Biological control of cabbage fly Delia radicum with entomopathogenic nematodes: selecting the most effective nematode species and testing a novel application method

Abstract We assessed the efficacy of entomopathogenic nematodes (EPN) as biocontrol agents against larvae of the cabbage fly (CF), Delia radicum L. (Diptera: Anthomyiidae) infesting radishes in a laboratory and a greenhouse experiment. In the laboratory experiment, we first assessed the efficacy of the EPN Steinernema feltiae (Filipjev), Steinernema carpocapsae (Weiser), Heterorhabditis bacteriophora (Poinar) and Heterorhabditis megidis (Poinar, Jackson & Klein) when applied in suspensions at a dose of 2000 IJs/radish. Each experimental unit consisted of three radishes artificially infested with 20 CF maggots in pots. Application of S. feltiae and S. carpocapsae was the most effective, leading to the lowest number of CF maggots recovered from the radishes. In the greenhouse experiment we compared the conventional application of EPN in a water suspension with EPN applied encapsulated in alginate beads. Alginate beads were applied during sowing (early) or one week (late) before CF infestation. The late bead treatment significantly reduced visible infestation rates and mining by the CF larvae in harvested radishes, and both bead treatments (early and late) led to a lower number of externally visible mines compared to control and suspension treatments. Interestingly, the application of empty beads without nematodes also mitigated CF infestation. However, after prolonged storage of the harvested radishes almost all of them were found to be infested, independent of treatment. In conclusion, application of EPN in alginate beads can be more effective than in suspension, most likely because it increases EPN persistence. Yet, their effective application is contingent on improving and optimizing the encapsulation process, as well as on selecting an appropriate application scheme in terms of timing and EPN dose.