Emergence of new resistances to Cydia pomonella Granulovirus: insights from 12 years of monitoring

Introduction: Codling moth resistance to the granulosis virus CpGV, a widely adopted biological control agent in apple orchards poses a significant challenge for growers. Since the first report of Type I resistance to the CpGV-M in Europe (2005), a monitoring system was set up in France to track resistance evolution and anticipate emergence to new viral variants. Methods: This study presents the results of 12 years of bioassay based monitoring of resistance to three commercially available CpGV isolates (CpGV-M, CpGV-V15 and CpGV-R5), representative of the virus genetic diversity. Results: Among 193 wild codling moth populations tested, 62.3% carried a significant number of individuals resistant to CpGV-M, 37.0% to CpGV-V15 and 13.3% to CpGV-R5. Despite mitigation recommendations implemented since the first detection of type I resistance, the prevalence of CpGV-M resistance has not declined over time. Similarly, resistance to CpGV-M + CpGV-V15 (Type IV) and CpGV-M + CpGV-V15 + CpGV-R5 (Type V) seems to be fairly stable in frequency from one year to the next. Populations carrying type I resistance have been detected in all French production basins and in northern Italy, while type IV and V resistances are localized to northwestern France and northern Italy. Discussion: The emergence of these resistances types underscore the urgency of revising deployment strategies for new granulovirus-based products. Effective solutions will require elucidating the underlying resistance mechanisms to inform countermeasures.