Microscopic study of nine galls induced in Populus nigra by aphids of the Iberian Peninsula

Aphids (Hemiptera, Aphididae) feed on the phloem and some of them induce the formation of conspicuous galls on their primary hosts. Histological studies were proposed to elucidate the evolutionary history of galling habit in Pemphigini tribe, assessing if gall complexity indicates the phylogenetic relations among gall inducers. Nine Eriosomatinae galls induced on Populus nigra from the Iberian Peninsula were studied microscopically. The galls are induced by Pemphigini of the genera Thecabius (2 galls) and Pemphigus (7 galls). Using multiple correspondence analysis of the observed microscopic characteristics, a hierarchical cluster was obtained showing the existence of 2 groups of galls. One group consists of the 2 galls induced by Thecabius and, within the genus Pemphigus, those belonging to the subgenus Pemphigus (P. populi and P. vesicarius). The other group consists of galls of the genus Pemphigus, subgenus Pemphiginus (P. bursarius, P. immunis, P. spyrothecae, P. protospirae and P. populinigrae). The gall histological complexity is related to Pemphigini phylogeny, confirming the importance of such studies in galling aphid taxonomy and possible pathways of galling habit evolution. Similarities are established between the galls of Pemphigini aphids studied here with others we studied microscopically, induced by Fordini and Eriosomatini. Finally, a classification of all Eriosomatinae galls is proposed, establishing 3 groups: galls that cause severe malformations (induced by derived galling taxa), those that cause less severe malformations, and those that cause mild malformations (pseudogalls). It also demonstrates the importance of the strategy of waste evacuation in derived galling species.