Persistence and Efficacy of Four Nuclear Polyhedrosis Viruses for Corn Earworm (Lepidoptera: Noctuidae) on Heading Grain Sorghum

The persistence and efficacy of Helicoverpa zea nuclear polyhedrosis virus (HzNPV) on heading grain sorghum was compared with three multiply-enveloped NPV's from other hosts to which the corn earworm, Helicoverpa zea (Boddie), is susceptible. Bioassay of NPV by feeding florets from sprayed heads to second instar corn earworm showed only low levels of virus remaining on heads in all of the treatments four days after application. The initial activity and persistence of a commercial preparation of H. zea NPV were greater than four unformulated virus preparations including unformulated HzNPV at rates of 3, 6, and 15 × 10¹¹ polyhedral inclusion bodies (PIB) per ml (P < 0.05). Initial activity and persistence were similar for unformulated preparations of HzNPV, Heliothis armigera (HaNPV), and Autographa californica (AcNPV). Activity and persistence of Anticarsia gemmatalis (AgNPV) were significantly less than for the other viruses (P < 0.05). In a small plot test on heading grain sorghum in which viruses were applied at 1.5, 3, and 6 × 10¹¹ PIB/ha, mortality of collected corn earworm larvae and larval population reduction were greater in the HzNPV treatments than in the three multiply-enveloped virus treatments. HaNPV and AcNPV preparations were more effective than AgNPV. All virus preparations required one wk or longer to significantly reduce larval populations at all rates (P < 0.05).