Laboratory evaluation of the virulence of Beauveria bassiana isolates to the sorghum shoot borer Chilo partellus Swinhoe (Lepidoptera : Pyralidae) and their characterization by RAPD-PCR

Beauveria bassiana has long been used as a mycopesticide. It has a wide hos t range; isolates have been reported to differ in host range and virulence to a given insect species. Identification of a molecular marker linked to a virulent phenotype to a target pest would be useful in screening for isola tes effective against it. Twenty B. bassiana isolates were tested for their virulence to the second instar larvae of Chilo partellus Swinhoe in labora tory bioassays and their DNA fingerprints were generated by RAPD-PCR. Three arbitrary categories of aggressiveness were chosen; isolates that caused > 70%, between 70 and 40% and < 40% larval mortality were grouped as highly, medium and less aggressive types, respectively. In the random amplified po lymorphic DNA (RAPD) analysis a 30% variability was observed among the isol ates; which clustered into three major groups. The groups based on virulenc e rating did not match with the RAPD clusters. One of the highly aggressive isolates clustered with less aggressive isolates in one cluster and the ot her grouped along with the medium aggressive isolates in a different cluste r. The B. bassiana isolates were classified phenotypically based on the tax onomic order of the original insect host and the climatic zone (tropical/te mperate) from which they were isolated. No correlation between the aggressi veness of the isolate and the relatedness of the original insect host to th e tested insect was observed; both the highly aggressive isolates were from coleopteran insects. A correlation was found between the RAPD grouping and the phenotypic classification of the isolates. All the lepidopteran isolat es grouped into one major cluster, most sub clusters were constituted by is olates from the same climatic zone.