Effects of Ultraviolet-C Exposure on Western Flower Thrips, Frankliniella occidentalis, and Ornamental Plants

Western flower thrips, Frankliniella occidentalis (WFT), is one of the most destructive insect pests of vegetables and ornamental crops globally. Ultraviolet-C (UV-C) exposure has been shown to reduce populations of arthropod pests, including whiteflies and two-spotted spider mites, but has not been fully assessed for WFT. The goal of this study was to determine if UV-C radiance could be a viable strategy for inclusion in integrated pest management (IPM) programs for WFT. The objectives were to (1) assess the relationship among UV-C dose (irradiance × duration) and mortality of WFT adults and second instar larvae, (2) determine the effect of UV-C on WFT fecundity and egg hatch, and (3) assess the effect of the WFT lethal dose of UV-C on three WFT-prone ornamental plants. A UV-C dose is measured in Joules, which equals power (watts) × exposure time. A dose-dependent relationship between UV-C exposure and mortality of WFT larvae and adults was observed. At the doses of 0.98 and 0.68 J/cm² (5 and 4 min exposure, respectively), 50% of the larvae died within 24 and 48 h, respectively. The UV-C dose needed to achieve 50% mortality was higher for adults than larvae, occurring at 5.2 and 4.4 J/cm² (35 min and 25 min exposure, respectively) within 72 and 120 h, respectively. The number of eggs laid by surviving WFT subjected to UV-C treatment was less than by those that were untreated, and the egg-laying period was significantly shorter among those treated with UV-C. When leaves containing WFT eggs were exposed to UV-C doses known to cause 30–40% mortality in adults, 86–98% fewer eggs hatched compared to untreated controls. Ornamental plants exposed to UV-C doses lethal to eggs, second instars, and adult WFT either showed no damage, or when damage occurred, plants recovered within 14–30 days. Additional studies under controlled greenhouse conditions are needed to elucidate the effectiveness of UV-C radiance against WFT over time and its compatibility with biological control and other IPM practices.