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SNELSON The persistence of a chemical can be studied by conventional residues techniques but these are generally laborious, time-consuming and sometimes imprecise, particularly during the earlier stages of the development of a new pesticide. The use of isotope-labelled compounds permits such persistence studies to be conducted quickly, accurately and economically and with every chance of determining the effects of chemical, physical and meteorological variables on the rate of degradation/disappearance.

Where chemicals have a capacity to be translocated in plants, animals or soil, isotope-labelled compounds can be used to determine the rate, degree and site of translocation without the necessity of developing complex, cumbersome and costly analytical techniques.

Isotope techniques cannot be substituted for conventional residues studies because of the need to determine the maximum levels resulting from approved uses under widely different agricultural, climatic and meteorological conditions. However, such studies are more valuable if the analytical techniques are confirmed against samples obtained from crops or commodities treated with isotope-labelled compounds. The extraction, recovery and clean-up stages can be conveniently checked by such comparisons. The nature of the residues, including bound residues, can be best determined using isotope-tracer studies.

Where there are any doubts concerning the accumulation of residues in fruits, seeds and foods of animal origin these can be dispelled by relatively simple studies involving isotope-labelled compounds. It is possible to similtaneously determine the distribution of residues in edible and non-edible portions of the commodity and the effect of time, temperature or other variables on such distribution.

Many pesticides, including insecticides, nematocides and fungicides, as well as herbicides, have to be applied by way of the soil. Others find their way onto the soil adventitiously. It is important to know the nature and fate of the compounds falling on or formed in the soil and the effects of soil type, moisture and tillage on the soil residues. Such studies are best carried out using isotope techniques which lend themselves to precise, small-scale laboratory studies that are quick, convenient and reproducible.

Many agricultural commodities are stored for varying periods after harvest.

Storage may involve controlled atmospheres, refrigeration, moisture control and application of post-harvest treatments with other chemicals. The influence of such storage conditions on the nature, level and distribution of the pesticide residue can be conveniently studied using isotope-tracer studies. Finally, it is important to know the effects of preparation, processing and cooking on the nature and level of the pesticide residue; here too we find many investigators favour the use of labelled compounds in order that results can be obtained from a variety of simulated processes without the complication that would have to be overcome if conventional residues methods had to be developed and validated.

I have been a member of the FAO panel of experts on Pesticide Residues more or less continuously since 1969 and I have been privileged to examine a great variety of studies of the type mentioned above. I have made a selection from studies evaluated by the Joint FAO/WHO Meeting of Experts on Pesticide Residues over the past three or four years to show how isotope techniques have been applied during studies to determine the fate of pesticides in the agricultural environment. These will be presented in alphabetical order by compound.



Metcalf et al. [2] demonstrated that aldicarb was completely oxidized to aldicarb sulfoxide in cotton foliage within four to nine days. Further hydrolysis yielded the sulfoxide oxime, and oxidation of the aldicarb sulfoxide to aldicarb sulfone occurred. Coppedge et al. [3] confirmed these findings, and identified the sulfoxide nitrile as a definite metabolite in cotton. Once formed, aldicarb sulfone is not reduced to provide a secondary source of aldicarb sulfoxide, nor could evidence of oxidative N-demethylation be found. The total radioactivity in the cotton plant is reduced with time through volatilisation and dilution by plant growth.

Field-grown cotton was treated with radioactive aldicarb in furrow at planting and by side-dressed applications [3]. The residues were identified, quantitated, and the rates of decline determined. The metabolic pattern of aldicarb was in agreement with that described by earlier investigaters. A second field study employed petiole injection and obtained similar results [4]. A complete di stribution of radioactivity in the cotton plant was described and the residue in the maturing fruit was characterised. After four weeks, no toxic residues were present in the bolls.

The sulfoxide oxime is further transformed to a mixture of water-soluble products. These consist primarily of sugar conjugates of sulfoxide alcohol, as well as similar quantities of sulfoxide and sulfone acids and sulfone amide [5].

