A New Genus of Four-Legged Mites from Palms in Vietnam: The Morphology and Phylogeny of Calventer arengii n. g. & sp. (Eriophyoidea, Phytoptidae)

Eriophyoidea (Acariformes), a superfamily of obligate phytophagous and highly host-specific mites, comprises four early-diverging lineages (Pentasetacidae, Phytoptidae s. str., Nalepellidae, and Eriophyidae s.l.) with unresolved inter-relationships. The clade Phytoptidae s. str. is restricted to angiosperms and includes a group of six genera associated with palms (Arecaceae) and classified in tribes Mackiellini and Phytoptini. Global distribution patterns of Phytoptidae s. str. are biased towards the Americas and Europe. Current knowledge of Asian palm-associated phytoptids is limited to a single record of Borassia from India and the new Calventer arengii n. g. et sp. (Sierraphytoptinae: Mackiellini) described here as a vagrant on the lower surface of fronds of Arenga westerhoutii Griff. (Arecaceae) from Vietnam. Maximum likelihood COI analyses did not support the monophyly of Mackiellini but revealed a sister-group relationship between Mackiella and Calventer and questioned the taxonomic position of Borassia. Notably, Calventer possesses one of the most reduced chaetoms in Phytoptidae, illustrating a recurring trend of setal loss across Eriophyoidea. Plesiomorphically, females of eriophyoids have 65 setae, with most being prone to homoplastic reduction; however, a stable subset (h2, 3a, emp, and ω) is consistently retained. Excluding these “stable setae,” the remaining chaetom can be expressed as a “setal formula” incorporated into generic diagnoses for the efficient, rapid delimitation of genera.