A weight-based phenology model for immature stages of the red-headed cockchafer, Adoryphorus coulonii (Burmeister) (Coleoptera : Scarabaeidae : Dynastinae), a pest of pastures in south-eastern Australia

A new method of modelling insect phenology which can be used when stadia occupancy times are unobserved is described. The method was motivated by a study of larvae of the red-headed cockchafer which were reared and weighed regularly for a range of constant temperatures using largely field-collected first, second and third-instar larvae. These larvae had undergone an unknown but significant proportion of their development in the field. The method first models development rate as a function of temperature using relative growth rates in mean larval weight. Predicted mean larval weight was then used as a physiological time scale in an ordinal regression model of the proportion of the population in each of first to third-instar stage. An algorithm to predict proportions in the field of each stage up to and including the pupal stage given a known starting date for the population and daily maximum and minimum soil temperatures is described and tested using data from two field sites. Growth rates under laboratory conditions were slower than those in the field probably due to the handling required for weighing. As a result, an adjustment to the rate parameter and a 10°C lower development threshold were required for the growth and phenology models to predict trajectories for mean larval weight and proportion of second-instar larvae which were consistent with observations from the field sites.