Addition of cinnamon oil improves toxicity of rotenone to Spodoptera litura (Lepidoptera: Noctuidae) larvae

Although rotenone is widely used as a pesticide, it has a low level of insecticidal activity on Spodoptera litura (F.) (Lepidoptera: Noctuidae). To gain a better understanding of the high tolerance to rotenone, a synergist (cinnamon oil) was added, and the comparative physiological impacts were assessed. After rotenone treatment, a considerable amount of rotenone was discharged in excreta, but extremely low levels were found in the ventral nerve cord and brain. By contrast, the rotenone + cinnamon oil treatment group showed an increased amount of rotenone in the ventral nerve cord and brain. The co-toxicity coefficient for rotenone + cinnamon was 213, indicating synergism. The midgut cells from insects treated with rotenone alone, and the controls, exhibited no significant differences, whereas those of the rotenone + cinnamon oil group had larger intercellular spaces. These findings suggest that rotenone alone could not effectively penetrate the midgut, perhaps accounting for its low toxicity to S. litura. The rotenone + cinnamon oil mixture apparently affected midgut cell spacing and membrane permeability, thus effectively increasing rotenone toxicity.