Population Ecology of the Light Brown Apple Moth, Epiphyas postvittana (Lepidoptera: Tortricidae)

Abstract

(1) The population ecology of the light brown apple moth, Epiphyas postvittana (Walker) (Tortricidae) was studied in Victoria for sixteen generations at one site and nine generations at another between 1971 and 1977. Life-tables are given for the two populations. Key factor analyses were carried out using data from one site which provided information on six summer generations, five autumn-winter generations and five spring generations. (2) Mortality of eggs has been identified as the key factor in spring and autumn-winter generations with mortality of first instar larvae and of eggs as the key factors in the summer generation and these two stages therefore determine trends from one season to the next. Mortality of other stages, particularly of larval instars II-V was found to be unimportant. Mortality of instar VI and pupal stages ranked next to that of eggs and/or first instar larvae in key factor analyses. (3) Predation by arthropods was identified as the key factor in determinations of sub-mortalities of the egg, instars II-V, instar VI and pupal stages. Other causes of mortality such as infertility, a Nuclear Polyhedrosis Virus disease, and a complex of parasitoids of the egg, larvae and pupae were found to be unimportant in determining population abundance, although some of these factors caused death of large numbers of individuals. (4) Failure to attain maximum potential natality or losses in fecundity ranked next to predation as a key 'mortality' factor of the egg stage. Only about 14-30% of the maximum egg-laying potential was achieved. Fecundity was dependent on the weather, quality and variety of food plants available and the succession of these plants. Summer generation moths laid fewer eggs for this reason, in contrast to those of autumn-winter and spring generations. (5) Population fluctuations of the moth are determined by climate and food acting in a density-independent manner, and by egg and/or first instar larval predation acting in a density-independent manner. Exceptionally dry and warm seasons, such as the 1973 summer, can greatly accentuate the effects of weather, almost eliminating summer generations. Milne's concept of population regulation appears to hold well for E. postvittana.