Abstract: Spodoptera frugiperda is an important invasive pest which causing serious damage to various crops. Following the invasion and expansion, the pest makes great threaten to the crops in China. Entomopathogenic nematodes, the natural enemies specialized to the pest, are considered a potential biological control agent for fall armyworm. In the present study, the symbiotic bacterium was isolated from the Steinernema carpocapsae strain N-Yz1, and identified to the genera of Xenorhabdus nematophila. The bacteria were the main toxic factor to the larvae of S. frugiperda, led to rapid larval death by injected 100 to 200 bacterial counts per larva. The infection of nematode N-Yz1 caused rapid lethality and high mortality to juvenile larvae (the second and third instar) of fall armyworm, with infected lethal individuals appearing within 24 h of inoculation and mortalities of 97.0% and 81.94% at the dosage of 30 IJs per larva. For the elder larvae (the fourth to fifth instar), the killing speed was relative slow, with initial lethal individuals appearing at the time of 48 h post inoculation and mortalities of 81.94% and 76.39% at the time of 72 h after nematode inoculation. It was positive correlation between inoculum dosage of nematode N-Yz1 and mortality ration of larvae. All the larvae of 4^th instar S. frugiperda were killed when the inoculation dose exceeds 60 IJs per larva 48 h post inoculation. N-Yz1 also had the ability to infecting pupae of S. frugiperda, at the dose of 50 and 100 IJs per pupa, the mortalities of 1^st day pupae were 53.57% and 55.38%. The activity of N-Yz1 infection was affect by temperature, with disability of infection at 35 ℃and peak mortality delayed to 120 h at 15 ℃. It was indicated that N-Yz1, a strain of Steinernema, is a potential agent for the controlling to the larvae and pupae of S. frugiperda..

Key words: Spodoptera frugiperda, Steinernema carpocapsa, Xenorhabdus nematophila