POPULATION-CHANGES OF THE SEEDFLY, PEGOHYLEMYIA-JACOBAEAE (DIPTERA, ANTHOMYIIDAE) INTRODUCED FOR BIOLOGICAL-CONTROL OF RAGWORT

Population changes and infestation levels of ragwort seedfly, Pegohylemyia jacobaeae, were recorded over two seasons in the central North Island, New Zealand. Adult seedfly emerged up to 6 weeks before ragwort flowered, resulting in competition for oviposition sites. Multiple oviposition by seedfly occurred in 22% and 16% of ragwort seedheads infested in 1982 and 1983, respectively, and fecundity was reduced. Poor synchronisation of damage and a lengthy flowering period resulted in 80-90% of ragwort seeds escaping predation. Of the uneaten seeds remaining in seedfly infested seedheads, 28-43% germinated.