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Insecticidal Activity of Selected Thai Plants on Diamondback Moth, Plutella xylostella (L.) (Lepidoptera: Yponomeutidae)
Dissertation Abstract:
The unilateral approach to overcoming diamondback moth (DBM) problems with synthetic insecticides by the farmers has itself generated other undesirable effects, which elicited interest on the possible use of botanical insecticide against DBM.
Ethanol extracts and volatile oils from specific parts oftive different plant species were evaluated for their insecticidal activities on the third instar larvae of DBM, Plutella xylostella (L.). The plants are Zingeber zerumhet (rhizome), Cos/us speciosus (rhizome), Hymenodictyon excelsum (bark), Gynandropsis gynandra (whole plant), and Mentha cardiafolia (whole plant). The high contact toxicity by topical application was observed on both ethanol extract and volatile oil from C. speciosus (LD50 = 15.62 and 13.80 g/mg body weight, respectively). The toxic properties of the ethanol extract from G. gynandra (LC50 = 70.82 mg/ml) and the volatile oil of C. speciosus (LC50 = 62.80 mg/ml) were also shown by leaf residue film method (LRFM). The volatile oil of Z. zerumbet showed an antifeedant activity but the reduction in feeding was 50 percent less than the control. The repellent activity was exhibited by the volatile oils from Z. zerumbet and G. gynandra at 100 mg/ml, which caused about 50 percent of DBM larvae to permanently move out of the treated cabbage leaf.
Insect growth regulatory activity was shown by the volatile oil from C. speciosus. The morphogenetic effective dose, EDw was 7.20:g/mg body weight and the abnormalities observed were deformed pupae, laval-pupae intermediates, and reduced pupal size. The active constituent, crude diosgenin, was isolated from C. speciosus, C. lacerus, and C. globosus. Based on the ED50, crude diosgenin was more active than purified diosgenin.
Diosgenin reduced fecundity and oviposition of the moth. Sterilant effect was markedly exhibited by the cross of treated female x treated male (TF x TM). Diosgenin has low ovicidal activity (LC50 of 34.51 mg/mI) but high oviposition deterrent activity. The DBM parasitoid, Cotes is plutellae, was 14 times less sensitive to diosgenin compared with DBM at 24 hours after exposure to the toxicant.