Heterogeneity in virulence relationships between Cryptophlebia leucotreta granulovirus isolates and geographically distinct host populations: lessons from codling moth resistance to CpGV-M

In South Africa, the baculovirus Cryptophlebia leucotreta granulovirus (CrleGV) is used to control the citrus pest Thaumatotibia leucotreta (Meyrick) (Lepidoptera: Tortricidae). However, the risk associated with continuous application of a single active ingredient is resistance. In order to manage resistance should it occur in field populations of T. leucotreta in South Africa, five new CrleGV genotypes from geographically distinct insect populations which were shown to exhibit some degree of phenotypic variation were recovered and genetically characterized. In droplet bioassays using seven CrleGV isolates against five T. leucotreta populations, some isolates were found to show higher virulence to some host populations than others. There were marked differences in the LD50 values of isolates and the number of occlusion bodies required per larva ranged between 0.79 and 3.12. The significance of these findings with respect to the application of CrleGV biopesticides and the management of resistance is discussed.