Effect of host sterols on the sterol composition and virulence of a nuclear polyhedrosis virus of Heliothis zea

The type of sterol in the diet of Heliothis zea affected not only the sterol composition of the insect larva but also the virulence and/or sterol composition of a single‐nucleocapsid nuclear polyhedrosis virus (HzSNPV). This baculovirus, which was purified by differential and sucrose density gradient centrifugation, had a sterol content of 40 ng per 10 6 polyhedra. When the sterol composition of HzSNPV was characterized by gas liquid chromatography, reversed phase‐high performance liquid chromatography, mass spectrometry, proton nuclear magnetic resonance spectrometry and/or ultraviolet spectroscopy, the sterols in the virus were similar to those of the host. The HzSNPV isolated from larvae fed Δ 5_ , Δ 0_ or Δ 5,7 ‐sterols contained primarily cholesterol, cholestanol or 7‐dehydrocholesterol, respectively. Changes in the sterol composition of HzSNPV affected its LD 50 , but not LT 50 , in larvae containing Δ 5 ‐sterols. The LD 50 of virus isolated from larvae containing Δ 0_ , Δ 5_ and Δ 7 ‐sterols decreased from 275,423 to 32,359 to 5,012 polyhedra/larva, respectively. The latter virus was also more virulent than the one that was isolated from larvae containing Δ 5,7 ‐sterol and had an LD 50 of 58,884 polyhedra/larva. In contrast, the LD 50 of an HzSNPV (Sandoz, Inc.) containing Δ 5 ‐sterol was not affected by the presence of Δ 5_ , Δ 0_ or Δ 5,7 ‐sterols in the tissues of the host (1,413; 1,288 and 355 polyhedra/larva, respectively). The results of this study indicate that the sterol composition of H. zea can affect the sterol composition of HzSNPV and therefore may affect the ability of this biological control agent to control its economically important insect host.