The increasing pest status of the bollworm, Helicoverpa armigera (Hübner) in South Africa, has prompted renewed interest in the use of bio-insecticides, especially as resistance is suspected to be developing against commonly used pyrethroids. Three entomopathogenic fungi, Beauveria bassiana (Balsamo-Crivelli) Vuillemin, Nomuraea rileyi (Farlow) Samson and Isaria fumosorosea Wize were compared in laboratory bioassays employing topical versus per os inoculation techniques. At a dose of 3.75 × 105 conidia per larva, N. rileyi out-performed both I. fumosorosea and B. bassiana, causing a mean of 87 ± 1.4 % mortality. Following treatment of larvae the proportional mortality among larvae and pupae was generally biased towards pupae with (pooled) treatment mean mortalities of 28 ± 8 % vs 56 ± 18 %, respectively. Notably, no difference was detected between the two inoculation techniques employed with any of the three fungal species assayed. The incubation periods for topical applications ranged from 4 days (PPRI 7201 and 8072) to 7 days (PPRI 7758) while per os treatments responded on Day 4 (PPRI 8072) and Day 5 (PPRI 7758 and 7201) post-inoculation. A general trend of accelerated pupation following treatment with N. rileyi was noted with both inoculation techniques. A novel per os inoculation substrate was tested that ensured more accurate dose acquisition and performance quantification. Compared to the other fungi, superior performance by N. rileyi underscores the potential of this species for development as a myco-insecticide against H. armigera.
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Vol. 20 • No. 1