Chemical control of the diamondback moth (Plutella xylostella L.) with methomyl and mevinphos and their residues in Chinese kale (Brassica alboglabra Bail)
1987
Pinyo Vititayanon
Evaluation of the field control for the effectiveness of the two insecticides against the diamondback moth, methomyl (64 and 128 gm ai/rai) was not significantly different from mevinphos (34.13 and 68.26 gm ai/rai) by number of insect counted and yield. In the laboratory, the determination of toxicity of insecticides to diamondback moth was carried out by leaf dipping method. Mevinphos with the median lethal concentration (LC50) value 21 micro g/ml was obviously more effective than methomyl with LC50 value 20,610 micro g/ml. Residue of both insecticides was determined by gas liquid chromatography (GLC) The residues of both compounds degraded rapidly. The residue was reduced due to late harvest. The range of detectable residue of methomyl was 0.14-23.24 ppm and 8.87 days half-life. The safety and suitability of harvesting time was recommended at 18 days after the last spray. Meanwhile the residue of mevinphos ranged from 0.2x10(exp-3) -101x10(exp-3) ppm and 0.97 day half-life, with appropiate harvesting time of 2 days after the last spray. Temperature, humidity, rainfall and sunshine remarkably affected the decomposition rate of the residues.
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