Persistence and Distribution of Wild-Type and Recombinant Nucleopolyhedroviruses in Soil

Persistence of recombinant and wild-type nucleopolyhedroviruses (NPV) was compared in field and laboratory microcosm experiments. Horizontal and vertical distribution of the viruses also was monitored in the field agricultural soil. Mixed populations of the bollworm, Helicoverpa zea, and tobacco budworm, Heliothis virescens, in cotton were sprayed five times during a growing season with wild-type H. zea NPV (HzSNPV.WT) or with a genetically modified H. zea NPV expressing an insect-specific neurotoxin (HzSNPV.LqhIT2). HzNPV.WT accumulated 2.3 times as many occlusion bodies (OB) as HzSNPV.LqhIT2 in soil by the end of the growing season in October 1997. Both NPVs were detected at all soil depths down to 26-35 cm. Both NPVs were randomly distributed among 0-2 cm soil samples throughout the plots according to analysis with Taylor's power law. By 4 August 1998, soil concentration of HzSNPV.WT was only 11-13 OB/g at depths from 0 to 14 cm, and the wild-type virus was not detected below 14 cm. HzSNPV.LqhIT2 was detected only in trace amounts at 0-2 cm at this time. Neither NPV was detected in bioassays of cotton leaves nor in insects sampled from the plots in 1998. Viral persistence also was monitored in laboratory soil microcosms. Three viruses-wild-type Autographa californica NPV (AcNPV.WT), A. californica NPV expressing a scorpion toxin (AcNPV.AaIT), and A. californica NPV expressing juvenile hormone esterase (AcNPV.JHE-S201G)-were introduced into soil microcosms by each of two methods, in water suspension or in host cadavers, for a total of six treatments plus controls. After 17 months, the number of viable OB remaining did not differ among the treatments. The results indicate that the only differences in soil populations of wild-type versus recombinant NPVs are due to the greater amounts of the wild-type viruses that accumulate, probably because they have a greater capacity to replicate in the host insect population.