Effect of potting media on the control of Otiorhynchus sulcatus larvae on outdoor strawberry plants using the entomogenous fungus Metarhizium anisopliae

Abstract

The potential of entomogenous fungi to control black vine weevil (Otiorhynchus sulcatus (Fabricius)) larvae has been demonstrated on a range of protected crops species, but very little attention has been paid to weevil control on outdoor crops, such as strawberry. Curative applications of Metarhizium anisopliae (Metschnikoff) Sorokin conidia to strawberry plants potted in field soil (5 × 108 conidia/pot) reduced the population of O. sulcatus larvae by up to 94%, whereas similar treatments reduced larval numbers in peat compost by a maximum of 53%. Significant differences in relative performance were observed among the four M. anisopliae strains (37-80, 101-82, 189-83, and 275-86) in the two media. Strain 189-83 was the most virulent isolate in both media, whereas strain 101-82 reduced the O. sulcatus population in field soil by only 15% and had no effect in peat compost. Larval establishment in the pots treated with 0.05% Triton X-100 (control treatment) was greater in peat compost than in field soil with nearly twice as many larvae in the former medium. The larval population was evenly distributed in most of the pots with the exception of the pots containing peat compost treated with strain 189-83 which showed significant heterogeneity. Spore density declined with increasing depth in pots containing peat compost. The situation was reversed in the pots of field soil and the highest spore concentrations were found at the base of the pots. More conidia were recovered from the pots containing peat compost than from the pots of field soil. There were also significant differences between strains in the number of colony forming units per sample. In both media, mean counts for strains 37-80 and 275-86 were significantly greater than those for strain 101-82; strain 189-83, however, had a low count in peat relative to 37-80 and 275-86, but had a higher count in field soil.