FIELD STUDIES OF THE COOCCLUSION STRATEGY WITH A GENETICALLY ALTERED ISOLATE OF THE AUTOGRAPHA-CALIFORNICA NUCLEAR POLYHEDROSIS-VIRUS

The first field study of a genetically altered virus in the United Sta tes was performed with an isolate of the Autographa californica nuclea r polyhedrosis virus is (AcMNPV), which lacks a polyhedrin gene. In th e first year of the study, three applications of 7.4 x 10(11) AcMNPV p olyhedra containing 48% genetically altered and 52% wild-type virus pa rticles (co-occluded) were made on a 0.1-ha circular plot of cabbage p lants. The application area was surrounded by a 0.7-ha circular buffer zone. Before each application, the plants in the application area wer e infested with 4,500 third-instar Trichoplusia ni (Hubner) larvae. Af ter each application, 100% of the T. ni test larvae sampled 5 d after infection were infected with AcMNPV and produced progeny polyhedra con taining an average of 42 +/- 17.6% genetically altered virus particles . At the end of the 1st yr, the progeny polyhedra population in the ap plication area was estimated at 1.6 x 10(13) polyhedra. In the 2nd yr, the application and buffer sites were replanted with cabbage plants. At four times during the growing season, the plants were seeded with T . ni larvae or eggs. Less than 2% of the test larvae became infected w ith AcMNPV. Polyhedra were extracted from soil samples collected in th e application and buffer areas. Using neonate larval bioassays with th e soil extracts, it was estimated that the soil in the application and buffer areas contained an average of 1,652 +/- 3,370 and 832 +/- 2,53 9 biologically active polyhedra per gram dry weight, respectively. Sev enty-five larvae infected with polyhedra extracted from application ar ea soil samples produced progeny polyhedra containing a mean of 9 +/- 19% genetically altered virus particles. In the 3rd yr, the applicatio n area soil samples contained an average of 1,671 +/- 3,274 biological ly active polyhedra per gram dry weight. Eighty-four progeny polyhedra l samples contained a mean of 6 +/- 14% genetically altered virus part icles. The co-occlusion strategy did not alter the environmental persi stence of the polyhedra containing both wild-type and polyhedrin-minus virus particles. However, the data show a decline in the percent of p olyhedrin-minus particles in the polyhedra and demonstrate that the pe rsistence of a polyhedrin-minus virus in a cycling virus population is limited by the co-occlusion process. The environmentally desirable at tributes of using the co-occlusion process for genetically enhanced ba culovirus pesticides and possible problems are discussed.