Submitted to: Society for Invertebrate Pathology Annual Meeting Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: May 17, 2004
Publication Date: August 1, 2004
Citation: Hatting, J.L., Wraight, S.P. 2004. Employing a novel bioassay methodology for comparison of the relative susceptibility of two russian wheat aphid clones to beauveria bassiana (hyphomycetes). Society for Invertebrate Pathology Annual Meeting Proceedings. 37:86.
Since its appearance in South Africa in 1978, the Russian wheat aphid, Diuraphis noxia, has become the principal pest of dryland wheat. As part of an integrated control approach, entomopathogenic fungi are being evaluated for biocontrol of D. noxia and other cereal aphids. From a commercial point of view, the quantitative expression of virulence (i.e., median lethal concentration or LC50) of these agents is a crucial step in development. A new bioassay methodology was developed employing direct spray inoculation of aphids and subsequent incubation on live, untreated plants, thus limiting secondary dose acquisition post inoculation. Initially, four strains of Beauveria bassiana were assayed at a single dose, and the best isolate was then assessed in two series of six-dose assays (five assays/series). LC50's, fiducial limits, and other regression parameters were used to evaluate the efficacy of the protocol and to investigate between-assay variability within two clones of D. noxia originating from the Free State (FS) and Western Cape (WC) provinces, respectively. Against the FS clone, an average LC50 estimate of 83 (95% CI: 58-120) conidia per mm2 for B. bassiana strain GHA was calculated. The data indicated high assay precision, reflected by an average coefficient of variation for slope of less than 20%, an average chi-square value of 5.5 ± 2.7 (4 df), and control mortality below 4%. A second set of assays was conducted against the WC clone for verification of assay precision and comparison of susceptibility to B. bassiana between the two D. noxia clones. The assay design accommodates testing of aphid species other than D. noxia and is expected to facilitate tritrophic studies on effects of host-plant resistance on fungal efficacy.