Biological Activity of Azadirachtin., Three Derivatives., and Their Ultraviolet Radiation Degradation Products Against Tobacco Budworm (Lepidoptera: Noctuidae) Larvae
Azadirachtin and three of its derivatives were injected orally into mature (fifth instar) larvae of Heliothis virescens (F.) (Noctuidae). At the lowest dose tested (0.5 µg) azadirachtin, 22,23-dihydroazadirachtin, and 2′,3′,22,23-tetrahydroazadirachtin, but not 3-deacetylazadirachtin, were significantly active. At a dose of 1.0 µg, all four compounds were effective in preventing pupation of the fifth instar and in causing larval mortality. At higher doses (2.0 and 4.0 µg), all four compounds were equally effective, causing 100% larval mortality. The same four compounds were exposed to ultraviolet radiation and analyzed for structural degradation and concomitant loss of biological activity. Of the four compounds tested, 2′,3′,22,23-tetrahydroazadirachtin was the most stable to ultraviolet radiation, remaining structurally unaltered and retaining full biological activity following 200 h of exposure. 2′,3′,22,23-tetrahydroazadirachtin photo degraded by 15% following 400 h of exposure, resulting in a significant decrease in biological activity. Although very little azadirachtin, 3-deacetylazadirachtin, and 22,23-dihydroazadirachtin remained intact following only 90 h of exposure to ultraviolet radiation, all three retained full biological activity. At least 200 h of exposure to ultraviolet radiation was necessary to significantly reduce the biological activity of azadirachtin, and at least 400 h of exposure was required to reduce the biological activities of 3-deacetylazadirachtin and 22,23-dihydroazadirachtin. These results suggest that one or more of the photodegradation products, likely involving the tigloyl moiety, are at least as biologically active as the native molecules.