Abstract

The virulence of new nine heterorhabditid isolates from South Carolina (Heterorhabditis megidis LEX, H. zealandica EDS and CHR, and H. bacteriophora WPS, SMP, PD, CFG, MF and CFM strains) against the beet armyworm was compared with two known heterorhabditid nematodes (H. bacteriophora Hb and HP88 strains) under laboratory conditions. The Petri-plate bioassay procedure was used to evaluate the susceptibility of the Spodoptera exigua larvae to the heterorhabditids at concentrations of 10, 25, 50, and 100 infective juveniles (IJs) per larva. Mortalities were counted for 4 days. At the final count, mortalities were 53.6-100, 72-100, 79.8-100, and 92.9-100% for all nematode species/strains at the concentrations of 10, 25, 50, and 100 IJs per larva, respectively. H. megidis LEX strain was superior and differed than others by having 100% mortality in all of the concentrations. It was second to cause early mortality. It had the highest mortality rate at 10 nematodes per larva and H. bacteriophora WPS, H. zealandica CHR, H. bacteriophora HP88 and H. zealandica EDS strains followed it with 92.9, 89.3, 85.7 and 82.1% mortality, respectively. LC₅₀ for most of the nematodes was relatively low (10 IJs per larva).  Virulence of H. bacteriophora WPS, HP88 and SMP and H. zealandica CHR strains were similar. The least virulent heterorhabditid was H. bacteriophora CFM strain with LC₅₀ value of 14.8 IJs per larva. The LT₅₀ value of H. bacteriophora WPS strain was the smallest and it was followed by H. megidis LEX, H. zealandica CHR and EDS, H. bacteriophora SMP, HP88, MF, PD, Hb, CFG and CFM strains, respectively.

Key words: Biological control, entomopathogenic nematodes, Heterorhabditis, Spodoptera exigua.