise contact with such potential causes of mortality by occupying plant regions that maintain low levels of endophyte infestation - sometimes called 'low endophyte space'. Suppression of fungal infestation has also been proposed as a selective advantage for high tannin levels in oak galls.

The significance of gall structure for cynipid gall communities (This topic is discussed in detail in our recent review in Trends in Ecology and Evolution, which you can download here)

Gallwasps are probably unmatched in the structural sophistication and diversity of their galls. Though enormously varied in form, position on the tree, and season of growth, each generation of each species has highly characteristic gall traits. As discussed above, it has been suggested that this diversity has evolved in response to selection for defence against attack by natural enemies (sometimes termed selection for 'enemy free space'). There is good reason to believe that gall traits should be sensitive to selection imposed by natural enemies. Natural enemies inflict high mortality in cynipid galls, and all of them attack through gall tissues. Because gall traits are determined largely by the gall-inducer, those that confer protection against attack by natural enemies should spread through natural selection.

Circumstantial support for the enemy hypothesis in oak gallwasps is provided by the fact that a number of apparently defensive traits have evolved repeatedly. Examples include spines, sticky coatings of resin, internal air spaces and larval chambers that roll freely within a hollow gall. However, the fact that many parasitoids and inquilines are still able to attack a wide structural diversity of host galls suggests that no gall structure provides an absolute refuge in the present. Gallwasps and their enemies, like the occupants of castles and besieging armies, are probably involved in an 'arms race' for attack and defence. Though parasitoids may not be wholly excluded, it remains possible that specific gall traits reduce the mortality inflicted by specific community members. The best-supported defensive gall traits are described below:

Nectar secretion

Nectar secretion has the clearest demonstrated significance of any oak cynipid gall trait for protection against natural enemies. Oak cynipids in the genera Andricus, Disholcaspis and Dryocosmus induce galls that secrete nectar and recruit ants. Oak trees never secrete nectar, and the nectaries on these galls are a clear example of gallwasp manipulation of its plant host. Four independent studies have demonstrated that ants significantly reduce parasitoid and inquiline attack on these galls.

Gall toughness

Tougher galls are thought to be harder to attack than softer galls, both because of the physical difficulty for the parasitoids of drilling into tougher galls to lay their eggs, and due to the risk of predation associated with prolonged egg laying. Two parasitoids of some of the toughest galls, Ormyrus nitidulus and Torymus auratus (= nitens) have particularly high concentrations of manganese in the blades of their drilling ovipositor. This is associated with increased toughness of chitin, and may be an adaptive response to gall hardness. Tough outer gall structures may also be important in excluding lepidopteran larvae.


LEFT: A parasitoid, Megastigmus stigmatizans, drilling into a knopper gall of Andricus quercuscalicis RIGHT: The tip of a Megastigmus ovipositor.

Gall wall thickness

Studies of intraspecific variation in gall structure in a range of systems show that parasitoids are restricted to attacking larvae within reach of their ovipositor, and that larvae in thicker-walled galls on average suffer a lower rate of parasitoid attack. In oak cynipid galls, parasitoids such as Torymus flavipes (= auratus) have differences in ovipositor lengths in their two annual generations that correlate with the gall wall thickness of their respective host galls. That parasitoid species can act as a selection agent for gall size has been demonstrated particularly well for the gall-inducing Tephritid fly Eurosta solidaginis. Here gall size is a reliable indicator of host quality, and parasitoid species select gall sizes according to the host size they require for successful development. This effect of wall thickness may explain why gallwasp larvae usually do not start to grow until the gall wall is fully developed: if parasitoids halt host growth on oviposition, such a strategy would exclude larger, short-ovipositored parasitoid species because small larvae just don't provide enough nutrition for successful parasitoid development.

In a further twist to this tale, some parasitoids lay their eggs early while the gall is small, and either delay egg hatching, or hatch and feed initially on plant tissue. Examples include Torymus cyaneus (Torymidae) and Eurytoma brunniventris (Eurytomidae). Eurytoma brunniventris can develop to maturity by feeding entirely on gall tissue. The ability to fed on plant tissue allows parasitoid larvae to migrate between insect food sources (as Eurytoma larvae do in Diplolepis rose galls), and also provides an alternative food source when insect hosts are too small to allow complete development. Torymus cyaneus attacks young host galls, but the larvae feed initially only on gall tissue. This allows the cynipid gall wasp and the gall to grow, providing a larger food resource when the parasitoid finally attacks the host larva, and a tougher gall as protection against hyperparasitoids.

The number of larval chambers per gall.

There is growing evidence that multilocularity (many larval chambers in a single gall) may represent a strategy associated with protecting larvae from parasitoid attack through induction of a larger gall. Though larvae in peripheral chambers remain vulnerable, those deeper within the structure are protected by a thicker shield of gall tissue and other larval chambers. A similar effect has been observed for inquilines that develop as multiple larvae within a host gall. The inquiline larvae are able to induce increased thickness of host gall tissues, and the more larvae there are, the thicker the gall wall.

Additional morphological traits

Further morphological characters that might affect parasitoid attack have been suggested although their effectiveness has not yet been tested:

  1. Coatings of sticky resins have evolved repeatedly in oak cynipid galls. These may prolong parasitoid oviposition, and so increase their risk of predation to the gall-inducer's benefit.
  2. Many galls change colour during development, normally from green to red. The extent to which the cynipid larva controls pigment synthesis remains unknown but there is evidence that parasitoid females recognise colour and use it in assessment of host quality.
  3. Coatings of hairs and spines of a range of lengths and densities have also evolved repeatedly in oak cynipid galls. It has been suggested that dense coatings of fine hairs or spines might influence parasitoid attack, while more open arrays of stout spines are probably more effective deterrents for vertebrate predators.
  4. Many oak galls contain airspaces between the larval chamber and outer gall tissues, another trait that has evolved repeatedly. An airspace means that the ovipositor of an attacking parasitoid is unsupported for part of its length, which reduces the force that a parasitoid can exert with the tip of its ovipositor without buckling. Having traversed such an airspace, an ovipositor has to penetrate the thin layer of sclerenchyma surrounding the larval chamber - often the toughest tissue in the gall. Only parasitoid species that possess a particularly toughened ovipositor are likely to be able to penetrate the sclerenchyma in such galls.