The use of double-stranded RNA (dsRNA) biopesticides is a novel crop protection technology that employs RNA interference to impede the synthesis of essential proteins in pest species. Recently, foliar-applied dsRNA biopesticides have been developed against a broad array of insect pests; however, to be effective, the stability of biopesticides upon application to leaf surfaces must be sufficient under exposure to conditions like sunlight. Furthermore, upon release to receiving aqueous environments, dsRNA biopesticides may undergo both direct and indirect photodegradation to be removed from the environment. In this study, we investigated the sunlight photolysis rates and mechanisms of dsRNA in solution and on surfaces. In solution, we examined dsRNA direct photolysis in phosphate buffer and indirect photodegradation by adding a photosensitizer (i.e., nitrate, dissolved organic matter, DOM). After 16.5 h irradiation, 12.7±0.3% of the dsRNA was degraded by direct photolysis, corresponding to a degradation rate constant of 8.22(±0.70) × 10⁻³ hour⁻¹ assuming first-order kinetics. In comparison, rate constants assuming first-order kinetics in the presence of DOM and nitrate were 4.09(±0.50) × 10⁻² hour⁻¹ and 3.62(±0.80) × 10⁻² hour⁻¹, respectively, indicating indirect photodegradation dominates. Next, we investigated the direct photolysis of dsRNA on a hydrophilic surface (i.e., glass) and a hydrophobic surface (i.e., polytetrafluoroethylene, PTFE). Over 24 h irradiation, the direct photolysis of dsRNA on the glass surface followed first-order kinetics with a rate constant of 0.11±0.01 hour^-1. We found that dsRNA degradation on glass was 4-fold higher than degradation on PTFE and 13-fold higher than direct photolysis in solution. The fast direct photolysis of dsRNA on surfaces implies that this degradation mechanism may have an important role in naturally attenuating foliar-applied dsRNA biopesticides. Furthermore, both direct photolysis on surfaces and solution-phase photodegradation (particularly indirect) may be important for assessing the environmental fate of dsRNA biopesticides regardless of application method.

Kun-Pu Ho

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