Virulence of new and mixed strains of the entomopathogenic nematode Steinernema riobrave to larvae of the citrus root weevil Diaprepes abbreviatus

Steinernema riobrave ranks as the most efficacious entomopathogenic nematode for inundative biological control (biopesticide) applications against larvae of the root weevil Diaprepes abbreviatus in Florida citrus groves and has been commercially available for several years. However, only one strain is known, and others could be more effective. Therefore, we used two soil sampling techniques (cores and shovel/bucket samples) and collected soil samples at 10 sites near the type locality of Weslaco, Texas. Soil samples were baited with either Galleria mellonella or D. abbreviatus larvae and produced 10 new isolates of S. riobrave from five sites, and an undescribed species of entomopathogenic nematode in the genus Heterorhabditis from one site. All new isolates were from G. mellonella larvae, and eight of the S. riobrave isolates were from soil cores. In virulence assays comparing the new S. riobrave strains, the commercial strain, and a mixed strain (formed by pooling the new strains), average larval mortality for five strains (including the mixed strain) was significantly greater than for the commercial strain. Larval mortality for the 12 strains ranged from 57.7 to 84.1% with the commercial strain and the mixed strain being the extremes, and the mixed strain killing 45.7% more than the commercial strain. Among the 10 new strains, larval mortality ranged from 65.8 to 79.3%, with two strains from the same site being the extremes and the only strains significantly different from one another. Thus, these strains have potential for improved biological control of D. abbreviatus and further comparisons are warranted.