The influence of temperature on the susceptibility of vine weevil, Otiorhynchus sulcatus (Fabricius) (Coleoptera: Curculionidae), larvae to Metarhizium anisopliae (Deuteromycotina: Hyphomycetes)

Mortality of Otiorhynchus sulcatus (Fabricius) larvae at 10^oC, 15^oC, 20^oC and 25^oC following treatment with 10⁷conidia ml“¹suspensions of six Metarhizium anisopliae (Metschnikoff) Sorokin isolates was temperature dependent. In all cases, the LT₅₀s were inversely related to temperature, but the nature of this response varied between isolates. Strain 101-82 was the most virulent isolate at 25^oC with an LT₅₀ of 3.7 days, but it was the least virulent isolate at 15^oC and it failed to kill any O. sulcatus larvae at 10^oC. In contrast, strain 159-83 had the lowest virulence at both 20^oC and 25^oC, whereas it was the most virulent isolate at 10^oC with an LT₅₀ of 20.0 days. The mortality rates followed a similar pattern and were positively related to temperature in all cases with the exception of strain 159-83 at 25^oC. Mycosis development was examined on larvae treated with strain 275-86 and significant differences were obtained between all four temperatures. Sporulation commenced after approximately 2.75 days at 25^oC, but took nearly 11 days at 10^oC. The infection rates also varied between temperatures; sporulation occurred on 98% of the treated larvae at 25^oC, but only on 93%, 87% and 49% of the larvae at 20^oC, 15^oC and 10^oC, respectively. The results of these bioassays demonstrate that temperature has a significant effect on the virulence of M. anisopliae. The differences between fungal strains also emphasises the importance of selecting isolates for specific situations on the basis of their temperature profiles.