Background: The fall armyworm, Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae) is a major threat to many crops, with larvae attacking the plants at all growth stages.

Materials: A study examined if nematodes (Steinernema carpocapsae (Sc) and Heterorhabditis bacteriophora (HP88)) could be effective against fall armyworm (S. frugiperda) infestations on okra plants. The alternations in enzymatic activities of the insect pest larvae were determined 48 hours after exposure to the nematodes.

Results: Under laboratory conditions, entomopathogenic nematodes, S. carpocapsae, exhibited greater virulence compared to H. bacteriophora against the second larval stage of Spodoptera frugiperda. The highest concentrations of nematodes recordS. carpocapsaeed the highest mortality in larvae of Spodoptera frugiperda. S. carpocapsae killed all larvae in their second instar, 100% of them, four days after treatment with the highest concentrations (300 and 200 IJs/ml). Also, when using H. bacteriophora, complete mortality was obtained 5- days post-treatment. In contrast, entomopathogenic nematodes and H. bacteriophora have the same virulence against the fourth larval stage of S. frugiperda.

Also, when using S. carpocapsae at a concentration (number) of 100 IJs/ml, four days after treatment, the greatest death rate (86.15%) was recorded.

The study examined the protein content and lipid peroxide activity of S. frugiperda larvae infected with entomopathogenic nematodes. The total protein content was highest in the untreated larvae, while those infected with H. bacteriophora had the lowest content. The fall armyworms (S. frugiperda) in their sixth larval stage had the most lipid peroxide activity when treated with both nematodes (S. carpocapsae and H. bacteriophora). However, the highest levels of chitinase activity were seen in worms treated with S. carpocapsae specifically during their fourth larval stage.