ESTABLISHMENT, SPREAD AND INITIAL IMPACT OF APHIS-CHLORIS KOCH (HEMIPTERA, APHIDIDAE), INTRODUCED INTO AUSTRALIA FOR THE BIOLOGICAL-CONTROLOF ST-JOHNS-WORT

The aphid, Aphis chloris Koch, quickly established following field rel ease at numerous sites throughout south-eastern Australia. Local popul ations built up to levels at which migratory alates were produced with in the first season. Dispersal of these alates over large distances le d to widespread colonization of st John's wort infested areas within 2 -3 years. Detailed observations at the primary release sire at Pierce' s Creek, ACT, showed that aphid populations undergo a seasonal cycle o f population growth throughout the summer, followed by a migratory pha se with alate production and dispersal in autumn and a population decl ine over winter. This pattern was repeated at other release sites moni tored over a 5-year period from 1987 to 1991 inclusive, without popula tions building up to damaging levels. Natural enemies, competition fro m 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, suc h 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.