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Polyacetylene derivatives (PADs) occurring in Rudbeckia hirta (Asteraceae) were isolated or chemically synthesized and examined for their insecticidal properties against mosquito larvae under different light regimes: dark (D), visible (VIS) and visible+near-ultra-violet (VIS+UV). A straight chain polyine, 1-tridecene-3,5,7,9,11-pentayne, was highly toxic under all light regimes tested although a thiarubrine, 3-(1-propynyl)-6-(3,5-hexadien-1-ynyl)-1,2-dithiacyclohexa-3,5-diene, and a thiophene, 2-(1-propynyl)-5-(3,5-hexadien-1-ynyl)-thiophene, showed a toxicity that significantly varied between the light regimes. The thiarubrine was more toxic against mosquito larvae under D or VIS+UV conditions while the thiophene had a more pronounced toxicity only in presence of VIS+UV irradiation. The distinctive insecticidal properties in darkness of the thiarubrine compare to the thiophene were also confirmed in trials with larvae of a herbivorous insect, Manduca sexta. Such a variability in the light-modulated toxicity to insects for different biosynthetically related PADs emphasizes a diversity in the insecticidal mechanisms of action. |
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