Biology of Ostenia robusta: observations on life history and behaviour

Ostenia robusta (Hutton) is an endemic dolichopodid fly that sometimes occurs in the soil under agricultural crops and pastures. Little is known about its biology and the role it may play in agro-ecosystems. A population of these flies near Southbridge, Canterbury, was monitored from 2013 to 2015. Adult flies emerged between December and March each year and were monitored with yellow sticky traps placed in wheat, ryegrass, radish and white clover crops. Adults will drink water under laboratory conditions but no predatory behaviour was observed and attempts to induce oviposition were unsuccessful. From dissection, maximum female egg load was approximately 60-65 eggs. Very few O. robusta larvae were found in 2014 and none were associated with pupae of Costelytra zealandica (White), a result contradictory to 2012 and 2013. The diet range of larval O. robusta remains unknown but this predator appears unlikely to play a significant role in the control of C. zealandica.