Prevalence of fungal infections in adult Delia radicum and Delia floralis captured on the edge of a cabbage field

Abstract

Brassiceye® traps baited with ethylisothiocyanate were modified and used to collect live adults of Delia radicum (L.) and Delia floralis (Fallen) (Diptera: Anthomyiidae) from the field to observe the prevalence of Entomophthora muscae (Cohn) Fresenius and Strongwellsea castrans Batko and Weiser. The traps were highly effective and selective for D. radicum and D. floralis. Of the flies identified, 98.4% in 1996 and 93.7% in 1997 were either D. radicum or D. floralis. In 1997 the maximum mean catch was as high as 82 flies per trap per day, and more than 80% of these were females. During both seasons E. muscae caused relatively high levels of mortality in adult populations of D. radicum and D. floralis. The fungus caused a total infection level of 17.9% in 1996 and 47.7% in 1997 with infection peaks of 82.4% in 1996 and 87.5% in 1997. Both years, a significant positive correlation was found between E. muscae prevalence and temperature. One infection peak was observed for S. castrans in 1996, and during that season the total S. castrans infection level was 18.0%. In 1997, the total S. castrans infection level was as low as 8.1%. There is no strong indication that the prevalence of E. muscae or S. castrans differs between either the fly species or sexes within species.