D3021: Assessing the virulence of Metarhizium spp. in sweetpotato for the control of Cylas formicarius elegantulus (Summers)

In the U.S., effective control of sweetpotato weevil [SPW, Cylas formicarius elegantulus (Summers)] occurs through quarantine and regular use of chemical insecticides. However, over-reliance on chemical control may lead to resistance. Entomopathogenic fungi have the potential as an alternative to chemical control. In this study, we assessed four different strains of Metarhizium isolated from soil collected in Louisiana (K048, K056, K082, and K101) and compared them to a well-known licensed strain (ARSEF 1095) for virulence against SPW. We evaluated virulence using two approaches, survival time and lethal concentration. The median lethal time and hazard ratio were determined using the cox proportional hazard model, and the median lethal concentration using probit. We found differences among the strains for both the hazard ratio and median lethal concentrations, even though these four strains were selected from over a hundred Metarhizium strains because of their high performance in an in-vitro assay. K048 and K056 had the best hazard ratios. Similarly, K048 proved highly effective with the lowest median lethal concentrations compared to other treatments. Overall, K048 was the most effective Metarhizium spp. tested with a hazard ratio of 8.85, a median lethal time of 4 days, and a median lethal concentration of 2.6 * 10⁵ spores/ml. We are currently in the process of testing these strains in the field and greenhouse. Pre-selecting the most virulent strain using a simple lab assay helps us focus on strains that will likely have the killing power to perform well in the field.