Biological and biochemical relationships between the nucleopolyhedroviruses of Mamestra brassicae and Heliothis armigera

The multiply embedded nucleopolyhedroviruses (MNPV) originally isolated from Mamestra brassicae (German and Dutch isolates) and Heliothis armigera have been studied comparatively to establish their relatedness, both in terms of biological activity and genomic homology. All three viral isolates replicated in M. brassicae, H. armigera, Heliothis zea, and Heliothis virescens, resulting in each case in progeny virus that was essentially similar to the inoculum. Dose-mortality studies carried out on M. brassicae and H. armigera indicate that these viruses do not differ significantly with respect to their virulence to these insects. The same studies also clearly indicate that the susceptibility of M. brassicae and H. armigera larvae to viral infection differs significantly with increasing larval age. The increase in LD(50) values from L1 to L4 is, in fact, over 40,000-fold for M. brassicae, while it is only 1300-fold for H. armigera. The results of the present study also confirm that all three isolates are genetically closely related. Due to their high degree of homology and almost identical biological activity, it is suggested that these isolates should be considered variants of a single virus species.