Inoculation of the entomopathogenic fungus Metarhizium brunneum to target the larvae of the cabbage root fly Delia radicum

Abstract: Inoculation methods of two isolates, NCRI 250/02 and KVL16-36, of the entomopathogenic fungus Metarhizium brunneum were evaluated as granular inoculum after cultivation on sterilized rice. The granules were added to cauliflower at the sowing stage in a peat substrate, incubated for 21 days in the greenhouse, transplanted to pots with field soil and kept in the greenhouse for 24 days. Plants were then uprooted and placed in sterile sand and inoculated with 20 eggs of the cabbage root fly, Delia radicum. The plants were kept for 14 days in climate room, and then root and sand was searched for larvae and pupae of D. radicum, which were incubated for fungal infections. Samples of roots were grinded to a homogenate and plated on selective media to evaluate root colonization. In addition, DNA was extracted from root samples and quantitative PCR was performed with specific primers to detect M. brunneum. The isolate KVL16-36 showed the highest level of root colonization compared to NCRI 250/02 at 56 days after inoculation, based on CFU counts. No clear correlation between CFU and qPCR detection was observed. Larvae and pupae were recovered at mean (± SE) of 7.4 (± 0.8) to 10.1 (± 0.8) per plant with no obvious reduction in the fungal treatments. However, some fungal infections were seen in these treatments after incubation. The tested method therefore ensures establishment of M. brunneum in the root system of cauliflower, but this fungal material is not sufficient to reduce the level of D. radicum larvae. However, the method must be further tested under more realistic field conditions and should be considered as a tool within an integrated pest management strategy against D. radicum.