Virulence and competitiveness of Cydia pomonella granulovirus mutants: Parameters that do not match

The LD₅₀, median survival time (ST₅₀) and virus production are virulence parameters that are commonly used to describe the biological characteristics of viruses. In this study, these parameters were determined for Cydia pomonella granulovirus (CpGV-M) and two naturally occurring mutants (CpGV-MCp4 and -MCp5) that carry Tc1-like insect transposable elements. The three virus genotypes were similar in their LD₅₀, ST₅₀ and virus production. However, the mutant genotypes MCp4 and MCp5 were very effectively out-competed by CpGV-M in direct competition experiments, where Cydia pomonella larvae were co-infected with known ratios of occlusion bodies or budded virus of CpGV-M and one of the two mutants. It was demonstrated that MCp5 and MCp4 could not be sustained in the virus population when the progeny viruses of different co-infections were used as inocula to infect next passage larvae. These results show that the virulence parameters LD₅₀, ST₅₀ and virus production alone do not adequately reflect the competitiveness of the virus and are thus not suitable to describe virus population dynamics. © 2005 SGM.