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.