Population changes of the seedfly, Pegohylemyia jacobaeae (Diptera: Anthomyiidae) introduced for biological control of ragwort

Abstract

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.

Keywords:

• ragwort seedfly
• Pegohylemyia jacobaeae
• Senecio jacobaea
• biological control