Formulation of a nucleopolyhedrovirus with boric acid for control of Spodoptera frugiperda (Lepidoptera: Noctuidae) in maize

The degree of control of the fall armyworm, Spodoptera frugiperda, by a multiple nucleopolyhedrovirus (SfMNPV) appears to be limited by the quantity of inoculum consumed by the insect and the delivery of the virus to the insect feeding site. The formulation of the virus with phagostimulants and/or viral synergists, such as boric acid, may help overcome this problem. The present study aimed to determine the degree of potentiation of boric acid toward SfMNPV in a granular phagostimulant formulation. In a laboratory bioassay the LC₅₀ value for second-instar larvae was reduced from 114 virus occlusion bodies (OBs)/mm² of diet surface for virus alone to 51 OBs/mm² of diet in the presence of 1% boric acid. The mean time to death of larvae exposed to virus mixed with 0.5 or 1% boric acid was not significantly different from that of larvae inoculated with virus alone. Increasing the concentration of boric acid at a single determined concentration of virus (80 OBs/mm²) resulted in a significant increase in the prevalence of virus-induced mortality. The boric acid alone did not cause S. frugiperda mortality at the concentrations tested. A field trial performed with S. frugiperda larvae held on plants within fine gauze bags indicated that application of maize flour granules containing virus + 1% boric acid caused a significant increase in virus-induced mortality compared to application of granules containing virus alone. A randomized block experiment performed later also resulted in a higher prevalence of virus-induced mortality in S. frugiperda larvae exposed to virus mixed with 1% boric acid in samples collected at 5 days postapplication and reared in the laboratory until death or pupation, but not in samples made at 1 day and 3 days postapplication. Differences in the prevalence of virus infection in insects collected at each time point may have been related to the consistency of the granular formulation, which turned into a paste and adhered to the surface of maize plants under conditions of heavy rainfall. Granules containing 1 and 4% boric acid were not toxic to the earwig, Doru taeniatum, in the laboratory. The same concentrations of boric acid sprayed onto maize plants did not significantly reduce the abundance of natural enemies or other nontarget insects at any sample time point. Boric acid offers an economical means of enhancing baculovirus activity with little apparent risk to nontarget arthropods.