Diazinon persistence

Diazinon is rapidly biotransformed and excreted in mammals. Estimated half-times of diazinon persistence were 6 to 12 h in rats (Anonymous 1972) and dogs (Iverson et al. 1975). Most of the diazinon metabolites were excreted in the urine as diethyl phosphoric add and diethyl phosphorothioic acid in dogs (Iverson et al. 1975), and as hydroxy diazinon and dehydrodiazinon in sheep (Machin et al. 1974). 

Diazinon persisted for about 15 days in a flooded soil (pH 6,6) that had been treated previously with the insecticide but, in a flooded soil that had never been exposed to diazinon, it persisted for about 60 days. Similarly, water from a diazinon-treated rice field inactivated the insecticide within 5 days after incubation. Microorganisms that developed in response to insecticide application accelerated its hydrolysis and the subsequent mineralization of the hydrolysis product, 2-isopropyl-6-methyl-4-hydroxy pyrimidine, to COg. A Fla-vobacterium sp., isolated from water of a treated rice field, had exceptionally high capability to metabolize diazinon as sole carbon source. This provides unequivocal evidence that microbes are involved in the rapid inactivation of diazinon in rice fields. 

Diazinon persists for six months or longer in nearly neutral unflooded soil (I, 8). Its life is shorter in acid soils (9) because of the rapid hydrolysis under acid conditions (10,13,14). Diazinon applied to a nearly neutral flooded soil persists only for 2 months (2). When applied to an acid soil (Luisiana clay with an initial pH 4.7) immediately after submergence, its persistence is shortened considerably (2). In such an acid soil if the application of diazinon is delayed to two or three weeks after submergence, the insecticide is not degraded rapidly because soil pH rises by about two units after reduction in waterlogged soils. In fact, diazinon controlled rice pests for 15 days after it was applied to Luisiana clay that was kept submerged for four weeks before application (15). 

Diazinon degrades rapidly in plants, with half-time persistence usually less than 14 days. However, persistence increases as temperatures decrease, and is longer in crops with a high oil content (Table 16.1). In water, diazinon breaks down to comparatively nontoxic compounds with little known hazard potential to aquatic species (Meier et al. 1976 Jarvinen and Tanner 1982), although the degradation rate is highly dependent on pH (Table 16.1). The half-time persistence of... 

Table 16.1 Persistence of Diazinon in Plants, Soil, and Water... 

Diazinon (phosphorothioic acid 0,0-diethyl 0-(6-mcthyl-2-(l-mcthylcthyl)-4-pyrimidinyl) ester) is an organophosphorus compound with an anticholinesterase mode of action. It is used extensively to control hies, lice, insect pests of ornamental plants and food crops, as well as nematodes and soil insects in lawns and croplands. Diazinon degrades rapidly in the environment, with half-time persistence usually less than 14 days. But under conditions of low temperature, low moisture, high alkalinity, and lack of suitable microbial degraders, diazinon may remain biologically active in soils for 6 months or longer. 

Kuhr, R.J. and H. Tashiro. 1978. Distribution and persistence of chlorpyrifos and diazinon applied to turf. Bull. Environ. Contam. Toxicol. 20 652-656. 

Wilkinson, F.C. 1980. The uptake of dipping fluid and persistence of diazinon on shower-dipped sheep. Austral. Veterin. Jour. 56 561-562. 

Getzin, L.W. Persistence of diazinon and zinophos in soil effects of antoclaving, temperature, moisture, and acidity, J. Econ. Entomol, 61(6) 1560-1565, 1968. 

Lichtenstein, E.P., Fuhremann, T.W., and Schulz, K.R. Effect of sterilizing agents on persistence of parathion and diazinon in soils and water, J. Agric. Food Chem., 16(5) 870-873, 1968. 

Sears, M.K., Bowhey, C., Braun, H., and Stephenson, G.R. Dislodgable residues and persistence of diazinon, chlorp3uifos and isofenphos following their application to turfgrass. Pestle. Scl, 20(3) 223-231, 1987. 

Sethunathan, N. and MacRae, I.C. Persistence and biodegradation of diazinon in submerged soils, J. Agtlc. Food Chem., 17(2) 221-225, 1969. 

Suett, D.L. Persistence and degradation of chlorfenvinphos, diazinon, fonofos, and phorate in soils and their uptake by carrots. Pestle. Sci., 2(3) 105-112. 1971. 

Diazinon toxicity results predominantly from the inhibition of acetylcholinesterase in the central and peripheral nervous system. The enzyme is responsible for terminating the action of the neurotransmitter, acetylcholine, in the synapse of the pre- and post-synaptic nerve endings or in the neuromuscular junction. However, the action of acetylcholine does not persist long as it is hydrolyzed by the enzyme, acetylcholinesterase, and rapidly removed. As an anticholinesterase organophosphate, diazinon inhibits acetylcholinesterase by reacting with the active site to form a stable phosphorylated complex which is incapable of destroying acetylcholine at the synaptic gutter between the pre- and post-synaptic nerve... 

The influence of soil pH on the persistence of diazinon was studied by Chapman and Cole (1982). 

Since diazinon is not a priority pollutant and has not been considered to pose serious threats from bioconcentration or bioaccumulation in fish and shellfish species, it has attracted far less attention in the United States than persistent organochlorines like DDT or chlordane in routine surface water monitoring networks. Carey and Kutz (1985) reported that the maximum diazinon residue collected in a national surface water monitoring program conducted from 1976 to 1980 was 2.38 ppb and that diazinon was detected in only 1.2% of the samples collected. More recently, Pereira and Hostettler (1993) conducted a study of the Mississippi River and its tributaries during 1991 and 1992. These authors reported that... 

Dr. P. Jeffers at the State University of New York at Cortland is gathering information to determine the persistence of organophosphorus compounds in groundwater and the effects of various soils on the degradation and transport of these compounds. Both neutral and base hydrolysis processes will be evaluated. Transport studies in soil columns will be conducted to determine the mobility of diazinon in soils. 

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. 

Most soils have a pH ranging from 4 to 9. The degradation of pesticides such as organo-phosphates and carbamates is affected by the pH of the soil. Most organophosphates are hydrolyzed under alkaline conditions, but diazinon is unstable in acid soils. Carbamates such as carbofuran are also hydrolyzed under alkaline conditions. The persistence of neonicotinoids is primarily determined by the pH. Imidacloprid and thiamethoxam are hydrolyzed under alkaline conditions, whereas thiacloprid and acetamiprid are less stable... 

Any factors that stimulate the growth of soil microorganisms or that increase the availability of pesticides in soil will enhance the degradation of the chemical. Felsot et al. (1981) found that the persistence of carbofuran was inversely correlated with microbial activity in corn-cropped soils. Tu and Miles (1976) reported that 88% of parathion was lost from soil in 7 months diazinon, 92% lost in 20 weeks paraoxon, 100% hydrolyzed in 12 hr mala-thion, 50-90% lost in 24 hr and carbofuran, 50% lost in 3-50 weeks. 

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