Reexamination of Rhopalosiphum (Hemiptera: Aphididae) using linear discriminant analysis to determine the validity of synonymized species, with some new synonymies and distribution data.

Although 17 species of Rhopalosiphum (Hemiptera: Aphididae) are currently recognized, 85 taxonomic names have been proposed historically. Some species are morphologically similar, especially alate individuals and most synonymies were proposed in catalogues without evidence. This has led to both confusion and difficulty in making accurate species-level identifications. In an attempt to address these issues, we developed a new approach to resolve synonymies based on linear discriminant analysis (LDA) and suggest that this approach may be useful for other taxonomic groups to reassess previously proposed synonymies. We compared 34 valid and synonymized species using 49 measurements and 20 ratios from 1,030 individual aphids. LDA was repeatedly applied to subsets of the data after removing clearly separated groups found in a previous iteration. We found our characters and technique worked well to distinguish among apterae. However, it separated well only those alatae with some distinctive traits, while those apterate which were morphologically similar were not well separated using LDA. Based on our morphological investigation, we transfer R. arundinariae (Tissot, 1933) to Melanaphis supported by details of the wing veination and other morphological traits and propose Melanaphis takahashii Skvarla and Miller as a replacement name for M. arundinariae (Takahashi, 1937); we also synonymize R. momo (Shinji, 1922) with R. nymphaeae (Linnaeus, 1761). Our analyses confirmed many of the proposed synonymies, which will help to stabilize the nomenclature and species concepts within Rhopalosiphum.