Statistical analysis of decay of organophosphorus and pyrethroid insecticides and carbaryl on paddy rice, maize, sunflowers and field peas

A model is developed to describe the decay of seven insecticides on stored paddy rice, maize, sunflowers and peas, and is shown to be more precise tha n each of six literature models on pesticide decay, It relates residues (R) , as a ratio of applied concentration (R-o), to time after insecticide appl ication (t), modifying the usual first-order kinetics equation In R = In R- o - kt by assuming that the rate constant k has a mixture of values accordi ng to a gamma distribution with mean K. The resulting equation is In R = In R-o - Kc In(1+t/c), where c is a constant determined empirically. For larg e c, this model reduces to the first-order model with K = k. The parameter K is further modelled as linear in temperature for maize and rice. The rate of loss of protectants was found not to be seriously affected by co-applic ation with other insecticides, and this is a useful finding for future stud ies on rates of loss of grain protectants. The apparent rate of loss was al so not seriously affected by the method of analysis, whether by gas chromat ography or high-performance liquid chromatography, and this finding support s previous validation of the methods. (C) 2001 Elsevier Science Ltd. All ri ghts reserved.