Virus loads in Douglas-fir tussock moth larvae infected with the Orgyia pseudotsugata nucleopolyhedrovirus

Abstract

AbstractWe studied Orgyia pseudotsugata multiple nucleopolyhedrovirus (OpMNPV) infections in larvae of the Douglas-fir tussock moth, Orgyia pseudotsugata McDunnough (Lepidoptera: Lymantriidae), to determine the quantity of OpMNPV particles that result in mortality. We observed a bimodal pattern of mortality in Douglas-fir tussock moth larvae that ingested diet plugs contaminated with 9.5 OpMNPV occlusion bodies. A mortality peak (80% of total mortality observed) occurred between day 5 and day 11 post ingestion, and a second, smaller mortality peak coincided with the onset of pupation. Virus loads, defined as the number of OpMNPV occlusion bodies in each sample of tested larval homogenate, were quantified using an indirect ELISA method. Virus loads that resulted in mortality were significantly greater than those quantified in larvae that were sacrificed during and after the peak mortality wave (P < 0.004 and P < 0.0001, Mann–Whitney U two-tailed rank test and SPSS®, respectively). This is the first known attempt to differentiate the quantity of virus produced during lethal infections from the virus loads in larvae that survive infection.