A bioassay of the pathogenicity of Verticillium lecanii conidiospores on the aphid, Macrosiphoniella sanborni

Methods of obtaining uniform batches of conidiospores and stock insects are described together with an assay technique using isolated apterae on leaf discs at virtually 100% relative humidity. Spore suspensions were applied to the apterae in a Buchner funnel and drained off by suction. The weighted mean slope for 11 bioassays was 2.26, and the weighted mean LC₅₀ was 2.33 × 10⁵ spores/ml of suspension with 95% fiducial limits of 1.61 to 3.38 × 10⁵. The within-assay variances were small (mean 0.0081), but in comparison the between-assay variance was large (0.0659) and partly due to a significant lowering of the LC₅₀s with time after the first assay, thought to be caused by an increasing susceptibility of the laboratory stock of aphids. The effects on precision of varying the numbers of replicate assays, doses within an assay, and insects per dosage are illustrated. One four-dose assay will detect differences of about X8 in the LC₅₀s of two batches of spores, duplicating assays X4; and replication 7 times, X2.