Studies On The Application Of The Extract From Azadirachta Indica A.Juss For The Control Of The Plutella Xylostella L.& Pieris Rapae L. And Its Mechanism

In this paper, the toxicity of the extract from neem tree(Azadirachta indica AJuss) to the diamondback moth (Plutella xylostella L.) and imported cabbage worm (Pieris rapae L) was evaluated in laboratory. The extracting rate of the methanol extract from neem was 13.8%,and the azadirachtin it obtained was 9.275mg/g,analysised by HPLC. In a laboratory study the methanol extract from neem is effective against the third instar larvae of the diamondback moth and cabbage worm. The LC50 (%) of the methanol extract against the larvae of the diamondback moth was 0.1944,15 days after treatment by feeding method. Meanwhile, the LCso(%) of the extract against cabbage worm was 0.1187.The activity of the neem extract on low concentration as an antifeedancy and growth disruption agent was proved. The antifeedant rate of the different concentrations against the third instar larvae of the diamondback moth were 37.5 % 46.7% and 61.9%,respectively,24h after treated by feeding method. Treated with the extract from neem , the duration of the larvae of diamondback moth was extendedobviously. At the same time, the food intake of the larvae was decreased. The results showed that the extract from the neem displayed strong antifeedancy and inhibition of the growth and development against the diamondback moth.The study on the histopathology of the midgut of cabbage worm showed that the active materials from neem can cause some remarkable pathological symptom, 48h after treated by feeding method. The normal structure of the midgut cells was destroyed and finally cause the death of the worm. The results of PAGE indicated that the extract from neem can obviously inhibit the activity of the esterase of the larvae ,48h after treatment. This mean that the extract from neem can not only kill the larvae of the diamondback moth and cabbage worm but also be used as a synergist to achieve better pest-killing effect.