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Abstract

The effectiveness of nine species of Chinese insecticidal plants have been tested against 40 species of insects. The toxicity of three species, namely, Millettia pachycarpa Benth., Tripterygium Forrestii Loes. and Rhododendron molle G. Don were studied in detail. The rotenone-bearing plants, Millettia pachycarpa, Pachyrrhizus erosus Urban and Derris Fordii Oliv., are stomach- as well as contact-insecticides. The finely ground seeds of Millettia pachycarpa, when applied as sprays in suspension, were highly effective against aphides, Pentatomids and leaf-beetles. The root bark of the wrinkle-fruited thunder-god-vine, Tripterygium Forrestii, contains an alkaloid and other unknown toxic principles. Data obtained from laboratory and field tests show that the root bark of this plant acts as a powerful repellant and as a stomach-insecticide. When applied as dusts it was found to be effective in protecting vegetables from the attack of leaf-beetles and certain species of lepidopterous larvae. The toxicity of the root bark of the bitter tree, Celastrus angulata Maxim., is similar to that of Tripterygium but it appeared to act also as a contact-poison. The root of Stemona tuberosa Lour. is a contact-poison, and alcohol extracts were effective against the body lice and plant lice. The stems and rhizomes of the black hellebore, Veratrum nigrum L., is a stomach- and a contact-poison : it is used by the farmers for house-fly control. The flower of Rhododendron molle was found to be a contact- and stomach-poison. Its toxicity is very specific, being effective only against certain species of lepidopterous larvae, Pentatomids and leaf-beetles. The rootstock of Steller a chaemejasme L. contains an active antibiotic principle which acts also as a repellant and as a contact-poison against tent caterpillars.

The toxicities of insecticides of vegetable origin were compared with modern synthetic insecticides under laboratory conditions. As a contact spray against the cruciferous leaf-beetle, Colaphellus bowringi Baly, γ-benzene hexachloride appeared to be more toxic than rotenone but rotenone was about four times more toxic than technical DDT and its killing action was much quicker than that of γ-benzene hexachloride and DDT. γ-Benzene hexachloride was 306 times more toxic than the seeds of Millettia pachycarpa. Based on the time-mortality curves, however, a 5% water suspension of the seeds of Millettia pachycarpa compared favourably with a o.1 % γ-benzene hexachloride spray when used against the ten-spotted grape leaf-beetle, Oides decempunclata Billberg. As a stomach-poison against the fifth-instar Small White butterfly, Pieris rapae L., Millettia seeds proved to be more toxic than phenothiazine.

Apart from the standard botanical insecticides such as derris, pyrethrum and nicotine, Millettia pachycarpa, Pachyrrhizus erosus, Tripterygium Forrestii, Celastrus angulata, Rhododendron molle and Veralrum nigrum are widely distributed in China and have great possibilities for extensive cultivation as an easily available source of cheap insecticide. These insecticidal plants have no phytotoxic effect and with the exception of Rhododendron and Veratrum they are not very toxic to human beings. The danger of chronic poisoning from the daily consumption of foodstuff treated with these insecticides seems remote. Recommendations are made for further investigations of these plants with special emphasis on the research of the chemistry of the toxic principles.