Response of hosts of varying susceptibility to a recombinant baculovirus insecticide in the field

In an attempt to improve the effectiveness of nucleopolyhedroviruses (NPV) as bioinsecticides of lepidopteran pests, the NPV of Autographa californica (AcMNPV) has been genetically modified to include a gene that expresses an insect-selective scorpion toxin, AaHIT. In a field trial, we compared the response of a highly susceptible 'target' host (Trichoplusia ni) and a representative less susceptible species (Mamestra brassicae), following spraying with wild-type AcMNPV and the genetically modified virus. Mortality and therefore risk of infection were significantly lower in the less susceptible species but did not differ between the wild-type and the recombinant viruses. Speed of kill was consistently faster in T. ni infected with the recombinant virus for samples collected 1, 3, and 5 days following spray application. The results for M. brassicae were more variable; time to death induced by both viruses was longer in M. brassicae than in T. ni but the genetically modified virus acted faster only in larvae taken 3 and 5 days following spraying. This contrasts with laboratory assays in which the recombinant virus took longer than the wild-type virus to kill M. brassicae. The results demonstrate that there are differences in virus productivity, distribution, and timing of virus release when two hosts with different susceptibilities are treated with wild-type and recombinant baculoviruses, which will impact on further rounds of virus replication. Thus, the effects of recombinant baculoviruses on less susceptible, potentially nontarget, hosts are not likely to be easy to predict, and they highlight the need for both more information on the behavior of baculoviruses in hosts of varying susceptibility and further studies on the impact of these factors on secondary transmission, (C) 1999 Academic Press.