Pathogenicity of blastospores and conidia of Paecilomyces fumosoroseus against larvae of the Mexican bean beetle, Epilachna varivestis Mulsant

Abstract

The effects of blastospores and conidia of Paecilomyces fumosoroseus were compared for insecticidal activity against 3-day old larvae of Mexican bean beetle (MBB), Epilachna varivestis Mulsant. Blastospores of P. fumosoroseus were produced using strain ARSEF 3581 grown in a liquid basal medium, whereas conidia were produced on potato dextrose agar. Initial insecticidal activity was demonstrated with a single-dosage dipped-leaf assay using a concentration of 1 x 10(8) conidia or blastospores/mL. Blastospores, conidia, and no fungus control treatments averaged 90%, 86%, and 19% larval mortality, respectively, after 3-day continuous exposure. Subsequently, a dosage response assay was completed to compare two assay conditions, dipped-leaf and topical application techniques. Five spore concentrations, 1.2x10(9), 2.4x10(8), 4.8x10(7), 9.6x10(6), and 1.9X10(6) spores/mL, were prepared and used for both assay techniques. For the dipped-leaf technique, 25 larvae were exposed to treated leaf disks, five larvae to each of five treated leaf disks per spore concentration. For the topical assay, 5 mu L was applied directly to each of 25 larvae for each spore concentration. After 3-day incubation, blastospores of P. fumosoroseus showed higher activity than conidia, and the topical application technique was more effective (lower LC50 values and lower X-2 values) than the dipped leaf assay technique. This research demonstrates the insecticidal activity of P. fumosoroseus on MBB and provides a comparison of two basic bioassay techniques that may be useful in future research on this topic.