Two unknown species of Mermithidae (Nematoda) parasitising fall armyworm Spodoptera frugiperda (Lepidoptera: Noctuidae) in Australia

Abstract Fall armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae) is an invasive and important pest of multiple crops including grains and vegetables. The moth was first detected in Australia in 2020 and has since established populations across the north of the continent with further spread expected. Surveys for parasites and pathogens impacting Australian fall armyworm populations were undertaken between 2020 and 2025. Here, we report on two distinct genetic lineages of parasitic nematode of the family Mermithidae collected from S . frugiperda caterpillars during this period. Because the specimens available were post-parasitic juveniles, they could not be reliably identified morphologically beyond the family level. Phylogenetic analyses using two ribosomal (18S, 28S) and one mitochondrial (COI) gene demonstrated the presence of two distinct species-level lineages (one in Queensland and one in the Northern Territory) but they did not resolve the taxonomic position of either. Thus, neither lineage can be reliably assigned to any known genus and are therefore given informal placeholder assignments: Mermithidae sp. FA-QLD and Mermithidae sp. FA-NT. Mermithid nematodes likely represent an important component of natural population regulation for fall armyworm and join a growing list of organisms which have been found parasitising this moth in Australia.