In this paper, we describe the development of a novel field detection system for the identification of Orgyia pseudotsugata nucleopolyhedrovirus (OpNPV) and OpNPV infections in Douglas-fir tussock moth (O. pseudotsugata) (DFTM) larvae, utilizing antibodies in a dipstick immunoassay. The dipstick method is sensitive to a minimum of 10ng of extracted virus protein, or approximately 1070 virus occlusion bodies, and is sufficiently sensitive to detect OpNPV infections in DFTM prior to mortality. Additionally, the method can be used to unambiguously detect virus in infected larvae without purification of the test sample. This research provides a novel tool for on-site assessment of the incidence of OpNPV in field populations of DFTM, and has the potential to improve the biological control of the DFTM by facilitating on-site pest management decisions.