Global Patterns in Gallwasp Diversity
There are about 1000 known species of oak gallwasp in 41 genera world-wide, predominantly in the Northern Hemisphere. Geographic variation in gallwasp diversity generally folows (and so is perhaps driven by) patterns in oak species richness. The greatest richness of oak gallwasps is found in the Nearctic (and particularly in Mexico), where there are an estimated 700 species in 29 genera. Oak gallwasps are thought to have undergone major radiations in Central America, alongside their oak hosts. Rose gallwasps (tribe Diplolepidini) and the Acacia gallwasps (tribe Eschatocerini) show similar patterns, suggesting that America may have acted a general centre of diversification for this related set of lineages. The palaearctic fauna is less species-rich. For example, California alone contains more oak cynipid species than the whole of Europe and Asia (ca. 140 species in 11 genera), and three times as many as are known from Asia (ca 50 species). However, both regional and global estimates of oak cynipid species richness can only be regarded as approximate for two reasons. First, the sexual and asexual generations of many cynipid lifecycles have yet to be linked (particularly in the Nearctic), with the result that many species are probably currently classified as two. As lifecycles are resolved, so species richness will fall. Second, the cynipine faunas of large areas of high potential species richness remain little known - particularly Central America and highland oak forests in China and Southeast Asia, and as these are sampled more intensively, species richness is expected to rise.
Gallwasp invasions and introductions
There are at least three independent examples of changes in gallwasp distributions resulting from human activity. Two cases represent introductions of individual species that are significant because they affect economically important hosts. Introduced European cork oak, Quercus suber, in California has been colonised by a European cynipid, Plagiotrochus suberi, and Chestnut (Castanea) in Japan, North America and very recently Italy, has been colonised by Dryocosmus kuriphilus, a native of China and Korea. The most significant example of cynipid range expansion is associated with human dispersal of a specific oak - the Turkey oak Quercus cerris, in Europe. This section Cerris oak is native to southern Europe - Italy, the Balkans and Asia Minor, and is the host for one or both generations of a wide diversity of cynipids. No cynipids dependent on Q. cerris (or any other section Cerris oak) for one or both generations in their lifecycle occur naturally in northern Europe.
Over the last 400 years Quercus cerris has been planted widely in northern and western Europe, and at least 9 oak gallwasps have subsequently invaded northwestern Europe, including eight host-alternating Andricus species (A. aries,A. corruptrix, A. gemmea, A. grossulariae, A. kollari, A. lignicola, A. lucidus and A. quercuscalicis) and two species currently thought to be wholly dependent on Q. cerris ( Aphelonyx cerricola and Neuroterus saliens). Andricus corruptrix, A. aries, A. lignicola, A. lucidus, A. quercuscalicis and Aphelonyx cerricola all reached Britain between 1950 and 2000, 2000 km from the nearest natural Q. cerris stands, apparently without direct human assistance. Andricus kollari has spread naturally across most of northern Europe, but was also deliberately introduced into Britain from the eastern mediterranean in the first half of the nineteenth century. These gallwasps have proved to be valuable study systems for work on the way in which natural enemies locate and attack new hosts (some papers on this subject are listed here).
