Entomologia Experimentalis et Applicata 97: 265–274, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 265 Prevalence of fungal infections in adult Delia radicum and Delia floralis captured on the edge of a cabbage field I. Klingen 1 , R. Meadow 1 & J. Eilenberg 2 1 The Norwegian Crop Research Institute, Plant Protection Centre, Department of Entomology and Nematology, Høgskoleveien 7, N-1432 Ås, Norway; 2 Royal Veterinary and Agricultural University, Department of Ecology, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark Accepted: May 11, 2000 Key words: Delia radicum, Delia floralis, Entomophthora muscae, Strongwellsea castrans, trap, microbial control, natural enemy, Entomophthorales, epizootiology, fungal pathogen, prevalence Abstract Brassiceye  traps baited with ethylisothiocyanate were modified and used to collect live adults of Delia radicum (L.) and Delia floralis (Fallén) (Diptera: Anthomyiidae) from the field to observe the prevalence of Entomophthora muscae (Cohn) Fresenius and Strongwellsea castrans Batko & 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. Introduction D. radicum, the cabbage root fly, and D. floralis, the turnip root fly, are considered to be economically im- portant pests in Brassica vegetables, root crops and canola in the north temperate region (Rygg, 1962; Coaker & Finch, 1971; Finch, 1993; Broatch & Vernon, 1997). Recent developments in forecasting attacks have improved the timing of pesticide appli- cation against D. radicum and D. floralis (Finch et al., 1996; Bligaard et al., 1999), but the number of avail- able chemical insecticides for root fly control has diminished and at present the control situation is not satisfactory. Efforts have been made to manage these pest insects by alternatives to chemical insecticides, but few are viable by themselves (Vernon & Macken- zie, 1998) and IPM systems have therefore been suggested as a control strategy (Finch, 1989, 1993). Predators and parasitoids are recognized as important naturally occurring control factors. Insect pathogenic fungi, however, are not always considered as such and are seldom included in IPM programs. Despite this, several studies of insect pest systems confirm that in- sect pathogenic fungi are important in agricultural and forestry systems (Eilenberg & Philipsen, 1988; Ha- jek et al., 1990; Six & Mullens, 1996; Hajek, 1997) and might be used in IPM systems (Pell et al., 1993; Furlong & Pell, 1995; Hollingsworth et al., 1995; Steinkraus et al., 1996). Adults of the first generation of root flies emerge in spring or early summer from the overwintering pu- pae. After a pre-oviposition period of about a week the females begin to lay eggs, mostly in the soil around the stem of the host plant. Larvae hatch within a week