Evaluation of low risk methods for managing Delia radicum, cabbage root fly, in broccoli production

The cabbage root fly (CRF; Delia radicum) is currently difficult to control as insufficient plant protection products are available, especially in organic farming. In this study the influence of two strains of the entomopathogenic fungi Metarhizium brunneum and Beauveria bassiana (Hypocreales), straw mulch, and nitrogen lime were investigated in regard to CRF and root damage reduction in field trials. Spinosad-based insecticide was used as a positive control, because of its known efficacy against CRF. Furthermore, rhizosphere competence and endophytism, mineral uptake, chlorophyll a fluorescence, and growth parameters of M. brunneum treated plants were examined under field and/or greenhouse conditions. Neither fungi nor straw mulch or nitrogen lime decreased CRF pressure significantly, but spinosad did. M. brunneum colonized the rhizosphere of broccoli plants but did not show endophytic characteristics. A significant increase of Mg and N concentrations was detected in Metarhizium-treated plants when grown in the greenhouse whereas no different nutritional values in plant tissue were observed in field grown plants. Plants from the greenhouse also exhibited higher maximum quantum efficiency of PS2 photochemistry and electron transport rate as well as promoted growth when inoculated with M. brunneum. We conclude that for brassicas where the CRF damaged parts are not consumed (e.g. cauliflower) the tested low risk CRF management tools might be adequate. But for brassicas, where CRF directly damages the produce (e.g. turnips), treatments having greater impact on CRF (e.g. spinosad) should be considered.