Effect of Norwegian entomopathogenic fungal isolates against Otiorhynchus sulcatus larvae at low temperatures and persistence in strawberry rhizospheres

Abstract

Biological control of belowground stages of the black vine weevil Otiorhynchus sulcatus F. (Coleoptera: Curculionidae) in strawberries in cool temperate regions using entomopathogens is challenged by low temperatures during the periods when larvae are vulnerable to infections. In a laboratory study we tested six indigenous Norwegian isolates of entomopathogenic fungi (one Beauveria bassiana, three Beauveriapseudobassiana, and two Metarhizium brunneum; Ascomycota: Hypocreales) for their efficacy against O. sulcatus larvae at 6, 12, and 18°C. At the lowest temperature only Beauveria spp. affected survival of O. sulcatus while all three fungal species reduced larval survival compared to the control treatment at 12 and 18°C. Two of the Norwegian isolates, one B. pseudobassiana and one M. brunneum, were then evaluated for long-term persistence (>1year) in the bulk soil and the rhizosphere soil of strawberries in a semi-field experiment. An exotic isolate of M. brunneum sharing origin with a widespread commercial biocontrol agent (F52/Met52 (Novozymes)) was included for comparison. All three isolates showed significantly higher abundances in the rhizosphere soil compared to bulk soil at 153, 366, and 471days after inoculation, thus indicating rhizosphere competence for B. pseudobassiana. Notably, CFU levels for both Norwegian isolates were much higher than for the exotic M. brunneum isolate. Selection of locally adapted isolates may therefore be of importance when considering biocontrol strategies of belowground pests in strawberry production.