Toxicokinetics of the phototoxic allelochemical α-terthienyl in three herbivorous lepidoptera

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Abstract

α-Terthienyl, a phototoxic secondary metabolite of various Asteraceae, shows considerable variation in its insecticidal activity to different herbivorous lepidopterans. The topical LD50 (lethal dose for 50% mortality) to Manduca sexta was 10 μg/g; to Heliothis virescens, 474 μg/g; and to Ostrinia nubilalis, 698 μg/g. To investigate this differential response, the toxicokinetics of α-[3H]terthienyl (3H-α-T prepared by a new exchange process was studied in the three species. Following either oral or topical administration, larvae of M. sexta were unable to excrete this allelochemical rapidly. However, H. virescens and O. nubilalis were able to rapidly clear the chemical from the body via the feces, preventing lethal concentrations from reaching the cuticle where light-mediated toxic interactions may occur. For example, after 48 hr of feeding on 3H-α-T-treated diet, the ratio of radiolabel in the body to that in feces was 58:42 for M. sexta, 32:68 for H. virescens, and 16:84 for O. nubilalis, respectively. Elimination of 3H after topical application is much more rapid in O. nubilalis (t12 = 8.5 hr) as compared with H. virescens (t12 = 22 hr) or M. sexta (t12 = 48 hr). This study suggests that rapid clearance of this phototoxic thiophene is one method by which tolerant insect herbivores deal with this type of allelochemical in host plants. The possible preadaptation of insects feeding on Asteraceae is discussed.

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