Insecticides of plant origin

Abstract

Limonoids, characteristic natural products of the Meliaceae, Rutaceae, and other Rutales, have marked biological activity against a variety of insects. In particular, the well-known compound azadirachtin is under development as a possible commercial insecticide owing to its potent antifeedant and growth-regulating properties. This compound inhibits the feeding, growth and survival of the variegated cutworm, Peridroma saucia, with an EC50 and LC50 of 0.36 and 2.7 ppm in diet, respectively. In contrast, azadirachtin has no antifeedant activity against nymphs of the migratory grasshopper, Melanoplus sanguinipes, but does inhibit molting in a dose-dependant manner with an oral ED50 of 11.3 µg/g insect fwt. The ED50 via intrahemocoelic injection is 3.2 µg/g, indicating that the gut poses a physical or physiological barrier to azadirachtin bioavailability. The lesser toxicity via the oral route may be largely due to local metabolism, as the oral toxicity can be synergized by piperonyl butoxide, an inhibitor of mixed-function oxidases. The topical ED50 is 4.5 µg/g insect, indicating that azadirachtin is well absorbed through the integument. Azadirachtin toxicity in this insect does not involve inhibition of sterol reductases or sterol transport. Of nine other limonoids tested, only cedrelone, anthothecol, and bussein inhibit larval growth of P. saucia, and only cedrelone inhibits the molting of the milkweed bug, Oncopeltus fasciatus. Foliar extracts of species in the subfamily Melioideae are on average more insecticidal than extracts from the Swietenioideae; some deserve further attention as sources of insecticidal phytochemicals.Limonoids, characteristic natural products of the Meliaceae, Rutaceae, and other Rutales, have marked biological activity against a variety of insects. In particular, the well-known compound azadirachtin is under development as a possible commercial insecticide owing to its potent antifeedant and growth-regulating properties. This compound inhibits the feeding, growth and survival of the variegated cutworm, Peridroma saucia, with an EC50 and LC50 of 0.36 and 2.7 ppm in diet, respectively. In contrast, azadirachtin has no antifeedant activity against nymphs of the migratory grasshopper, Melanoplus sanguinipes, but does inhibit molting in a dose-dependant manner with an oral ED50 of 11.3 µg/g insect fwt. The ED50 via intrahemocoelic injection is 3.2 µg/g, indicating that the gut poses a physical or physiological barrier to azadirachtin bioavailability. The lesser toxicity via the oral route may be largely due to local metabolism, as the oral toxicity can be synergized by piperonyl butoxide, an inhibitor of mixed-function oxidases. The topical ED50 is 4.5 µg/g insect, indicating that azadirachtin is well absorbed through the integument. Azadirachtin toxicity in this insect does not involve inhibition of sterol reductases or sterol transport. Of nine other limonoids tested, only cedrelone, anthothecol, and bussein inhibit larval growth of P. saucia, and only cedrelone inhibits the molting of the milkweed bug, Oncopeltus fasciatus. Foliar extracts of species in the subfamily Melioideae are on average more insecticidal than extracts from the Swietenioideae; some deserve further attention as sources of insecticidal phytochemicals.