Persistence and distribution of wild-type and recombinant nucleopolyhedroviruses in soil

Persistence of recombinant and wild-type nucleopolyhedroviruses (NPV) was c ompared in field and laboratory microcosm experiments. Horizontal and verti cal distribution of the viruses also was monitored in the field agricultura l soil. Mixed populations of the bollworm, Helicoverpa zea, and tobacco bud worm, Heliothis virescens, in cotton were sprayed five times during a growi ng season with wild-type H. tea NPV (HzSNPV.WT) or with a genetically modif ied H, tea NPV expressing an insect-specific neurotoxin (HzSNPV.LqhIT2). Hz NPV.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 d etected at all soil depths down to 26-35 cm. Both NPVs were randomly distri buted 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 lea ves nor in insects sampled from the plots in 1998. Viral persistence also w as monitored in laboratory soil microcosms. Three viruses-wild-type Autogra pha californica NPV (AcNPV.WT), A. californica NPV expressing a scorpion to xin (AcNPV.AaIT), and A. californica NPV expressing juvenile hormone estera se (AcNPV.JHE-S201G)-were introduced into soil microcosms by each of two me thods, in water suspension or in host cadavers, for a total of six treatmen ts plus controls. After 17 months, the number of viable OB remaining did no t differ among the treatments. The results indicate that the only differenc es 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.