Cynipini

IRENE LOBATO-VILA, ALBA SALA-NISHIKAWA, GEORGE MELIKA, GRAHAM N. STONE, CHANGTI TANG, MAN-MIAO YANG, ZHIQIANG FANG, YING ZHU, YIPING WANG, SUNGHOON JUNG, JAMES A. NICHOLLS & JULI PUJADE-VILLAR, 2022, A catalogue, revision, and regional perspective of Eastern Palaearctic and Oriental oak gall wasps and their inquilines (Hymenoptera: Cynipidae: Cynipini, Synergini, Ceroptresini), Zootaxa 5161 (1), pp. 1-71 : 50-52

Cynipini host-plant associations

Of the 103 valid Cynipini species whose host associations are known, 81 induce galls on Quercus : 28 on hosts in subgenus Quercus (all section Quercus ), 51 on hosts in subgenus Cerris (19 in section Cerris , 2 in section Ilex , and 30 in section Cyclobalanopsis ), and two hosts in oaks of undetermined section ( Table 3 ). In contrast, only 18 species induce galls on other Fagaceae genera ( Lithocarpus , Castanea and Castanopsis ). The host plant associations of the remaining species are unknown.

A striking feature of EPO Cynipini is that while individual species are specific to a particular non-oak genus or oak section, the host ranges of several genera span multiple divergent lineages of Fagaceae ( Table 3 ). This is most pronounced in Dryocosmus , which includes species that gall three genera of Fagaceae ( Castanea , Castanopsis , Quercus ) and two oak sections in different subgenera ( Cyclobalanopsis and Quercus ). Other genera show a similar pattern, if less dramatically: the purely East Asian genus Cycloneuroterus includes species that gall Quercus , Castanopsis and Lithocarpus , while Neuroterus includes species galling each subgenus of Quercus and one species on Lithocarpus , and Andricus primarily galls section Quercus oaks but includes one species galling Castanea .

While not all EPO Cynipini genera show such host diversity, those that do contrast markedly with pattern in the WP, in which (with some well-proven exceptions) members of a single Cynipini genus attack hosts in a single oak section ( Stone et al. 2009). For example, WP Dryocosmus and Neuroterus only gall oaks in the sections Cerris and Quercus , respectively. The exceptions in the WP are a monophyletic clade of Andricus species that show obligate host alternation between a sexual generation on section Cerris and an asexual generation on section Quercus , and a monophyletic clade of Callirhytis that alternate between the same host taxa but in the opposite direction. It is notable that no EPO species are known that show such Quercus / Cerris host alternation, or any other type of host alternation, despite co-occurrence of both host oak sections and other Fagaceae over large areas.

One possible reason for the high host range of some Cynipini genera in the EPO regions is that the morphological characters currently regarded as diagnostic do not define monophyletic groups, leading to paraphyletic or polyphyletic genera. This is discussed in more detail for specific genera below. However, if we take the pattern at face value, then high host richness could be an evolutionary result of geologically long-term exposure of cynipid lineages to very high host lineage diversity.

Cynipini lifecycles and gall induction sites

Excluding the three species known to be purely parthenogenetic ( Andricus pseudoflos , A. targionii and Dryocosmus kuriphilus ), alternating generations have been linked for only 23 of the remaining 100 EPO Cynipini species (23%). If we assume that all of these 100 Cynipini species are cyclically parthenogenetic, then at least 77 additional gall and wasp generations remain to be discovered. Of the 77 (77%) species known from a single generation, and again excluding species known to be purely parthenogenetic, 19 (24.7%) are known only from an asexual generation and 58 (75.3%) are known only from a sexual generation. These values are interesting because in the WP Cynipini fauna, most species known only from a single generation are known from their asexual generation ( Nieves-Aldrey 2001; Roskam 2019). This difference may reflect regional differences in the detectability of the relevant galls. In the WP the mature asexual generation galls of many (though not all) Cynipini are relatively large and often patterned and/or coloured ( Stone et al. 2002; Csóka et al. 2005; Roskam 2019), and persist on the plant for long periods, which may aid their detection. This is less true for the EPO Cynipini fauna so known far, though this does include some very detectable asexual generation galls (such as the large ball or spiny galls induced by asexual generations of Trichagalma ).

With this in mind, knowledge of gall structures and locations on the host plant for EPO Cynipini is very incomplete. Currently known species between them induce galls on all major plant organs ( Table 1) and (with some exceptions) each generation of a specific species tends to gall a single plant organ ( Table 1). In contrast, alternating generations of the same species often gall different organs on the same plant. Leaves are the most frequent gall induction site ( Table 1); most Andricus sexual generations induce integral leaf blade galls, while asexual generations of Belizinella and of Cerroneuroterus induce detachable galls on the underside of leaves. All Cerroneuroterus sexual generations develop in catkins, and the asexual forms of Cyclocynips develop inside stems. As far as we know, the genus showing the greatest diversity of gall induction locations in the EPO is Dryocosmus , which contains species that gall all plant organs except the roots.