Gabriel GUILLET, Claude BOURRET-BERNARD, Catherine PODESZFINSKY, Juraj
HARMATHA, Tony DURST, Sherry DELORME, Bernard J. R. PHILOGENE, John T. ARNASON
University of Ottawa, Department of Biology, 30 Marie Curie, Ottawa, Canada, KlN 6N5
Over the last l7 years, our laboratory has focused on the investigation of the insecticidal properties of polyacetylenes and thiophenes which are present in many species of the Asteraceae. Some of these allelochemicals, which are also referred to as phototoxins due to their light-intuced biological activities, were shown to be as toxic to insects as synthetic insecticides. However, plants bearing polyacetylenes and/or thiophenes in their tissues do not appear to iely exclusively on these pOisou1ous compounds to ensure their protection against either herbivores or pathogens. In the present study, we present experimental evidence that monoterpenes, lignans and sesquiterpene lactones, may enhance the insecticidal properties of phototoxins.
Foliar glands in Porophyllum gracile and P. macrocephalum are filled with several thiophenes including a-terthienyl and highly volatile monoterpeu1es that are associated with thc typical smell of these species. The monoterpenes alone were shown to be repellent and deterrent to Melanophus rubrum-femur (Orthoptera Acrididae) adults. To test if these monoterpenes affect the toxicity of a-terthienyl sequestered in the same foliar glands the growth rate of third instar Ostrinia nubilalis (Lepidoptera Pyralidae) larvae was determined by bioassays using each individual cotnpound vs both compounds used simultaneously. The results showed that volatile monoterpenes and a-terthienyl that are co-occuring in the leaves of Porophyllum spp exert additive effects in reducing larval growth rate of O. Nubialis.
The a-terthienyl phototoxicity creates a light-induced oxidative stress resulting in peroxidation of lipids. It is expected that any process that would deplete the antioxidant pool would enhance the susceptibility of an organism to a-teuthienyl. In agreement with this prediction, it was shown that sesquiterpene lactones bearing either an a-methylene y-lactone or a cyclopentenone functions can covalently bind to glutathione leading to a reduction of this antioxidant in Manduca sexta larvae (Lepidoptera: Sphingidae) by up to 50%. Such a decrease in the level of glutathione was also correlated with an increase of lipid peroxidation in larvae treated with a-terthienyl and exposed to near-UV light. Simultaneous application of a-terthienyl and sesquiterpene lactones resulted in a higher larval mortality of M. sexta than treatment with a-terthienyl alone.
Finally, the monolignol investigated, dillapiol, was shown to reduce the ability of insects to excrete and metabolize the a-terthienyl resulting in a reduction of the LC50 to Aedes atropalpus (Diptera Culicidae) and Ostrinia nubalis larvae by up to 25-fold. Physiological investigations have suggested that the aptitude of dillapiol to synergize the insecticidal properties of a-terthienyl is related to two independent mechanisms. The first involves the inhibition of the polysubstrate monooxygenases complex which normally metabolizes potentially toxic allelochemicals to polar metabolites that are more easily excreted by insects. The second mechanism of action may rely on the ability of dillapiol to interfere with a recently discovered insect P-glycoprotein homolog which excretes unmetaholized allelochemicals from the haemocoel into the midgut lumen.