Jürg Huber

Rapid emergence of baculovirus resistance in codling moth due to dominant, sex-linked inheritance.

Insect-specific baculoviruses are increasingly used as biological control agents of lepidopteran pests in agriculture and forestry, and they have been previously regarded as robust to resistance development by the insects. However, in more than a dozen cases of field resistance of the codling moth Cydia pomonella to commercially applied C. pomonella granulovirus (CpGV) in German orchards, resistance ratios exceed 1000. The rapid emergence of resistance is facilitated by sex-linkage and concentration-dependent dominance of the major resistance gene and genetic uniformity of the virus. When the gene is fixed, resistance levels approach 100,000-fold. Our findings highlight the need for development of resistance management strategies for baculoviruses.

Molecular and biological characterization of new isolates of Cydia pomonella granulovirus from Iran

The Cydia pomonella granulovirus (CpGV) is a very effective biological control agent of the codling moth, Cydia pomonella L. (Lep.: Tortricidae). Only a few CpGV isolates originating from Mexico (M), England (E), and Russia (R) have been described so far. In a field survey at different locations in Iran, CpGV isolates were collected from single or pooled codling moth larvae. The isolates, designated I1, I7, I8, I15, I22, I28, I30, I66, I67, I68, and I70 showed genetic (DNA restriction endonuclease profiles) and biological (bioassays) differences. Most isolates could be attributed to genome types similar to those found in CpGV-M, -E, and -R. Some of them were clear mixtures of different genotypes. Thus, the CpGV isolates found in the North–West of Iran make an important contribution to the known diversity of CpGV. The occurrence of novel, naturally occurring CpGV isolates emphasize the necessity of further studies towards the diversity and evolution of CpGV.

High stability and no fitness costs of the resistance of codling moth to Cydia pomonella granulovirus (CpGV-M)

Resistance against the biocontrol agent Cydia pomonella granulovirus (CpGV-M) was previously observed in field populations of codling moth (CM, C. pomonella) in South-West Germany. Incidental observations in a laboratory reared field colony (CpR) indicated that this resistance is rather stable, even in genetically heterogeneous CM colonies consisting of both susceptible and resistant individuals. To test this hypothesis, the resistance level of CpR that was 1000times less susceptible to CpGV-M was followed for more than 60 generations of rearing. Even without virus selection pressure, the high level of resistance, expressed as median lethal concentration, remained stable for more than 30 generations and declined only by a factor of 10 after 60 generations. When cohorts of the F32 and F56 generations of the same colony were selected to CpGV-M for five and two generations, respectively, the resistance level increased to factor of >1,000,000 compared to a susceptible control colony. Laboratory reared colonies of CpR, did not exhibit any measurable fitness costs under laboratory conditions in terms of fecundity and fertility. Resistance testing of seven selected codling moth field populations collected between 2003 and 2008 in commercial orchards in Germany that were repeatedly sprayed with CpGV products gave evidence of different levels of resistance and a more than 20-fold increase of the resistance in 1-3 years when selection by CpGV-M was continued. A maximum 1,000,000-fold level of resistance to CpGV-M that could be induced in the laboratory under virus pressure had been also observed in one field population. The high stability of resistance observed in the genetically heterogenous colony CpR indicates that resistance to CpGV-M is not very costly.

Comparative pathogenesis of the Helicoverpa armigera single-nucleocapsid nucleopolyhedrovirus in noctuid hosts of different susceptibility

Neonate larvae of the noctuid moth Spodoptera exigua were susceptible to an infection by Helicoverpa armigera single-nucleocapsid nucleopolyhedrovirus (HaSNPV). Biological activity (LD(50),ST(50)) of the virus was considerably reduced as compared to its activity in the homologous host, H. armigera. Pathogenesis was studied using a recombinant HaSNPV carrying a green fluorescent protein gene, which induces fluorescence in infected cells to mark infection. In larvae of H. armigera, fluorescence was pronounced in the fat body after 2.9 days post infection and could also be detected in several other tissues. In contrast, fluorescence was not observed in tissues of S. exigua until 9 days post infection and was restricted almost exclusively to cells of the ganglia. Examination of serial sections of wildtype HaSNPV-infected S. exigua-larvae revealed a similar pattern of tissue tropism. Apparently, HaSNPV does not undergo the usual steps in host invasion and infection in this insect species, but targets specifically to nervous tissue.