The nuclear polyhedrosis virus (NPV) from the beet armyworm, Spodoptera exigua (Hübner) (SeMNPV), was the most active virus tested against the beet armyworm (LC₅₀ = 4.1 PIBs/mm²), followed by nuclear polyhedrosis viruses from the alfalfa looper, Autographa californica (Speyer) (AcMNPV; LC₅₀ = 92.6 PIBs/mm²), and the celery looper, Anagrapha falcifera (Kirby) (AfMNPV; LC₅₀ = 195.7 PIBs/mm²). In the case of the nuclear polyhedrosis virus from the bollworm, Helicoverpa armigera (Hübner), LC₅₀s could only be obtained for five/six replicates, whereas LC₅₀s could only be obtained for two/six replicates for the nuclear polyhedrosis virus from the wax moth, Galleria mellonella (L.) (GmMNPV). When an optical brightener Tinopal LPW was added to virus suspensions, LC₅₀ values were reduced by 130-fold for both SeMNPV and AcMNPV and by 300-fold for AfMNPV. The addition of Tinopal LPW greatly increased the activities of HaMNPV and GmMNPV. In terms of speed of kill, Tinopal LPW reduced the LT₅₀s for all nuclear polyhedrosis viruses by 30–40%.