Lightbrown apple moth - Population dynamics

The population dynamics of lightbrown apple moth has not been analysed in New Zealand but studies have been conducted in Victoria, Australia. The key factors affecting population change were egg mortality in spring and autumn/winter generations, and 1st stage larval (and egg) mortality in summer. Pupal and final (sixth) stage caterpillar mortalities were next in importance. Predation by a wide variety of predators played the key role in egg, young larval, and pupal mortalities. On the other hand, losses of the 2nd to 5th larval stages from a variety of causes were unimportant in affecting population change. Infertility, the nucleopolyhedrosis virus, and the complex of egg and larval parasitoids were minor factors in population dynamics.

Another major 'mortality' factor influencing the population density was failure of females to achieve full fecundity, which was determined by weather, the quality, variety and succession of food plants. These factors often combined to reduce egg laying in the summer. In summary, fluctuations in populations in Australia are determined by the density independent effects of weather and food plants affecting fecundity, and the imperfectly density dependent effects of egg and larval predation.

This information provides only a guide to possible factors which could determine the population density of lightbrown apple moth in New Zealand. For example, our summer weather and availability of food plants could increase the ability of lightbrown apple moth to reach its full potential egg production each summer. There is circumstantial evidence of leafroller population decline following the introduction of Trigonospila brevifacies and Xanthopimpla rhopaloceros from Australia, particularly in populations outside orchards, and this requires investigation. High losses of young larvae dispersing from egg batches are known to occur in New Zealand, as they do in Australia.