Persistence of conidia and potential efficacy of Beauveria bassiana against pinhole borers in New Zealand southern beech forests

Abstract

Three native species of pinhole borer Treptoplatypus caviceps, Platypus apicalis and Platypus gracilis (Curculionidae: Platypodinae) are occasional pests of indigenous forests in New Zealand. These species predominantly attack ‘southern beech’, usually colonising fallen logs or stumps, but populations can reach densities which threaten healthy trees when large amounts of breeding material are available. The larvae live in tunnels which penetrate deep into the trees. Few control processes or agents are known in New Zealand. A search for naturally occurring biological enemies of pinhole borer resulted in the isolation of several insect-pathogenic fungi, the majority of isolates being Beauveria bassiana. Several strains of B. bassiana were isolated from pinhole borer in South Island southern beech forests, as determined by sequencing of the ITS-5.8s region of rDNA. Previous research demonstrated that B. bassiana is pathogenic to both pinhole borer larvae and adults in the laboratory. This paper reports on further investigations into the practical use of B. bassiana as a biopesticide for localised control of pinhole borer outbreaks. Aqueous suspensions of conidia were applied to logs and persistence was examined in a southern beech forest. B. bassiana colony-forming units were isolated up to 7 months after application, with better survival on the sides of logs than either the top or bottom. It was also demonstrated that conidia could penetrate several centimetres into the tunnels. In the field there was some pinhole borer mortality, as evidenced by cessation in frass production and a reduction in pinholes, but the level was too low to be considered acceptable for control purposes.