Hostplant Relationships

Not unsurprisingly, the majority of oak gallwasps gall oaks, genus Quercus, in the subfamily Fagoidea of the plant family Fagaceae. Small numbers induce their galls on hosts in the other subfamily of the Fagaceae, the Castanoidea, including chestnuts (Castanea), chinquapins (Castanopsis, Chrysolepis) and tanbark oaks (Lithocarpus).

The genus Quercus is divided into two genera - the strictly Asian subgenus Cyclobalanopsis, and the more widespread subgenus Quercus. Little is known about the cynipids associated with Cyclobalanopsis oaks. The subgenus Quercus is divided into four sections. American oaks belong to three sections - Quercus sensu strictu (white oaks), Lobatae (red oaks), and Protobalanus (golden cup oaks), while Palaearctic (European and Asian) oaks include members of the section Quercus sensu strictu, and an endemic Eurasian taxon, the section Cerris. Cynipids gall hosts in all sections within the subgenus Quercus and with the exception of the host alternating species described below, each cynipid species is generally associated only with a closely related group of oak species.

Oak distributions and patterns of species richness have probably played a major role in the distribution and species richness of oak gallwasps. The subgenus Quercus contains approximately 500 species world-wide, distributed throughout the temperate regions of the Northern Hemisphere, and extending southwards as far as Indonesia and Ecuador. Oaks are more species-rich in the Americas (ca. 300 species) than in the Palaearctic (ca. 175 species), and the greatest richness is found in Mexico (ca 135- 200 species). The Western Palaearctic (the region including Europe) has a low diversity of oaks, with approximately 40 species, and large areas of northern Europe are dominated by just two species - Quercus robur and Quercus petraea. The Eastern Palaearctic contains ca. 130 species, and oaks are a major component of climax forests in highland areas from the eastern Himalayas to the Philippines. The southernmost record of an oak cynipid in Asia is from Quercus spicata in Java.

Host-alternating cynipids

Host alternation (heteroecy) in gallwasps is known only from the oak gallwasps, and within this tribe is known only for western palaearctic species in the genera Andricus and Callirhytis. In both cases, the alternation is between oaks in the sections Cerris and Quercus. In all host alternating Andricus species, the asexual generation females oviposit on the section Cerris host and the sexual generation females oviposit on the section Quercus host, while in Callirhytis the situation is reversed. Cynipids with host-alternating life cycles have distributions restricted to areas where both host taxa occur together, a requirement that has had significant effects on geographical patterns in cynipid species richness in the Western Palaearctic (see Gallwasp invasions and introductions, below). Host alternation is generally extremely rare in insects, though in aphids it has evolved many times. An evolutionary analysis of host use in Andricus (Cook et al. 2002) suggests that it has only evolved once in this genus from a lifecycle with both generations on section Quercus hosts.