Per Os Infectivity of Preoccluded Virions from Polyhedrin-Minus Recombinant Baculoviruses

With polyhedrin-minus baculoviruses, preoccluded virions (POV), membrane-bound nucleocapsids which are normally occluded within polyhedra, accumulate in high concentrations within infected nuclei. Using the droplet feeding assay with Trichoplusia ni neonate larvae, POV were shown to be highly infectious when administered per os. Samples from Spodoptera frugiperda (Sf-21) tissue culture cells infected with a polyhedrin-minus isolate of the Autographa californica nuclear polyhedrosis virus (AcMNPV) contained higher per os infection levels than samples infected with a polyhedrin-plus (wild-type) AcMNPV isolate. Under the conditions used, the nonoccluded virions that bud from infected cells did not infect the test larvae. Survival time bioassays were performed with wild-type, polyhedrin-minus, and polyhedrin-minus/lac Z AcMNPV virus isolates. Removal of the polyhedrin gene caused a significant increase in median survival time (ST50) of infected larvae. The ST50 was further increased by the insertion and expression of the lac Z gene. The protocol developed should be of general use in evaluating the pesticidal properties of all recombinant baculoviruses lacking a polyhedrin gene.