Abstract
Current knowledge of the field resistance of codling moth (CM, Cydia pomonella, L) against Cydia pomonella granulovirus (CpGV) is based mainly on the interaction between the Mexican isolate CpGV-M and CpRR1, a genetically homogeneous CM inbreed line carrying type I resistance. The resistance level of laboratory-reared CpRR1 to CpGV-M was recently found to have decreased considerably, compared to the initially high resistance. To understand the background of this phenomenon, CpRR1 larvae were exposed over several generations to CpGV-M for re-selection of the original resistance level. After five and seven generations of selection, new CpRR1_F5 and CpRR1_F7 lines were established. The resistance ratio of these selected lines was determined by full range bioassays. The CpRR1_F5 strain regained a higher level of resistance against CpGV up to 104-fold based on LC50 values compared to susceptible larvae (CpS), which indicated that the absence of virus selection had resulted in a reduction of resistance under laboratory rearing conditions. In addition, some fitness costs of fecundity were observed in CpRR1_F5. Single-pair crossings between CpRR1_F5 or CpRR1_F7 with susceptible CpS moths revealed a dominant but not fully sex-linked inheritance, which suggests a partial loss of previous resistance traits in CpRR1.
Highlights
The baculovirus Cydia pomonella granulovirus (CpGV) is an important and commercially successful biopesticide agent, which is used to control larvae of codling moth (CM, Cydia pomonellaL.) in pome fruit production [1,2,3]
Re-selection of CpRR1 larvae to its previous resistance level was achieved by consecutive mass crossings of CpRR1 adults followed by exposure of L1 larvae to CpGV-M occlusion bodies (OBs)
Starting from the obtained progeny CpRR1_F1, two rounds of selection followed at the same concentration but with neonate (L1) larvae exposed to virus OBs, resulting in generation of CpRR1_F3
Summary
The baculovirus Cydia pomonella granulovirus (CpGV) is an important and commercially successful biopesticide agent, which is used to control larvae of codling moth (CM, Cydia pomonellaL.) in pome fruit production [1,2,3]. CpGV preparations containing the Mexican isolate CpGV-M had failed [1,4]. When individuals of this population were reared in the laboratory for two years on virus-free diets, the established colony still exhibited an at least 100-fold resistance to CpGV-M. More than 40 local orchards with CM populations presumed to carry type I resistance have been discovered in Austria, Czech. It has been suggested that in most of these populations, type I resistance to CpGV-M had developed because resistance-breaking CpGV isolates provided successful control of CM in most orchards with CpGV resistance [10]
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