Allomermis solenopsi n. sp. (Nematoda: Mermithidae) parasitising the fire ant Solenopsis invicta Buren (Hymenoptera: Formicidae) in Argentina George O. Poinar Jr Æ Sanford D. Porter Æ Sha Tang Æ Bradley C. Hyman Received: 30 August 2006 / Accepted: 13 November 2006 Ó Springer Science+Business Media B.V. 2007 Abstract Allomermis solenopsi n. sp. (Mermithi- dae: Nematoda) is described from the fire ant Solenopsis invicta Buren (Hymenoptera: Formicidae) in Argentina. Diagnostic characters of the new species include stiff and erect processes on the surface of the mature egg, small female amphids, extension of the latero-medial rows of male genital papillae beyond the middle rows, an obliquely truncate spicule tip and a ventrally swollen male terminus. This is the first record of Allomermis Steiner, 1924 from South America and the first host record for members of this genus. Previous records of mermithids from Solenopsis spp. are summarised. The placement in Allomermis was confirmed by molecular analyses based on nuclear 18S ribosomal DNA sequences, the first such molecular framework for the Mermithidae. The possible life-cycle of the parasite is discussed, with the aim of using A. solenopsi as a biological control agent for fire ants in the United States. Introduction Ant parasitism by mermithid nematodes has been ongoing for some 40 million years (Poinar, 2002) and these associations occur throughout the world today. Mermithids have been reported from at least 17 species of Neotropical ants; however, only four species have been described and, of these, only the description of Meximermis ectatommi Poinar et al. (2006) included the adults of both sexes. Mermithids have also been reported from various species of Solenopsis Westwood (Table 1); however, none were identified or described. Thus it was of considerable interest when, during a study of native parasites of Argentinean populations of fire ants with the objective of appraising potential biological con- trol agents for release in the United States, popula- tions of Solenopsis invicta Buren were discovered to be parasitised by an undescribed mermithid species. Since mermithids are known to kill their hosts and have been used previously in the biological control of insect pests (Petersen & Willis, 1972), the present G. O. Poinar Jr (&) Department of Zoology, Oregon State University, Corvallis, OR 97331, USA e-mail: poinarg@science.oregonstate.edu S. D. Porter USDA-ARS, CMAVE, 1600 SW 23rd Drive, Gainesville, FL 32608, USA S. Tang Á B. C. Hyman Interdepartmental Graduate Program in Genetics, Genomics, & Bioinformatics, University of California, Riverside, CA 92521, USA B. C. Hyman Department of Biology, University of California, Riverside, CA 92521, USA 123 Syst Parasitol (2007) 68:115–128 DOI 10.1007/s11230-007-9102-x