Biology of the mountain pinhole borer, Platypus subgranosus Schedl, in Tasmania

Information on the life cycle and associations of the mountain pinhole borer, Platypus subgranosus Schedl (Coleoptera : Curculionidae : Platypodinae) is reviewed. Existing unpublished data combined with data collected in this study are used to provide quantitative descriptions of aspects of P.subgranosus biology. The within-tree spatial pattern of attack was found to be highly aggregated both at high and low density of attack. Aggregation was very high around host tissue infected with the pathogenic fungus Chalara australis Walker and Kile. A minimum spacing between pinholes of approximately one centimetre was indicated but densities were not sufficiently high for this to result in spatial regularity. Use of larval head capsule width and body length in a non-hierarchical classification procedure confirmed the presence of five instars. Initial oviposition occurred when the gallery ranged between 5 and 25cm in length. The natality rate at initial oviposition was roughly one egg per centimetre beyond an initial length of 4cm. Initial oviposition in galleries established in the late summer/autumn period occurred at roughly the same date irrespective of establishment date. This was paralleled by a faster rate of gallery development for autumn compared to late summer establishment. Timing of emergence exhibited an analogous trend to that of initial oviposition. It is postulated that these trends as well as a trend for emergence and subsequent gallery establishment to occur less commonly in spring/early summer than late summer/autumn are a response to high mortality of eggs and early instars in summer from desiccation. For a sample of galleries established in the late summer/autumn period, initial oviposition occurred in winter one to eight months after gallery establishment with eggs usually laid in a batch with median size of seven. First to third instars appeared through the following spring and early summer with fourth and fmal instars appearing in summer and subsequently the fmal instar predominating through winter until pupation in the following spring. Emergence began in early summer. A new model of insect phenology based on conditional probabilities is developed and compared to existing ordinal regression and gamma entry time models. The sex ratio of emergents over the population is very close to unity but individual galleries can deviate markedly from this with an excess of either sex. The mean number of emergents per gallery was 19.7 with a maximum of 92. The gallery failure rate was 8% but negligible mortality of immature stages was observed. Development time ranged from ten months to two years depending on the timing of gallery establishment as predicted by the linear day-degree model. Threshold temperature for development and total day-degrees above this threshold from gallery establishment to emergence, DD, were estimated from field data at 11°C and a mean of DDii of 4047 respectively. A new estimation procedure based on maximum likelihood is developed to estimate the parameters of the day-degree model under ambient temperatures. Both gamma and inverse normal distributions were found to adequately describe the empirical distribution of DD ii . Only for the gamma, though, was the estimation algorithm successful. The implications of P .subgranosus biology for rainforest ecology and management are discussed.