The virulence of six Egyptian steinernematid and heterorhabditid nematodes against Galleria mellonella larvae and Periplaneta americana adults was compared at different concentrations and exposure times in the laboratory. The filter paper bioassay was used to screen the effect of different nematode isolates on the two insect hosts. Nematodes were able to infect and kill both hosts. The degree of virulence of each nematode species against the two hosts varied considerably. P. americana adults were less susceptible than G. mellonella larvae at all concentrations and exposure times. Steinernema sp. E was the most virulent nematode against G. mellonella larvae causing 100% mortality at all exposure times. This nematode was followed by Steinernema sp. 2D that produced the lowest LC50 values of 2.40, 2.17, and 1.24 IJs/ treatment at 24, 48, and 72 h post exposure, respectively. Heterorhabditis sp 8Z, Heterorhabditis sp 15Z and Heterorhabditis sp 2 M were less virulent than the previous ones. Steinernema sp. 2Q was the least virulent nematode with LC50 values of 13.79, 7.48, and 3.32 IJs/ treatment at 24, 48, and 72 h post exposure respectively. Against P. americana, again Steinernema sp. E was the most virulent nematode strain with the lowest LC50 values of 324.05, 215.62, and 153.62 IJs/ treatment at 24, 48, and 72 h post exposure, respectively. This nematode was followed by Heterorhabditis sp. 8Z, Heterorhabditis sp. 2 M, Heterorhabditis sp. 15Z and Steinernema sp. 2Q. Steinernema sp. 2D was the least virulent nematode with the highest LC50 value of 1725.88 IJs/ treatment at all time intervals. Signs and symptoms of steinernematid and heterorhabditid infection in P. americana were recorded and these included extensive aggressive self-grooming, scratching and chewing activities. Eventually the IJs successfully penetrated the cockroach and established in the body cavity causing the paralysis and the death of P. Americana. Steinernema sp. E appears to be very promising entomopathogenic nematode for use in P. americana control. The data indicate the importance of testing a number of Egyptian nematode species/strains against specific insect pests in the laboratory before commencing field trials for biological control.
Key words: Steinernema, Heterorhabditis, Cockroach, Greater Wax Moth
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