CONTROLLED ENVIRONMENT STUDY OF THE DEGRADATION OF ENDOSULFAN IN SOILS

Abstract                                                                         Back to Table of contents

The degradation rate of endosulfan applied to a Black Earth soil from South East Queensland, was studied in controlled environment chambers. The degradation rates of a- and b-endosulfan were determined for the soil samples kept at the constant temperature of 30˚C under two water contents of 20% and 30% and submerged under water. The degradation rates of both isomers in all three moisture conditions followed first order kinetics with the rate constants differing significantly between submerged and non-submerged soils. For non-submerged soils the degradation rates of the two endosulfan isomers were lower for the soil with higher water content. The b-endosulfan isomer disappeared from the samples at a faster rate than the a-isomer at both water potentials. However the rates of decline for both isomers were much slower than those suggested in the literature.

Degradation of both endosulfan isomers in the submerged soil took place at a slower rate than in the non-submerged conditions, and in contrast to the results with unsaturated soil, the rate for b-endosulfan was slower than the a-isomer. Adding endosulfan to soil appears to reduce the rate of degradation of other organochlorine pesticides already present in the soil. The half-life of dieldrin was significantly increased in the presence of endosulfan. This effect was much more pronounced at the higher of the two water contents investigated. The applicability of these results to the soils of cotton farms in Queensland and NSW is under investigation.

Conclusion

Degradation of sorbed endosulfan at constant temperature appears to be well described by a first order kinetics equation under every moisture condition studied. In the soil studied, the degradation rates for both endosulfan isomers, however, are much slower than expected from the literature, half-lives being comparable with those of the most persistent members of the organochlorine family of pesticides. The main reason for such longer-than expected half lives could be the low pH value of the soil used in the experiments. It may be noted that in Australia, though not necessarily elsewhere, cotton soils have a high pH.

The degradation rates of both a- and b-endosulfan in the moist (non-submerged) soils were affected by soil moisture content, both rate constants being smaller for 30% water content than 20%. This effect was more pronounced for a-endosulfan than the b-isomer.  Similar effects of water content were observed for aldrin and dieldrin present in the soil. Combining these results with those given by Ghadiri et al. (1995) suggests that the soil water content of around 20% (-220 kPa potential) may be the optimum moisture condition for the degradation of endosulfan and other organochlorine pesticides studied. The degradation of all these pesticides decrease when the soil is wetter or drier than this optimum value.

When submerged under a deep layer of water, the degradation of b-endosulfan in soil is significantly slower than in the non-submerged condition, while the reverse was true for the a-isomer. Such an unexpected behaviour of the endosulfan isomers in the submerged soil necessitates further studies of these chemicals in the bedloads of the rivers and lakes which receive agricultural runoff.

A negative interaction was observed between the newly applied endosulfan and other organochlorine pesticides already in the soil. Applying endosulfan to the soil containing aldrin and dieldrin reduced the rate of degradation of the latter two pesticides. Interactions of this type could become an important factor in predicting the degradation rates and the half-lives of a various sorbed pesticide added to the soil. Interaction of similar nature might have contributed to the persistence of DDT and DDE in the cotton soils of Australia several years after their final application.

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THE BREAKDOWN OF LAMBDA-CYHALOTHRIN IN COTTON GROWING SOIL

Abstract                                                                         Back to Table of contents

The breakdown of lambda-cyhalothrin in cotton growing soil was studied in an indoor incubation experiment. Lambda-cyhalothrin was incubated at 30˚C in the dark in soil with different water contents. The degradation rate was similar at three water contents ranging from 30% to 90%. Degradation in shaken aqueous soil slurry was faster than in unsaturated soil, possibly due to the better redistribution of the lambda-cyhalothrin. The study indicated that degradation of this compound followed first-order kinetics and the half-life of this compound in different soil water contents was calculated to be in the range of 19-37 days. Degradation was apparently biological, as it was prevented by heat sterilisation and sodium azide. The isomerization of lambda-cyhalothrin also occurred in this experiment.

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ASSESSING POPULATION STRUCTURE IN THE COTTON BOLLWORM, HELICOVERPA ARMIGERA (LEPIDOPTERA: NOCTUIDAE), USING VARIATION IN THE A+T-RICH REGION OF MITOCHONDRIAL DNA

Abstract                                                                         Back to Table of contents

Helicoverpa armigera is an introduced pest that attacks a wide variety of agricultural crops in Australia, especially cotton.  Genetic variation in mitochondrial DNA (mtDNA) is being examined amongst individuals from different geographical locations to assess population structure.  A 630 bp mtDNA fragment is amplified using primers in conserved genes that span the hyper variable A+T-rich region. Using one of the same primers, direct sequencing of the amplified product yields 260 bp of sequence from individual moths.  To date twenty-two polymorphic nucleotide positions within this sequence has allowed identification of 33 different haplotypes among 119 moths.  The relatively high degree of haplotype diversity suggests a large effective population size and/or a high mutation rate.  Some haplotypes appear to be relatively common and occur in most regions while other, usually rarer, haplotypes are unique to a particular region.  Chi-squared analysis shows Australia-wide heterogeneity that indicates some degree of population differentiation.  Further sampling is underway to test the consistency of this observation, and to assess the value of this marker for determining moth movement patterns.

