Attract and kill: Testing the potential of an entomopathogenic fungus to convert a trap crop into a dead-end trap crop against a soil-borne pest of vegetables

Trap crops and entomopathogenic fungi can provide partial solutions for integrated pest management, by attracting and killing insect pests, respectively, but both solutions have some limitations restricting their practical field applications. Both solutions have been tested against a major soil-borne pest of brassicaceous vegetables, the cabbage root fly Delia radicum. Chinese cabbage is very attractive to this pest, but it is also a high-quality host plant for developing larvae of D. radicum, which limits the application as a trap crop in the field. The entomopathogenic fungus Metarhizium brunneum can infect D. radicum larvae in the soil, but M. brunneum has not proved to be sufficiently effective in reducing damages caused by cabbage root fly. In the present work, we evaluated whether the entomopathogenic fungus M. brunneum can be used to regulate D. radicum populations together with trap crops by inoculating Chinese cabbage and broccoli plants at sowing with M. brunneum colonized rice grains before transplantation of small plants to field soil. The evaluation was performed under natural fly infestation. In both plant treatments, D. radicum infestations were high with no or only moderate effect of the fungus inoculation on the number of larvae and pupae recorded, despite evidence of successful fungal infections. On broccoli plants, our results clearly demonstrated that the M. brunneum application was inefficient in reducing number of D. radicum stages in the soil and resulting plant mortality. However, in the trap crop, Chinese cabbage, M. brunneum inoculation reduced the number of D. radicum imagos emerging from the plants by 36%. Hence, the strategy is likely to have effects on the next D. radicum generation. This result is the first to indicate complementarity between the 'attract' and 'kill' strategies to control pest development inside a favorable trap crop and prevent future pest population outbreaks. Also, from both plant inoculation treatments, some emerging D. radicum imagos developed M. brunneum infection, which may assist the transmission of the entomopathogenic fungus among adult populations.