Cydia pomonella granulovirus (CpGV) has been used for 15 years as a bioinsecticide in codling moth (Cydia pomonella) control. In 2004, some insect populations with low susceptibility to the virus were detected for the first time in southeast France. RGV, a laboratory colony of codling moths resistant to the CpGV-M isolate used in the field, was established with collection of resistant insects in the field followed by an introgression of the resistant trait into a susceptible colony (Sv). The resistance level (based on the 50% lethal concentrations [LC(50)s]) of the RGV colony to the CpGV-M isolate, the active ingredient in all commercial virus formulations in Europe, appeared to be over 60,000-fold compared to the Sv colony. The efficiency of CpGV isolates from various other regions was tested on RGV. Among them, two isolates (I12 and NPP-R1) presented an increased pathogenicity on RGV. I12 had already been identified as effective against a resistant C. pomonella colony in Germany and was observed to partially overcome the resistance in the RGV colony. The recently identified isolate NPP-R1 showed an even higher pathogenicity on RGV than other isolates, with an LC(50) of 166 occlusion bodies (OBs)/microl, compared to 1.36 x 10(6) OBs/microl for CpGV-M. Genetic characterization showed that NPP-R1 is a mixture of at least two genotypes, one of which is similar to CpGV-M. The 2016-r4 isolate obtained from four successive passages of NPP-R1 in RGV larvae had a sharply reduced proportion of the CpGV-M-like genotype and an increased pathogenicity against insects from the RGV colony.