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DEVELOPMENT OF LABORATORY AND FIELD ELISA TESTS FOR DETECTION OF ENDOSULFAN IN WATER AND SOIL

Abstract                                                                         Back to Table of contents

Environmental monitoring of endosulfan residues is currently being achieved by gas chromatographic analysis, but there is a need for faster and cheaper analysis that can be conducted in the field as well as in the laboratory. To satisfy this need, two enzyme immunoassay methods, a laboratory assay based on microwell plates and a field test based on the use of small plastic tubes, have been developed for the detection of endosulfan residues in water and soil. These assays have the detection limit of 0.2 ppb of endosulfan, and the detection range of 0.2 ppb – 10 ppb for laboratory assay and 0.2 ppb – 20 ppb for field assay. The field test takes 15 minutes to complete using dropper bottles containing enzyme conjugate and colour development reagents, and can be performed at the site of possible contamination. The laboratory test takes longer to run, but the ability to analyse large numbers of samples simultaneously gives the method high throughput. Water samples can be analysed directly and soil samples are simply extracted with 90% methanol. The tests also detect endosulfan sulfate with similar sensitivity to endosulfan but are four to ten times less sensitive to endosulfan diol, and therefore can potentially determine endosulfan and endosulfan sulfate from the total endosulfan residues present in the environment.

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THE FATE OF ENDOSULFAN SPRAYED ON COTTON

Abstract                                                                         Back to Table of contents

An extensive analysis of the total burden of endosulfan in cotton growing soils has been conducted over the past three years on four cotton fields in the Namoi valley of northern New South Wales. The study indicates that the maximum total burden of endosulfan residues, for analyses conducted at about 6-weekly intervals through the year, is usually not greater than a single application of endosulfan (750 g a.i./ha). This declines to lower levels at other times and particularly when cotton is rotated with wheat. There is no evidence that endosulfan residues are accumulating with current spraying rates; volatilisation is proposed to be a major cause of dissipation of endosulfan. Despite these findings, the concentrations of endosulfan in soil detected, particularly during the spraying season, are significant and could pose an environmental threat if transported off-farm in return waters.

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MIGRATION OF COTTON PESTS: PATTERNS AND IMPLICATIONS FOR MANAGEMENT

Introduction                                                                Back to Table of contents

Cotton has a large pest complex (Matthews, 1989), and many of the major pests are migratory to some degree. Kennedy (1975) distinguished insect migration from dispersal because of the persistent, straightened-out movement, and unresponsiveness to appetitive stimuli, shown by migrants. The term migration has been used in many senses, but in this review it will describe the long-range end of a continuum of movement (Farrow and Daly, 1987). This continuum ranges from trivial, appetitive movement below the biological boundary layer (for example, within or above a cotton crop) to prolonged flight in the geostrophic layer of the atmosphere, where winds can be strong and downwind displacements are of the order of tens to thousands of kilometres…

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CONTROLLED ENVIRONMENT STUDY OF THE DEGRADATION OF ENDOSULFAN IN SOILS

Abstract                                                                         Back to Table of contents

The degradation rate of endosulfan applied to a Black Earth soil from South East Queensland, was studied in controlled environment chambers. The degradation rates of a- and b-endosulfan were determined for the soil samples kept at the constant temperature of 30˚C under two water contents of 20% and 30% and submerged under water. The degradation rates of both isomers in all three moisture conditions followed first order kinetics with the rate constants differing significantly between submerged and non-submerged soils. For non-submerged soils the degradation rates of the two endosulfan isomers were lower for the soil with higher water content. The b-endosulfan isomer disappeared from the samples at a faster rate than the a-isomer at both water potentials. However the rates of decline for both isomers were much slower than those suggested in the literature.

Degradation of both endosulfan isomers in the submerged soil took place at a slower rate than in the non-submerged conditions, and in contrast to the results with unsaturated soil, the rate for b-endosulfan was slower than the a-isomer. Adding endosulfan to soil appears to reduce the rate of degradation of other organochlorine pesticides already present in the soil. The half-life of dieldrin was significantly increased in the presence of endosulfan. This effect was much more pronounced at the higher of the two water contents investigated. The applicability of these results to the soils of cotton farms in Queensland and NSW is under investigation.

Conclusion

Degradation of sorbed endosulfan at constant temperature appears to be well described by a first order kinetics equation under every moisture condition studied. In the soil studied, the degradation rates for both endosulfan isomers, however, are much slower than expected from the literature, half-lives being comparable with those of the most persistent members of the organochlorine family of pesticides. The main reason for such longer-than expected half lives could be the low pH value of the soil used in the experiments. It may be noted that in Australia, though not necessarily elsewhere, cotton soils have a high pH.

The degradation rates of both a- and b-endosulfan in the moist (non-submerged) soils were affected by soil moisture content, both rate constants being smaller for 30% water content than 20%. This effect was more pronounced for a-endosulfan than the b-isomer.  Similar effects of water content were observed for aldrin and dieldrin present in the soil. Combining these results with those given by Ghadiri et al. (1995) suggests that the soil water content of around 20% (-220 kPa potential) may be the optimum moisture condition for the degradation of endosulfan and other organochlorine pesticides studied. The degradation of all these pesticides decrease when the soil is wetter or drier than this optimum value.

When submerged under a deep layer of water, the degradation of b-endosulfan in soil is significantly slower than in the non-submerged condition, while the reverse was true for the a-isomer. Such an unexpected behaviour of the endosulfan isomers in the submerged soil necessitates further studies of these chemicals in the bedloads of the rivers and lakes which receive agricultural runoff.

A negative interaction was observed between the newly applied endosulfan and other organochlorine pesticides already in the soil. Applying endosulfan to the soil containing aldrin and dieldrin reduced the rate of degradation of the latter two pesticides. Interactions of this type could become an important factor in predicting the degradation rates and the half-lives of a various sorbed pesticide added to the soil. Interaction of similar nature might have contributed to the persistence of DDT and DDE in the cotton soils of Australia several years after their final application.

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