After gaining entrance into the potato plant, aldicarb residues move primarily by xylem transport with higher concentrations appearing in the foliage [6].

Stem injection studies have shown that only limited quantities of aldicarb and its metabolites move downward into the tuber. The toxic carbamate residues appearing in the tuber do not persist, but are actively degraded to non-toxic water-soluble products similar to those formed in the foliage [7, 8].

Systemic movement and concomitant metabolism of aldicarb resulted in a preferential accumulation of the residues in peanut foliage [9]. A small fraction of the observed radioactivity was found in the fruits. Translocation of residues to the forming fruit is facilitated by the polar nature of the metabolic products present in the maturing plants. These water-soluble metabolites were the prédominent component of the terminal residues in the foliage and constituted 90 to 95% of the recovered radioactivity in the kernels.

Aldicarb sulfoxide, aldicarb sulfone and the non-toxic water-soluble metabolites constituted the major portion of the residual l 4 C-materials in sugar beets [10]. Most of the absorbed radioactivity was found in the foliar portion of the plant throughout the growing season. At maturity (140 days after treatment) total H c - r e s i d u e s were 27.2 mg/kg in the foliage and

2.5 mg/kg in the roots. The corresponding values for total toxic residues was 11 mg/kg in the foliage and 0.6 mg/kg in the roots.

The chemical changes that occur in soil are essentially the same as those in plants, animals and insects. Series of parallel experiments have been performed under the same environmental conditions with single factors varied to assess their roles in controlling the persistence of aldicarb in the soil.

These factors include soil types, moisture, pH, and temperature (3, 5, 11].

Under greenhouse and field conditions, aldicarb and its breakdown products leave the soil with unexpected rapidity; a half-life of seven days was 68 SNELSON observed [11]. This emission is definitely linked with the degree of soil moisture and consists primarily of an upward movement. This phenomenon has been studied in an elaborate series of percolation experiments with different soil types in assorted sizes of columns [11] and under field conditions [5].

Only in pure sand is downward movement readily achieved through water action.

The dissipation of aldicarb in the soil is sufficiently rapid and complete to ensure that recommended rates will offer no hazard of contamination of subsequent crops in a treated area [12, 13, 14].

Following the administration of a single dose of aldicarb to a lactating cow, approximately 83* of the dosage was eliminated in the urine within 24 hours.

A minor quantity of residue was eliminated in faeces and a small residue was observed in the milk (less than 3% of the administered dose was observed in milk over a 5-day interval). Increasing the number of days of treatment from one to fourteen did not change the magnitude or the elimination pattern of aldicarb in milk or excretory products. Approximately 1% of the administered dose was secreted in milk with 95% of the administered dose being eliminated by the other routes. Small levels of residues were observed in tissues with the liver showing the major terminal residues. Continuous exposure of cows to aldicarb in the diet did not significantly alter its absorption and excretion patterns [15, 16].

Aldicarb and/or aldicarb sulfone administered as a single oral dose to laying hens was rapidly excreted in the faeces. Minute quantities of terminal residues were observed in eggs on the first day after treatment, but the residue level declined rapidly. Tissue residues were maximal within six hours of treatment after which a rapid decline was observed. Continuous administration of aldicarb for 21 days did not change the pattern of rapid excretion or of terminal residues in eggs or tissues [17].


Cartap is a recently introduced insecticide which is readily hydrolyzed to nereistoxin, a naturally occurring insecticidal substance isolated from the marine segmented worms, Lumbrineris heteropoda. Extensive information is available on the chemistry and synthesis of nereistoxin and its derivatives, of which cartap hydrochloride is the most potent. One of the most important applications of cartap is for the control of stem borers in rice.

