MULTIPLE-MODES OF INSECTICIDAL ACTION OF 3 CLASSES OF POLYACETYLENE DERIVATIVES FROM RUDBECKIA-HIRTA

Polyacetylene derivatives (PADs) occurring in Rudbeckia hirta (Asterac eae) were isolated or chemically synthesized and examined for their in secticidal properties against mosquito larvae under different light re gimes: dark (D), visible (VIS) and visible + near-ultra-violet (VIS UV). A straight chain polyine, 1-tridecene-3,5,7,9,11-pentayne. was hi ghly toxic under all light regimes tested although a thiarubrine, ,5-h exadien-1-ynyl)-1,2-dithiacyclohexa-3,5-diene, and a thiophene, 2-(1-p ropynyl)-5-(3,5-hexadien-1-ynyl)-thiophene, showed a toxicity that sig nificantly varied between the light regimes. The thiarubrine was more toxic against mosquito larvae under D or VIS+UV conditions while the t hiophene had a more pronounced toxicity only in presence of VIS+UV irr adiation. The distinctive insecticidal properties in darkness of the t hiarubrine 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. (C) 1997 Elsevier Science Ltd.