The persistence of pesticides

Considerable research effort has been paid to the problems of the persistence of chemical means for the plant protection. Many of the studies have been aimed at the persistence of pesticides in the soil. Insecticides based on chlorinated hydrocarbons are particularly persistent, for instance DDT, BHC isomers and so-called polychlorinated cyclodiene compounds, aldrin, dieldrin, andrin, heptachlor. In many countries the use of these pesticides has been either restricted or even prohibited on account of their persistence in the environment. For chemical protection of plants they are gradually being replaced by organophosphate and carbamate substances, which are more toxic, but are less stable in the environment (e.g. parathion, dichlor-vos, carbaryl, propoxur). 

From the standpoint of their persistence in the soil, the pesticides can be categorized as follows  

Pesticides containing metals and chlorinated hydrocarbons such as DDT and aldrin belong to the highly persistent substances persistent ones are urea and triazin pesticides medium persistent ones include amides and derivatives of benzoic acid less persistent pesticides are based on phenoxy- 

---

Baker RD, Applegate HG. 1974. Effect of ultraviolet radiation on the persistence of pesticides. TexJ Sci 25 53-59. 

Adapted from Chapman, R.A. and C.M. Cole. 1982. Observations on the influence of water and soil pH on the persistence of pesticides. Jour. 

Gambrell et al. [223] have discussed the recovery of DDT, Kepone and Permethrin added to soil suspensions incubated under controlled redox potential and pH conditions to determine the effect of time on the levels of the insecticides and their degradation products. Samples were analysed by gas chromatography, pH and redox potential affected the persistence of pesticides to different degrees. The recovery of DDT was affected by redox potential but not by pH. 

Different levels of pesticides can be found at harvest on grapes depending on the chemical characteristic of the active ingredients. Moreover, the persistence of pesticides can depend on photodegradation, thermodegradation, codistillation, and enzymatic degradation. 

The persistence of pesticides in soil is also influenced by their formulations. Granules are generally the most persistent. Wettable powders and dusts are usually less persistent than emulsifiable concentrates (McEwen and Stephenson, 1979). 

Soil moisture has a profound effect on the persistence of pesticides. Increases in soil water content increase the desorption of the pesticide by water molecules that bind competitively with the pesticide, thus displacing it. Table 11.1 shows the effect of soil moisture on the toxicity of insecticides. It can be seen that diazinon is 134 times more toxic in moist soil than in dry soil, indicating that insecticides can be more available to soil insects when the soil has more moisture. 

---