It was established that the virulence of the North American baculovirus strain LdMNPV-45 is almost two orders of magnitude higher than the virulence of the Asian strain LdMNPV-27 and does not depend on the test host population (gypsy moth). The Asian strain carries deletions in bro-p and vef-1 genes (82 and 91%, respectively). In accordance with the published data, the product of the latter can greatly increase the virulence of the virus. This result indicates that the population polymorphism of the virulence of baculoviruses can be explained by the vef-1 gene deletion.