An up to 10,000-fold resistance against the biocontrol agent Cydia pomonella granulovirus (CpGV) was observed in field populations of codling moth, C. pomonella, in Europe. Following different experimental approaches, a modified peritrophic membrane, a modified midgut receptor, or a change of the innate immune response could be excluded as possible resistance mechanisms. When CpGV replication was traced by quantitative PCR in different tissues of susceptible and resistant insects after oral and intra-hemocoelic infection, no virus replication could be detected in any of the tissues of resistant insects, suggesting a systemic block prior to viral DNA replication. This conclusion was corroborated by fluorescence microscopy using a modified CpGV (bacCpGV(hsp-eGFP)) carrying enhanced green fluorescent gene (eGFP), which showed that infection in resistant insects did not spread. In conclusion, the different lines of evidence indicate that CpGV can enter but not replicate in the cells of resistant codling moth larvae.
Affectivity ofPreliminary bioassays were conducted to determine the virus activity against the 1 st and 2 nd instar larvae of the cotton leafworm, Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae). First instar larvae were more susceptible to the virus than the second instar larvae. Mortality response of the larvae was greater at the highest concentration of the virus than at lowest concentrations. The lethal concentration that killed 95% of the tested larvae (LC95%) was 2.32 x 10 , 2 x 10 4 , 2 x 10 5 and 2 x 10 6 PIB/ml). However, second instar (2 nd ) larvae were less susceptible to the virus than the first instar larvae with the mortality rate of 0, 4, 9.39, 25.33, 43.24, and 71.63%, for the same tested concentrations, respectively. In control treatment, no mortality was recorded. The high concentration levels caused a high rate of mortality. The same trend of mortality is similar to those found by other studies.
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