Establishment, Spread and Initial Impact of Aphis chloris Koch (Hemiptera: Aphididae), Introduced into Australia for the Biological Control of St John's Wort

The aphid, Aphis chloris Koch, quickly established following field release at numerous sites throughout south-eastern Australia. Local populations built up to levels at which migratory alates were produced within the first season. Dispersal of these alates over large distances led to widespread colonization of St John's wort infested areas within 2-3 years. Detailed observations at the primary release site at Pierce's Creek, ACT, showed that aphid populations undergo a seasonal cycle of population growth throughout the summer, followed by a migratory phase with alate production and dispersal in autumn and a population decline over winter. This pattern was repeated at other release sites monitored over a 5-year period from 1987 to 1991 inclusive, without populations building up to damaging levels. Natural enemies, competition from existing biological control agents, climatic factors and emigration appear sufficient to limit such outbreaks. While caged plants did show a decline in vigour and seed production under heavy aphid attack, such damage was intermittent in open field situations. The present study suggests that, while A. chloris is now widespread, it will not make an important contribution to the overall control of St John's wort.