title: Dissipation of acephate, chlorpyrifos, cypermethrin and their metabolites in a humid-tropical vegetable production system.
abstract: BACKGROUND
  High amounts of insecticides are often used in intensive tropical vegetable production systems. Their persistence and residues in vegetables and soils need to be studied to ensure food safety and environmental stability. The dissipation of acephate, chlorpyrifos, cypermethrin and their metabolites was studied in green mustard [Brassica juncea (L.) Coss.] and soils. Two treatments, Impact 75 (acephate) and Agent 505 (cypermethrin plus chlorpyrifos), were applied 4 times at weekly intervals.
  
  
  RESULTS
  Dissipation of acephate, chlorpyrifos and cypermethrin in green mustard and topsoils followed first-order kinetics, with half-lives of between 1.1 and 3.1 days in green mustard and between 1.4 and 9.4 days in topsoils (26 degrees C). Higher vapour pressure of insecticides and higher rainfall appeared to stimulate dissipation from the vegetable, with least effect of rainfall on chlorpyrifos. Dissipation rates in the vegetable were faster or similar (cypermethrin) to rates observed for temperate areas. Preharvest intervals of 13, 4 and 3 days were required for acephate, chlorpyrifos, cypermethrin and their metabolites to comply with the tolerance levels. The pesticide dissipation rates in soils varied by less than a factor of 3 between sites. The metabolites methamidophos and TCP derived from acephate and chlorpyrifos amounted to less than 10 and 25% by mass of the parent compounds in soils. Vegetable shading possibly retarded pesticide degradation in soil.
  
  
  CONCLUSIONS
  The dissipation of pesticides and their metabolites in the vegetable was rapid and faster than the dissipation in temperate climates. The degradation rates of pesticides in the soil were equal to or slightly faster than the degradation rates in temperate soils.
authors.affiliation: Agriculture Research Centre, Semongok, Department of Agriculture Sarawak, Borneo Height Road, 93720 Kuching, Sarawak, Malaysia. chailk@sarawaknet.gov.my
authors.forename: Lian-Kuet
authors.initials: LK
authors.lastname: Chai
authors.forename: Norhayati
authors.initials: N
authors.lastname: Mohd-Tahir
authors.forename: Hans Christian
authors.initials: HC
authors.lastname: Bruun Hansen
mesh_terms: D004390:Chlorpyrifos / Q000737:chemistry / Q000378:metabolism; D006813:Humidity; D007700:Kinetics; D009149:Mustard Plant / Q000737:chemistry* / Q000378:metabolism; D009946:Organothiophosphorus Compounds / Q000737:chemistry / Q000378:metabolism; D010573:Pesticide Residues / Q000737:chemistry / Q000378:metabolism*; D063088:Phosphoramides; D011722:Pyrethrins / Q000737:chemistry / Q000378:metabolism; D012987:Soil / Q000032:analysis; D055548:Vapor Pressure
chemical_list: D009946:Organothiophosphorus Compounds; D010573:Pesticide Residues; D063088:Phosphoramides; D011722:Pyrethrins; D012987:Soil; C017160:cypermethrin; C001969:acephate; D004390:Chlorpyrifos