In rice plants grown in hydroponic solutions or under simulated field conditions containing 3 5 s - c a r t a p, absorption and distribution was rapid with cartap accumulating in all parts of the plant. Extensive degradation of cartap was found to occur rapidly with incorporation of 3 5 s into natural components (sulphur-containing amino acids). Prior to complete degradation, cartap was observed to be degraded through a series of similar oxidative pathways as in mammals with the principal exception being that the methylation reaction, as generally occurred in mammals, did not appear to occur in plants. The pathway of metabolism in plants thus results in a series of metabolites that substantially differ from that seen in mammals. Nereistoxin appears to be prédominent in plants and undergoes sulphur oxidation to the oxide (sulfinyl), dioxide (sulfonyl) and sulfate, as well as N-demethylation.

Thus, the metabolic pathway in plants appears to result in terminal residues that may be substantially different from those observed in mammals. This difference appears to result from the initial equilibrium established with nereistoxin and dihydronereistoxin both of which follow different pathways in plants and animals to their ultimate degradation [18, 19, 20, 21, 22].

IAEA-SM-263/30 The absorption and distribution of 3 5 s-labelled cartap hydrochloride in rice plants grown under various hydroponic conditions were studied By autoradiography. After 35g w a s a bsorped from the roots, it was distributed throughout the laminae and high concentrations were observed in the leaf sheaths. Accumulation occurred in the leaf apices. Absorption through the leaf sheaths was also observed, and was found to occur faster in young leaves than in older leaves. Distribution occurred through the vascular tissues.

When applied to the leaves, the insecticide diffused from the point of application to the leaf apices and accumulated in the leaf sheaths. 3 5 s was detected in the digestive organs, neural tissues and spiracles of the intoxicated rice stem borers [18].

In 66-day old rice seedlings, the major region of absorption was observed to be the roots. As much as 15% of the applied 35s radioactivity was found to have been taken up by the plant, mainly through the roots. As much as 500 mg/kg accumulated in the rice plant seedlings after six days immersion [19].

When paddy cultivation conditions were simulated, 35 S-labelled cartap hydrochloride was rapidly absorped, a maximum level in most tissues being reached after three days. Metabolism to water-soluable components occurred readily. Accumulation of 35g i n the panicle was also observed [20]. It was subsequently found [21] that under conventional field practice, the amount of 35s-radioactivity was high in the hull and rice bran but low in the milled rice. Most of the metabolites were water-soluble and many were amphoteric.

These metabolites were tentatively identified as methionine sulphoxide, methionine sulphone and S-methyl cysteine sulphoxide [22].


Considering the length of time the cyclodiene compounds have been available, the extent and variety of their use and the degree of concern over their persistence, the number and variety of studies that appear to have been conducted using nuclear techniques is relatively small.

Korte [23], after applying 1 4 C-aldrin, dieldrin, endrin, heptachlor, telodin and dihydroheptachlor to microorganisms, mosquito larvae, mammals and, in the case of aldrin, dieldrin and endrin to higher plants, measured remarkable conversion ratios, except for dieldrin, in microorganisms. Some of the break-down products were isolated and identified and had a lower mammalian toxicity than the parent compounds. Long-term feeding experiments with mammals showed that in all cases a sex-dependent saturation level of storage was reached after some time and that after discontinuing the application the radioactivity was eliminated. The investigators observed that when these compounds were applied to plants, a distribution of the insecticide and its metabolites was observed in all plant parts and in the soil as well. More than 50% of the radioactivity disappeared through evaporation and transpiration.

Brooks [24] provided some illustrations of the variety of biotransformations that occur in a small selection of organochlorine compounds, of the relationship of these conversions to those that occur chemically and of the products that might conceivably be expected from what is known about organochlorine chemistry. The author pointed to the important contribution of anaerobic microbial biotransformations to the final disposal of organochlorine compounds and pointed to the observation of a ring cleavage reaction among such sturdy molecules as the cyclodienes as a basis for expecting a considerable degree of biodégradation of such compounds in nature.

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