Density-dependent virus mortality in populations of Wiseana (Lepidoptera: Hepialidae)

Abstract

The establishment of a host-virus equilibrium between Wiseana sp., a soil-dwelling pasture pest, and its nuclear polyhedrosis virus (NPV) was studied. NPV could not be detected in very young field-collected larvae; however, when these larvae were reared in the laboratory, on a semisynthetic lima bean diet, a large proportion of them (>50%) were found to be infected with virus. For larvae weighing >200 mg, the level of infection of reared larvae was similar to that found in field-collected larvae. This suggested that larvae were being infected at an early stage of their life cycle but that the virus could not be detected. In young pastures virus mortality was shown to have a density-dependent relationship in which infected larvae in generation n could be related to the rate of virus infection in generation n + 1. The data could be fitted to a curve: Y = A (1 + B × X C) , where Y is the infection rate, X is the number of diseased larvae, and A, B, and C are constants. In older pastures (>10 years) a density-dependent virus mortality was not apparent, presumably due to the accumulation of virus in the soil from previous infected generations.