Phenology of autumn gum moth, Mnesampela privata (Guenee) (Lepidoptera: Geometridae)

The autumn gum moth, Mnesampela privata (Guenee) (Lepidoptera: Geometridae), is an economically important pest of young eucalypt plantations, particularly those of Tasmanian blue gum, Eucalyptus globulus Labill., and shining gum, E. nitens (Deane & Maiden) Maiden, which are grown in commercial pulpwood plantations in southern Australia. M privata is also a pest of flooded gum, E. grandis W. Hill ex Maiden, plantations grown under irrigation in southern Australia outside that tree's natural range. A major difficulty associated with M. privata management is detecting large populations before host trees are severely damaged. Early detection of populations is difficult because outbreaks are sporadic and may be localised, and also because the seasonal occurrence of pest activity varies in some parts of its geographic range. Damaging larval populations on the Australian mainland are largely restricted to the autumn period, April to June, by which time most larvae have pupated. In contrast, pest incidence usually extends from autumn to spring in lowland areas of Tasmania, whereas at altitudes above 500 m ASL in Tasmania, pest incidence usually extends from summer to autumn. The primary objectives of this study were to identify the underlying mechanisms influencing the phenology of M privata and to identify possible causes of geographic variation in the phenology of the species, thereby increasing our ability to predict the seasonal occurrence of pest activity throughout the distribution of the species. Laboratory studies found that the phenology of adult M privata was influenced by alternative developmental pathways initiated at specific points in the pupal stage. A diapause or non-diapause pathway was possible early in the pupal stage before adult differentiation began. Pupal diapause was induced by short daylengths and low temperatures experienced during larval development and averted by long daylengths and high temperatures, suggesting that M privata is a long-day species (Type I of Beck's classification). Also possible before adult differentiation began (in non-diapause pupae) was an aestival or non-aestival pathway. Low temperatures led to continued development, high temperatures to aestivation. Unlike diapause, which could only occur before adult differentiation began, aestivation could also occur late in the pupal stage, enabling pharate adults to delay eclosion. The generation time of M. privata was approximately 1268 degree-days (DD) above a developmental threshold of 5°C, consisting of 784 DD for egg-larval development and 484 DD for pupal development. However, the period between oviposition and the fourth instar, or `time-to-L4' (requiring 390 DD > 5°C) is especially important as it represents the lead time between peak oviposition and the onset of severe damage. At mean daily temperatures of around 12°C, typical of field temperatures experienced by M privata during egg-larval development, time-to-L4 is around 56 days. Hence, large populations of M. privata must be detected and controlled within about two months of peak oviposition in order to minimise damage to host trees. Mnesampela privata females selectively oviposited on expanding terminal shoots, but did not appear to discriminate on the basis of whether or not conspecific eggs were present. It is considered that such oviposition behaviour may lead to overexploitation of resources, but does not fully explain the eruptive population dynamics of the species. Sedentary adult behaviour was observed on two evenings in elevated plantations in Tasmania during summer, suggesting that low temperatures in autumn in those areas might preclude nocturnal reproductive activity. The incidence of larval feeding activity in Victorian blue gum plantations surveyed at the end of summer was extremely low, suggesting that extensive damage by M privata during summer is highly unlikely outside of Tasmania. Larval feeding damage in the surveyed areas is thus likely to be restricted to the autumn-winter period. It was concluded that pupal diapause and pupal aestivation are the primary mechanisms regulating the phenology of M privata and that the interaction between these mechanisms and local environmental conditions causes geographic variation in the phenology of the species. The findings of this study will provide a basis upon which reliable predictions may be made in relation to M. privata phenology.