Pathogenicity of isolates of Metarhizium to the New Zealand grass grub (Costelytra giveni) in different soil types

Grass grub (Costelytra giveni) (Coleoptera: Scarabaeidae), an endemic pest of improved pasture in New Zealand, is a damaging root herbivore which lives in soils for most of its lifecycle. The pest is resistant to most microbes, but larvae are susceptible to some strains of fungi belonging to the genus Metarhizium. However, fungal diseases are rarely sufficient to control the pest naturally. In this study, Metarhizium spp. were infecting around 5% of field-collected larvae. Metarhizium strains were obtained from naturally infected insects, directly from soil and from existing collections. Molecular characterisation showed these isolates were belonged to the species, M. anisopliae, M. novozealandicum, M. robertsii. M. guizhouense, M. pemphigi, M. brunneum, M. frigidum and M. pinghaense. Among these isolates M. novozealandicum C14, M. novozealandicum F99, M. anisopliae M2 M. anisopliae F672 and M. pinghaense JK caused high grass grub larval mortality after 35 days in bioassays in soil. Metarhizium novozealandicum C14 caused the highest larval mortality in second and third instar grass grub in both semi-sterilised and non-sterilised soil. The soil type (Templeton, Temuka and Wakanui) did not affect the pathogenicity of Metarhizium. The LT₅₀ and LC₅₀ values of M. novozealandicum C14 were lower than for other Metarhizium isolates, with an LC₅₀ of less than 2.5 x 10⁷conidia/10 g soil and an LT₅₀ of less than 25 days