Host-insect survival time and disintegration in relation to population density and dispersion of recombinant and wild-type nucleopolyhedroviruses

Wild-type and recombinant nucleopolyhedroviruses (NPVs) were compared with respect to host-insect (Trichoplusia ni) survival time and disintegration time, production of polyhedral occlusion bodies (POB), and viral population density and dispersion on collard leaves. Viruses tested were variants of Autographa californica NPV (AcNPV): wild-type (AcNPV.WT), AcNPV expressing a scorpion toxin (AcNPV.AaIT), and two viruses expressing juvenile hormone esterase (AcNPV.JHE-S201G and AcNPV.JHEKK). Preliminary results indicated that a plaque isolate (AcNPV.C6) did not differ significantly from AcNPV.WT in insect survival time, disintegration time, or production of POB. T. ni survival time was significantly shorter for insects infected with AcNPV.AaIT than for those infected with AcNPV.C6, AcNPV.JHEKK, or ACNPV.JHE-S201G; the latter three NPVs did not differ from one another. Disintegration of larval cadavers was most rapid in those killed by ACNPV.C6, followed by AcNPV.JHEKK, AcNPV.JHE-S201G, and AcNPV.AaIT. AcNPV.C6 produced 8.3 X 10(8) POB/insect, which was 1.5, 2.9, and 3.1 times the number per larva produced by ACNPV.JHEKK, AcNPV.JHE-8201G, and AcNPV.AaIT, respectively. T. ni larvae infected with one type of NPV were released onto collard plants in a greenhouse, and leaves were collected after 1 week and bioassayed. Population densities of AcNPV.WT, AcNPV.JHEKK, and AcNPV.AaIT were 126, 19, and 5 POB/38.5 mm2, respectively, and these three means were significantly different from one another. Infected insects voided viable NPV in liquid discharged from the anus in the late stages of the disease, regardless of the viral isolate used, which probably accounted for a portion of the POB numbers on leaves. Analysis with Taylor's power law indicated that the populations of ACNPV.JHEKK and AcNPV.AaIT, but not that of AcNPV.WT, had significantly clumped or contagious dispersion patterns. Thus, the insertion of a gene to express scorpion toxin or juvenile hormone esterase altered the biology (virulence--in terms of host survival time--and host disintegration time) of ACNPV, which, in turn, altered three basic ecological characteristics of the virus: reproduction rate, population density, and dispersion.