A Modified Method to Visualize Infection Sites of Spores of the Entomopathogen Beauveria bassiana (Deuteromycotina: Hyphomycetes) on the Exoskeleton of Citrus Root Weevil Diaprepes abbreviatus (Coleoptera: Curculionidae) Adults

Abstract

Beauveria bassiana is a widespread entomopathogen which is infectious to a great variety of insects. A commercial preparation of this fungus was used to study its potential as a biocontrol agent of the citrus root weevil, Diaprepes abbreviatus. Laboratory reared adult weevils were placed in clean plastic bags with 0.05 g of powdered inoculum per bag, shaken for thirty seconds, and placed in holding cages. In order to determine the spore concentration, germination, and position of ingress, inoculated weevils were dipped in a collodion solution after 30 min, 6 h, 12 h, 18 h, and 30 h. Collodion peels from various areas of the exoskeleton were removed, stained with lactophenol cotton blue, and observed microscopically. At 12 h post-inoculation, spores began to swell, and at 18 h close to 25% germinated on all body parts except on the elytra. After 30 h, between 45% and 75% of the spores germinated, depending on the body part. Most active spore germination occurred around the eyes (75%), followed by the ventral abdomen (65%), the dorsal pronotum (60%), scales (45%), and dorsal surface of the elytra (7%). Spores were washed from the surface of inoculated weevils with distilled water and plating onto potato dextrose agar. Spore density on the weevils' surface was determined to be 15,815 ± 479 spores initially after inoculation, and was reduced to 5,782 ± 136 spores after 3 h.