Larvicidal and Structure-Activity Studies of Natural Phenylpropanoids and Their Semisynthetic Derivatives against the Tobacco Armyworm Spodoptera litura (FAB.) (Lepidoptera: Noctuidae)

The larvicidal activity of 18 phenylpropanoids, 1-18, including phenylpropenoate, phenylpropenal, phenylpropene, and their semisynthetic analogues, were evaluated against the tobacco armyworm, Spodoptera litura (FAB.), to identify promising structures with insecticidal activity. Amongst various phenylpropanoids, isosafrole, a phenylpropene, showed the best activity, with an LC50 value of 0.6 mu g/leaf cm(2), followed by its hydrogenated derivative dihydrosafrole (LC50=2.7 mu g/leaf cm(2)). The overall larvicidal activity of various phenylpropene derivatives was observed in the following order: isosafrole (6) > dihydrosafrole (16) > safrole (12) > anethole (4) > methyl eugenol (11) > eugenol (13) >beta-asarone (8)> dihydroasarone (18) > dihydroanethole (15). Dihydrosafrole might be a promising compound, although presenting a lower larvicidal activity than isosafrole, because of its better stability and resistance to oxidative degradation (due to the removal of the extremely reactive olefinic bond) in comparison to isosafrole. Such structure-activity relationship studies promote the identification of lead structures from natural sources for the development of larvicidal products against S. litura and related insect pests.