Selection of an Isolate of the Insect Pathogenic Fungus Metarhizium anisopliae Virulent to the Lettuce Root Aphid, Pemphigus bursarius

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Abstract
The virulence of hyphomycete entomopathogenic fungi was measured in laboratory bioassays against the lettuce root aphid, Pemphigus bursarius, a serious pest of field lettuce grown in the UK. Of 25 isolates of fungi examined, only one isolate, Metarhizium anisopliae 391.93, killed lettuce root aphids consistently. This fungus was isolated originally from the closely related saltmarsh aphid, P. trehernei. The median lethal concentration of conidia at 10 days post6- 1 inoculation estimated from five independent bioassays was 2.45 × 106 conidia ml−1. The fungus had no significant effect on the mean number of offspring/aphid produced, but it sporulated 6 profusely on host cadavers, producing approximately 4 × 106 conidia/cadaver 14 days after treatment, and diseased aphids died attached to plant roots. It thus has the potential to spread through densely packed colonies of P. bursarius feeding on the roots of susceptible or partially resistant plants.