Entomopathogenic nematodes Heterorhabditis bacteriophora Poinar and Steinernema carpocapsae Filipjev are important agents of biological control. Identification of host defensive reactions against these agents can affect the application of these nematodes. Current study was performed to identify cellular defence reactions of the Spodoptera exigua against H. bacteriophora and S. carpocapsae. Seven individuals of infective juvenile were injected to beet armyworm larvae and defensive cellular reactions was addressed at 0.5, 1, 2, 4, 8, 12, 16, 20 hour post injection. The total hemocytes counts of blood cells in injected larvae with S. carpocapsae after 12 hours were increased to maximum amount. After this peck, this level was decreased. While the total hemocyte counts of hemocytes in the injected larvae with H. bacteriophora at 4 hours post injection were in the highest level. The immune system of S. exigua larvae could recognize infective juveniles of H. bacteriophora as foreign agent. By 0.5 hour after injection, 60% of injected infective juveniles were with attached hemocytes. After initial step of hemocytes attachment, complete encapsulation and melanization of H. bacteriophora were occurred by 12 and 20 hours after injection, respectively. The level of hemocytes reactions against H. bacteriophora including attachment and encapsulation was 35%. Unlike the strong immune reaction of fifth instar larvae of S. exigua against H. bacteriophora in term of hemocyte defence, this event was weak against S. carpocapsae. The hemocytes of the larvae could not recognize infective juveniles of S. carpocapsae. It concluded that the cellular defense reactions of S. exigua against S. carpocapsae were weaker than those against H. bacteriophora.

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