Preliminary Studies On Insecticidal Activity Of Podophyllotoxin Against Plutella Xylostella(L.)

Podophyllotoxin was one of the main insecticidal ingredients in Sabina vulgaris Ant., an insecticidal plant.The results of our previous studies had proven that podophyllotoxin had excellent insecticidal activity against cabbage worm (Pieris rapae ) and armyworm moth (Mythimna separata) and other pests. In order to further reveal insecticidal activity and mechanism of podophyllotoxin, we preliminary studied insecticidal activity, toxic symptoms and insecticidal mechanism of podophyllotoxin by using vegetable insect diamondback moth Plutella xylostella as tested species. The main results as follows:1. Podophyllotoxin had high antifeedant and toxic activity, as well as obvious growth inhibitor activity. In no-choice test, its AFC50 of 48h was 0.2617 mg·mL-1. Its LC50 of 72h was 1.9061 mg·mL-1. When the tested insects were treated at the dosage of 0.125 mg·mL-1, their related growth rate reduced 41.18%(48h) compared with control, pupation rate was only 43.33%, but pupates could't eclose normally.2. After treated by podophyllotoxin, the toxic symptoms of larva of Plutella xylostella were developed gradually. 4 hours after treatment, tested insect ingested leaf blades in dribs and drabs. Partial tested insect ceased to feed after ingesting a little of leaf, later, the moving capacity of larva of Plutella xylostella decreased, displayed mild palsy, but they can move mildly after touching, can not recover, their body color became yellow gradually, then died slowly. As for other tested insect, they ingested intermittently, displayed abnormal molt, some insects can not exuviate their head shells completely, some insects did not exuviate their tail part, and another can exuviate successfully but pupated abnormally. Furthermore, partial insects displayed prolapse of anus or convulsion gently.3. Podophyllotoxin had different influence on metabolic enzymes. The activity of esterases and carboxyl esterases(CarE) in tested insects was activated, and then inhibited. The activity of acid phosphatase (ACP) was inhibited, and then activated. The alkaline phosphatase(AKP)'s activity was inhibited obviously, and the inhibition would became intensive gradually according to the exposure time. Both glutathione S-transferase(GSTs) and cytochrome P450 were induced, and then inhibited.4. Podophyllotoxin had obviously influence on midgut digest enzymes.The activity of amylase in tested insects was activated. Lipase's activity was activated earlier, and then recovered. Protease was inhibited obviously.5. Histological observations showed that podophyllotoxin had some effects on epithelial cells of midgut. Epithelial cells of the midgut of treated larvae were arranged loosely, cell membrane disintegrated, cytoplasmic density decreased, the microvilli of columnar cells and goblet cells shed, endoplasmic reticulum expanded slightly, and many vacuoles appeared in the cytoplasm. Mitochondrion did not had obvious pathological changes.Based on above results, it can be concluded that one of the main action targets of podophyllotoxin was epithelial cells of the midgut, and the protease of midgut may also be one of its action targets.