Fear no weevil: Biological control of sweetpotato weevil with entomopathogenic fungi

The aim of this study was to develop a system for identifying entomopathogenic fungi with potential to be used as biological control agents for sweetpotato weevil (SPW), Cylas formicarius elegantulus (Summers). Over 250 Metarhizium spp. and Beauveria spp. were isolated from environmental samples collected from Uganda, Tanzania, and the United States. In order to select the most virulent strains from among these isolates, we developed a series of in vitro assays to assess the virulence. A subset of 38 fungal strains and 2 negative controls were used to run a validation study and select cut-offs. The validation study used a randomized, incomplete block design with 3 replicates and two application rates. Adult SPW were exposed to sweetpotato roots treated with a fungal solution of 10⁷ or 10⁶ spores/ml. Mortality was assessed after 7 days. Based on the results of this study, we determined that the screen could proceed using a single dose (10⁷) and a cut-off of 70% mortality. Using these methods, 213 strains have been tested and 32 have qualified for advancement. To confirm the on-plant predictability of the in vitro assay, a sweetpotato vine-cutting assay was developed. Thirty of the fungal strains have since been tested on-plant. We’ve observed a high level of similar percent mortality between the two assays indicating that the in vitro assay is highly predictive of on-plant activity. Future research will test lead strains in greenhouse and field trials to validate the activity of these fungal strains.