Natural occurrence of the entomopathogenic fungi Beauveria bassiana as a vertically transmitted endophyte of Pinus radiata and its effect on above- and below-ground insect pests

Background. The New Zealand forest industry would greatly benefit from a successful way of controlling insect pests. The entomopathogenic fungus, Beauveria bassiana could hold such potential and has previously been shown to be capable of endophytic colonisation of the Monterey pine Pinus radiata. Nevertheless clarifications on its mode of transmission, persistence and action in this plant are required. In this study we investigated B. bassiana transmission and persitence in P. radiata and whether this fungus is beneficial to P. radiata by testing its effect as a plant endophyte on the fitness performance of above and belowground insect feeders. Methods. Both culturing and molecular approaches were used to detect the occurrence B. bassiana in pines. Transmission electron microscopy of positive germinating seeds was also used to locate the fungus. Bioassays were conducted on root and needle feeding insects using Beauveria positive and endophyte free pine seedlings. Results. Beauveria bassiana was detected in seedlings which had not previously been exposed to the fungus, indicating a vertical mode of transmission. The fungus could colonise all parts of the pines, but did not always persist. We found that the presence of the fungus negatively affects the fitness of the below-ground insect feeding on the plant by reducing their survival by over 10% and their weight by 5%. This study also showed that the mode of action of endophytic B. bassiana in pine is likely to be by feeding deterrence of insects induced locally by fungal metabolites, rather than by direct fungal infection of the insects. Discussion. A vertically transmitted beneficial endophyte of pine could be used as a cost effective approach to control insect pest in these commercially grown trees.