Congo red (a diazo dye) and Tinopal LPW (a stilbene fluorescent brightener) were assayed to determine their effects on the activity of the gypsy moth, Lymantria dispar (L.), nucleopolyhedrovirus (LdMNPV) and cypovirus (LdCPV) infecting the gypsy moth. In the case of LdMNPV, Congo red reduced the median lethal concentration (LC₅₀) by approximately 26-fold, whereas Tinopal LPW reduced the LC₅₀ by approximately 360-fold. In tests using late second-stage larvae treated with LdCPV, LC₅₀s were reduced by approximately 12-fold and approximately 430-fold by Congo red and Tinopal LPW, respectively. Fourth instar larvae were challenged with LdMNPV/water, LdMNPV/Congo red, and LdMNPV/Tinopal LPW and the activities of progeny virus from all treatments were similar.