The insecticide chlorpyrifos is extensively used in the humid tropics for insect control on crops and soils.
Chlorpyrifos degradation and mineralization was studied under laboratory conditions to characterize the
critical factors controlling the degradation and mineralization in three humid tropical soils from
Malaysia. The degradation was fastest in moist soils (t 1/2 53.3e77.0 days), compared to dry (t 1/2 49.5e120
days) and wet soils (t 1/2 63.0e124 days). Degradation increased markedly with temperature with acti-
vation energies of 29.0e76.5 kJ mol ?1 . Abiotic degradation which is important for chlorpyrifos degra-
dation in sub-soils containing less soil microbial populations resulted in t ½ of 173e257 days. Higher
chlorpyrifos dosages (5-fold) which are often applied in the tropics due to severe insects infestations
caused degradation and mineralization rates to decrease by 2-fold. The mineralization rates were more
sensitive to the chlorpyrifos application rates reflecting that degradation of metabolites is rate limiting
and the toxic effects of some of the metabolites produced. Despite that chlorpyrifos is frequently used
and often in larger amounts on tropical soils compared with temperate soils, higher temperature,
moderate moisture and high activity of soil microorganisms will stimulate degradation and
mineralization.