Abstract
The susceptibility of Heliothis punctiger to specified doses of a commercial formulation of nuclear polyhedrosis virus (NPV), Elcar, is described in terms of larval age, length, and weight and time-mortality response. Our bioassays indicated that the susceptibility of newly molted thirdinstar larvae was independent of the temperature history and rates of larval development. Consequently, a generalized expression relating susceptibility to age, at a range of temperatures, was derived. In addition to a progressive decrease in susceptibility with increasing larval instar, fluctuations in susceptibility at various periods within instars were recorded. Both of these changes in susceptibility were apparently mediated by the gut, as no such changes were recorded when nonoccluded virions were injected into fourth-instar larvae of different ages and newly molted fifth-instar larvae. However, a large reduction in susceptibility to injected virus was recorded at 24 hr postmolt in the fifth (final) instar. This indicated that a different resistance mechanism, not mediated by the gut, was then operating.
Published Version
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