Cynipid galls and their associated parasitoids

Cynipid and other galls have proven useful models in community and population ecology for mainly three reasons: Firstly they are sessile, at least during the larval development, which makes the measurement of densities and distributions relatively easy. Secondly, inquilines (guests) and parasitoids that attack gall inhabitants accumulate inside the gall, and although parasitoids will consume their host, it is often possible to identify the remains. Thus, during the dissection of galls it is often possible to identify the whole community and often the trophic relationships therein. Finally, these communities are largely closed so that the member species are tightly linked and interdependent.

Megastigmus stigmatizans ovipositing into a gall of Andricus quercuscalicis

Askew (1984) gives an excellent overview of work that involved cynipid galls from determinants for the distribution of herbivores on host plants to parasitoid community studies.

Since then, CEH has been involved in various studies trying to understand the determinants of parasitoid community structure using the invasion of the British Isles by four alien species as a natural experiment of community assembly. We studied the galls of Andricus quercuscalicis in detail, but also carried out whole community studies to investigate how the aliens would link to native species and what their impact might be.

The most fascinating aspect of cynipid galls is the diversity of species specific and often generation specific morphologies. Theories about the evolution of a gall making life history pointed at the constant nutrient supply and the constant microclimate as a selective advantage. Both probably have some bearing, but would not account for the wide diversity of gall shapes. The enemy hypothesis suggests that gall morphologies and other gall traits such as their location on the host plant or their phenology might reduce mortalities suffered from the attack of natural enemies. Presumably the most important class of natural enemies are parasitoid wasps.

However, all cynipid galls appear to be attacked by parasitoids today and it has been pointed out that galls are often conspicuous targets even to our eyes and might attract parasitoids rather than protect from them. It is possible that gall morphologies have provided protection in the past, but have lost that aspect since parasitoids have adapted to overcome these defences. If so, the enemy hypothesis should be impossible to test today.

In single species studies, however, various workers could determine that parasitoids are only able to exploit galls during so called "windows of vulnerability" suggesting that at all other times the gall might well provide protection from parasitoid attack. It only appears that galls can be attacked all the time where a sequence of different parasitoid species have slightly different "windows of vulnerability" to exploit.

Together with G.N. Stone (Edinburgh) and J. Cook (Imperial College), CEH has studied whole communities of cynipid gall wasps and their parasitoids. Using a molecular phylogeny, we explore whether or not particularly convergently evolved gall traits can in fact reduce or prevent mortalities from parasitoid attack. To do so, we also have to assess the gall morphologies at the time when the parasitoid attack occurs, since characters such as gall size, the toughness of the gall walls, and even the development of spines and hairs, will change throughout the development of the galls. Also, most of the parasitoid and inquiline species that attack hosts in cynipid galls are reported to be generalists.

To assess the impact of alternative hosts on attack rates in galls included in the phylogeny, we sampled the whole community, to build quantified webs to assess the attacks of the parasitoid species throughout the communities. This work is on-going, but the theory also in the context of other non-cynipid galler systems have been reviewed by Stone et al (2002) and Stone and Schönrogge (2003).

Publications

Stone G N Schönrogge K (2003) The adaptive significance of insect gall morphology. Trends in Ecology & Evolution, 18, 512 - 521.

Stone G N, Schönrogge K, Atkinson R J, Bellido D, Pujade-Villar J (2002) The population biology of oak gall wasps (Hymenoptera: Cynipidae). Annual Review of Entomology 47:633 - 668.

Askew R R (1984) The biology of gall- wasps. In: Anathakrishnan TN (ed) The biology of galling insects, 1 edn. Oxford and IBH publishing Co., New Dehli, pp 223-271.