Behavior of a recombinant baculovirus in lepidopteran hosts with differentsusceptibilities

Insect pathogens, such as baculoviruses, that are used as microbial insecti cides have been genetically modified to increase their speed of action. Non target species will often be exposed to these pathogens, and it is importan t to know the consequences of infection in hosts across the whole spectrum of susceptibility. Two key parameters, speed of kill and pathogen yield, ar e compared here for two baculoviruses, a wild-type Autographa californica n ucleopolyhedrovirus (AcNPV), AcNPV clone C6, and a genetically modified AcN PV which expresses an insect-selective toxin, AcNPV-ST3, for two lepidopter an hosts which differ in susceptibility, The pathogenicity of the two virus es was equal in the less-susceptible host, Mamestra brassicae, but the reco mbinant was more pathogenic than the wild-type virus in the susceptible spe cies, Trichoplusia ni. Both viruses took longer to kill the larvae of M. br assicae than to kill those of T. ni. However, whereas the larvae of T. ni m ere killed more quickly by the recombinant virus, the reverse mas found to be true for the larvae of M. brassicae. Both viruses produced a greater yie ld in M. brassicae, and the yield of the recombinant was significantly lowe r than that of the wild type in both species. The virus yield increased lin early with the time taken for the insects to die. However despite the more rapid speed of kill of the wild-type AcNPV in M. brassicae, the yield was s ignificantly lower for the recombinant virus at any given time to death. A lower yield for the recombinant virus could be the result of a reduction in replication rate. This was investigated by comparing determinations of the virus yield per unit of weight of insect cadaver. The response of the two species (to both viruses) was very different: the yield per unit of weight decreased over time for M. brassicae but increased for T. ni, The implicati ons of these data for risk assessment of wild-type and genetically modified baculoviruses are discussed.