On the competitive fitness of baculoviruses in insects

Mixed infections of baculoviruses in insect hosts are quite common in nature.
This leads to ‘within-host’ and ‘between-host’ competition between virus variants. Because
both levels of selection will contribute to overall biological fitness, both must be included in
assessments of the fitness of fast-acting recombinant baculoviruses. We investigated
baculovirus fitness parameters in single and mixed infection of insect larvae, in single and
serial passage experiments in lepidopteran hosts (Helicoverpa armigera, Spodoptera exiqua
and Trichoplusia ni) in laboratory, greenhouse and field settings. Median time to death in
third instar larvae of H. armigera (Hübner) was lower in insects challenged with a mixture of
wild type (HaSNPV-wt) and mutant (_egt, HaSNPV-LM2) Helicoverpa armigera SNPV,
than in larvae infected with only HaSNPV-wt. The results from a behavioral study on cotton
(glasshouse, field) indicated that the transmission of HaSNPV-LM2 is not modified by the
absence of the egt gene, whereas in the case of the HaSNPV-AaIT (_egt, + AaIT) lower
virus yield as well as altered caterpillar behavior could compromise virus fitness. Virus
transmission in greenhouse and field was not reduced, when HaSNPV-LM2 was used in
mixed infections with HaSNPV-wt. However, a reduction of ‘between host’ transmission
was recorded when H. armigera larvae were co-infected with HaSNPV-wt and HaSNPVAaIT.
Serial passage experiments with S. exigua and T. ni showed positive selection for wild
type SeMNPV and AcMNPV over genetically modified variants (_egt, + AaIT in the case of
SeMNPV, and _egt in the case of AcMNPV) over passages. These findings can help to
understand long-term dynamics of virus genotypes in virus-insect-host plant systems. They
can also help foresee potential consequences of the introduction of genetically-modified or
exotic baculoviruses in agro-ecosystems.