PATHOGENICITY OF STEINERNEMA-RIOBRAVIS AGAINST CORN-EARWORM, HELICOVERPA-ZEA (BODDIE)

The pathogenicity of a new entomopathogenic nematode species, Steinern ema riobravis against prepupae and pupae of corn earworm, Heliocoverpa (= Heliothis) zea (Boddie) found soil samples of com fields in the Lo wer Rio Grande Valley of Texas, was tested under laboratory conditions . Exposure to 10, 20, 40, 80, and 100 infective juveniles per prepupa in filter paper resulted in mortalities of 40, 55, 85, 90, and 100 % r espectively. The LC50 of S. riobravis for H. zea prepupae was thirteen nematodes per prepupa. The nematodes multiplied similarly in prepupae and pupae of com earworm with average nematode production per prepupa and pupa cadavers of 321 000 and 300 000, respectively. Production of nematodes was independent of concentration of infective juveniles fro m 5 to 100 per host. The overall yield of nematodes per prepupa and pu pa was 311 000. The highest average yield of nematodes per insect cada ver was 375 000 which occurred at an exposure concentration of 40 infe ctive juveniles per prepupa. These results indicate a high degree of i nfectivity and parthogenicity of Steinernema riobravis and its symbiot ic Xenorhabdus bacterium to com earworm and suggest it may have a grea t potential against prepupal and pupal stages of H. zea.