Isolation and characterization of Beauveria spp. associated with exotic bark beetles in New Zealand Pinusradiata plantation forests

Abstract

A survey of entomopathogenic fungi associated with Hylastes ater and Hylurgus ligniperda was undertaken in six Pinus radiata forest sites in New Zealand. In each site soil, bark and frass associated with beetle galleries were sampled, as well as a selection of live beetles, larvae and mycosed cadavers. The density of Beauveria spp. (as measured by colony forming units) varied both within and between the sites and substrates sampled. The highest CFU density detected was 4 × 10 6 CFU/g dry frass, which was collected from a stump at the Mahurangi site in the North Island. A number of isolates were selected and characterized using PCR amplification of a terminal region of the EF1-α fragment. This analysis revealed that two Beauveria species, Beauveria caledonica and Beauveria bassiana, were commonly associated with bark beetle habitat. B. caledonica was the only species isolated from mycosed beetles, and was also recovered from soils, bark and frass, though not live insects. B. bassiana was also found in soils, bark, frass and live insects; however, it was not isolated from mycosed cadavers in this study. It is speculated that B. caledonica is the common ‘natural’ pathogen of bark beetles in their breeding habitat. A third species, Beauveria malawiensis, was isolated once from soil. This is the first report of this species from New Zealand. Thirteen isolates (selected from all substrates sampled and live beetles) were selected for laboratory bioassays. Under laboratory conditions all isolates were pathogenic to H. ater and H. ligniperda adults; there was little difference among the isolates in terms of efficacy despite the paucity of B. bassiana infection observed in the field. The mechanisms behind the lack of field infection observed for B. bassiana are not known, but are likely to be linked to limiting environmental factors in the breeding sites. These results indicate potential risks associated with the selection of strains for use in biological control programmes solely on the basis of laboratory bioassays, especially if the mode of action and biology of the species used are not fully understood. The importance of selecting strains which are environmentally competent is evident. Overall, however, this study demonstrates that entomopathogenic fungi can play a significant role in the regulation of H. ater and H. ligniperda populations in pine plantations.