Cross-Resistance of the Codling Moth against Different Isolates of Cydia pomonella Granulovirus Is Caused by Two Different but Genetically Linked Resistance Mechanisms.

Cydia pomonella granulovirus (CpGV) is a widely used biological control agent of the codling moth. Recently, however, the codling moth has developed different types of field resistance against CpGV isolates. Whereas type I resistance is Z chromosomal inherited and targeted at the viral gene pe38 of isolate CpGV-M, type II resistance is autosomal inherited and targeted against isolates CpGV-M and CpGV-S. Here, we report that mixtures of CpGV-M and CpGV-S fail to break type II resistance and is expressed at all larval stages. Budded virus (BV) injection experiments circumventing initial midgut infection provided evidence that resistance against CpGV-S is midgut-related, though fluorescence dequenching assay using rhodamine-18 labeled occlusion derived viruses (ODV) could not fully elucidate whether the receptor binding or an intracellular midgut factor is involved. From our peroral and intra-hemocoel infection experiments, we conclude that two different (but genetically linked) resistance mechanisms are responsible for type II resistance in the codling moth: resistance against CpGV-M is systemic whereas a second and/or additional resistance mechanism against CpGV-S is located in the midgut of CpR5M larvae.

Acquired resistance to a nucleopolyhedrovirus in the smaller tea tortrix Adoxophyes honmai (Lepidoptera: Tortricidae) after selection by serial viral administration.

A laboratory colony of Adoxophyes honmai was selected for resistance over 156 generations by feeding neonate larvae of every generation with the LC60 or LC70 of its nucleopolyhedrovirus, Adoxophyes honmai nucleopolyhedrovirus (AdhoNPV). A significant difference in LC50 values between the selected (R-strain) and unselected (S1- and S2-strain) strains was first observed after three generations of selection, and the resistance level then increased continuously. The highest degree of acquired resistance, based on the ratio of the LC50 values of R- and S1-strains, was more than 400,000-fold. After selection was stopped at either the 21st or the 149th generation, LC50 values did not decrease significantly, suggesting that resistance of the R-strain to AdhoNPV was stable. To assess which of the two routes of baculovirus infection is affected by resistance to AdhoNPV, 5th instar larvae of the R-strain were inoculated orally and intrahemocoelically with AdhoNPV and their susceptibility was compared to that of S-strain. The ratio of the LC25 values of selected and unselected strains was 91-fold when budded viruses were injected into 5th instar larvae, but was 107,000-fold after oral inoculation. These results indicate that the resistance mechanism of the R-strain of A. honmai disrupts both midgut primary infection and hemocoelic secondary infection.

Midgut-based resistance to oral infection by a nucleopolyhedrovirus in the laboratory-selected strain of the smaller tea tortrix, Adoxophyes honmai (Lepidoptera: Tortricidae).

A strain of Adoxophyes honmai resistant to Adoxophyes honmai nucleopolyhedrovirus (AdhoNPV) was established from a field-collected colony by repeated selection. Fifth-instar larvae of this resistant strain (R-strain) had over 66 666-fold greater resistance in terms of 50 % lethal concentration values to oral infection of AdhoNPV than non-selected strain larvae (susceptible for AdhoNPV; S2-strain). In this study, the mechanism of resistance to AdhoNPV was determined in R-strain larvae. An assessment of viral genome replication in AdhoNPV-infected S2- and R-strain larvae by quantitative PCR showed no viral genome replication occurring in R-strain larvae. Transcription of AdhoNPV ie-1, vp39 and polyhedrin genes was also not detected in R-strain midgut cells. Besides, a fluorescent brightener had no effect on AdhoNPV infection in either S2- or R-strain. However, binding and fusion of occlusion-derived virus with R-strain were significantly lower than those of S2-strain. These findings suggest that R-strain Adoxophyeshonmai larvae possess a midgut-based resistance to oral infection by AdhoNPV in which midgut epithelial cells are infected less efficiently.