A new ecotype of Plagiotrochus amenti Kieffer (Hymenoptera: Cynipidae) revealed by morphological and molecular analysis, with discussion of its potential as an emerging pest of holm oak

Abstract

The cynipids induce some of the more complex plant galls found in nature. While the majority of species are not harmful to their host plants, some have been reported to damage their hosts while some species even constitute pests, as is the case of the invading chestnut gallwasp species Dryocosmus kuriphilus Yasumatsu. Other potentially damaging species are some species of the genus Plagiotrochus Mayr, a genus associated with evergreen Mediterranean species of Quercus such as the cork oak (Q. suber L.) and the holm oak (Q. ilex L.). The best-known is P. amenti Kieffer, a species linked to the cork oak. Biologically, P. amenti is peculiar because is the only Plagiotrochus species thought to be strictly associated with Quercus suber In this study, the presence of an unknown Plagiotrochus species in an experimental plantation in Badajoz (Spain) on a specific clone of Quercus ilex ilex was monitored and investigated. Morphological examination and sequencing of two molecular markers, one mitochondrial (cytb) and another nuclear (opsin), revealed that the studied population is conspecific with typical P. amenti. However, given its different host plant it represents a distinct ecotype of this species, a fact also reflected in having cytb haplotypes that form a distinct lineage sister to haplotypes found in individuals from Q. suber hosts. This result represents the first reported case of a species of Plagiotrochus that breaks the strict host-specificity characterizing this genus, being the only one to gall both section Ilex and section Cerris oaks. Furthermore, the holm oak-associated ecotype appears to have lost its sexual generation and exhibits strictly asexual reproduction, instead of the cyclical parthenogenesis found in the typical cork oak-associated P. amenti ecotype. The importance of the detection of this new ecotype as a possible emergent pest on holm oak is discussed.