Black Tea and Lignin as Ultraviolet Protectants for the Beet Armyworm Nucleopolyhedrovirus

Abstract

A major constraint to the use of baculoviruses for biocontrol of insects is their sensitivity to UV degradation. In this study, we evaluated black tea (Lipton®, London, UK) and lignin (Reax 85A™, MeadWestvaco, Charleston, SC) as potential UV protectants for beet armyworm Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae) multiple-embedded nucleopolyhedrovirus (SeMNPV). The original activity remaining (OAR%) from SeMNPV upon exposure to various lengths of time (up to 5 h) to a source of UVA and UVB was evaluated in bioassays using beet armyworm third-stage larvae under laboratory conditions. Beet armyworm mortality was measured after larvae fed on artificial diet treated with SeMNPV. Mortality of beet armyworm due to SeMNPV, with no UV protectants added, was reduced to 23, 11.3 and 2.1% upon UV exposure for 15, 30 or 60 min, respectively. To investigate the mechanism of reduction in the efficacy of SeMNPV when exposed to UV was due to the degradation of full-length viral genomic DNA, a modified DNA isolation technique was developed to measure levels of the full length viral genomic DNA of SeMNPV through electrophoresis on an agarose gel. The efficacy of SeMNPV on beet armyworm was lost after 2 h of UV exposure, and the full-length genomic DNA also was degraded to levels that were not visible on agarose gel. However, both black tea and lignin provided nearly 100% UV protection for SeMNPV as measured in bioassays even after 5 h of UV irradiation. SeMNPV efficacy against beet armyworm in samples containing black tea or lignin resulted in no significant visual reduction of the full length viral genomic DNA. To investigate the mechanism of UV protection for SeMNPV from black tea and lignin, absorption spectra of both protectants were measured with a spectrophotometer. High rate of absorption in the UV range, especially at the range of UVB (280–320nm), was detected for both materials. The absorption rate was higher with lignin than with black tea. Whereas lignin was a good absorber for both UVB and UVA radiation, black tea was primarily an absorber of UVB. Therefore, both black tea and lignin are potential natural UV protectants in the formulation of baculovirus-based biopesticides.