Improving the biocontrol potential of Steinernema feltiae against Delia radicum through dosage, application technique and timing

BACKGROUND: The potential of the entomopathogenic nematode (EPN)Steinernema feltiae Filipjev as a biocontrol agent against the cabbage maggot Delia radicum (L.),was assessed in three field tests, focusing on EPN dosage, application technique and timing.
RESULTS: Spraying cabbage plant trays with different doses of infective juveniles (IJs) (50 000, 100 000 and 200 000 per plant) generated a similar reduction of plant mortality. Spraying plant trays with 200 000 IJs of Steinernema feltiae per plant temporarily reduced the number of maggots around the plants’ roots, while neither spraying a lower dose (50 000 IJs/plant) nor soil drenching with 200 000 or 50 000 IJs/plant) reduced maggot numbers. When applied as a plant tray spray, IJs of S. feltiae took 1–2weeks to spread through the soil surrounding the roots. The pathogenicity of the EPNs, as evaluated by a Galleria mellonella bait test, was highest (up to 100% mortality) until up to five weeks after application, and declined to control levels after 4–7 weeks. Follow-up drench applications with EPNs, applied one and/or two weeks after the first EPN application, did not influence control of Delia radicum.
CONCLUSION: Plant tray spraying provides better placement of Steinernema feltiae than soil drench treatments for control of Delia radicum. Plant mortality was not dose-dependent in the presented trials, unlike the reduction of maggot numbers. Further research into timing and application technique of follow-up treatments with S. feltiae is required to increase efficacy to commercial standards.