Laboratory Selection of Metarhizium spp. Isolates for Control of Vine Weevil Larvae ( Otiorhynchus sulcatus)

Abstract

The susceptibility of Otiorhynchus sulcatus larvae to 19 isolates of the genus Metarhizium was examined at 20°C. All the Metarhizium anisopliae var. anisopliae isolates were pathogenic, with LT 50S ranging from 6.09 to 8.72 days. The M. anisopliae var. majus strains were also pathogenic, but less virulent than the var. anisopliae strains, whereas nearly all the Metarhizium flavoviride and Metarhizium album strains had low virulence or were nonpathogenic. There were significant differences between the LT 50s of the virulent var. anisopliae strains and there were also significant differences between the LT 50s of the var. majus strains. There was no indication that the var. anisopliae isolates were specific to their original hosts because only one of the four most virulent strains was isolated from O. sulcatus. Two of the other highly virulent strains originated from Coleopteran hosts and the fourth strain was isolated from a Lepidopteran larva. Mycosis development was correlated with strain virulence and there were significant differences between strains. Mycelial emergence was observed 1.5 days after the death of larvae treated with a highly virulent var. anisopliae strain, whereas it occurred after 3 days with a less virulent var. majus strain. Sporulation occurred 2 to 3 days after mycelial emergence on larvae infected by most strains with the exception of one var. anisopliae and one var. majus strain. Sporulation on larvae treated with these two strains was variable and extensive mycelial development was often observed. The results of the bioassay assessments suggest that the most virulent strains may have potential as biological control agents for O. sulcatus and this now needs to be evaluated in pot and field trials using a range of plant species under different growing systems.