Aseperi, Adeniyi (2020) Occurrence and fate of neonicotinoid insecticides in soils of contrasting characteristics, and in flowers of cultivated and marginal plants from selected sites in Britain. (PhD thesis), Kingston University, .
Abstract
Neonicotinoids, popularly known as “neonics”, belong to a class of insecticides commonly used as plant protection agents against several insect pests. After the commercialisation of the first neonic, imidacloprid, in 1991, the sales of neonicotinoids expanded at a rapid pace; presently, the sales of neonicotinoids account for about 28% of the insecticides’ market value. The efficacy of neonicotinoids as insecticides is attributed to their properties such as high-water solubility, polarity, selective toxicity and being systemic in nature. Neonicotinoids are absorbed by plants and are readily transported via the vascular system due to their systemic nature; this enables their presence in plant’s tissues such as root, stem, leaves, flowers as well as the pollen and nectar. Consequently, the entire parts of the plant are protected from sucking and piercing insect pests. The mode of action of neonicotinoid insecticides has been recognised as acting as agonist at the postsynaptic nicotinic acetylcholine receptor (nAChRs) sites in the nervous system. The question of bee health and the use of neonics has generated a lot of debate and resulted in a two years moratorium (2013 - 2015) for the limited use of three neonicotinoids on selected crops. Recently, the use of these neonicotinoids, namely, imidacloprid, clothianidin and thiamethoxam has been completely banned in Europe after sustained reports from research and regulatory agencies such as the European Food Safety Authority (EFSA) implicated neonicotinoids as the killers of bees. A number of environmental factors and the physicochemical properties of the chemicals influence their sorption and transport in soils. There is usually negative correlation between mobility factor (Mf) and soil organic matter, however, there are exceptions particularly neonicotinoids eliciting different behaviour in different soil types. The need to understand the behaviour of these insecticides in the environment is desirous in predicting their fate, occurrence and remediation after their application. There are reports on the determination of different neonicotinoids in soils in UK. However, there is lack of information on the role played by soils with varied characteristics from different geographical location in UK on their adsorption capacities, sorption kinetics and leaching potential. The primary objective of this work was to assess the sorption and column mobility of neonicotinoid insecticides using field soils (unamended) with varied characteristics in determining their adsorption capacity, partition coefficients, leaching profile, kinetic and isotherm data. To achieve this objective, the work focussed on the investigation of the sorption behaviour of five neonicotinoids, namely dinotefuran, thiamethoxam, thiacloprid, imidacloprid and acetamiprid in five soils with contrasting characteristics. Also, the soil column leaching of the most and least adsorbed neonicotinoids on soils with the least and most organic carbon content was carried out in the laboratory, under control conditions, mimicking field studies. The presence of neonicotinoids in surface water, contaminated through run off, have been reported as a major source of exposure to aquatic ecosystem. Therefore, the UV photolytic degradation of neonicotinoids in aqueous environment was investigated. Also, the amount of neonicotinoid in flowers of selected cropped and marginal plants and wheat grains, then five months after, their respective soil from the same sites was determined to compare the rate of disappearance with those of UV photolytic degradation in water environment. The use of analytical method and instrumentation analysis was crucial to this study to ensure accurate and reliable data are obtained. Therefore, the Quick, Easy, Cheap, Effective, Rugged and Safe (QuEChERS) was modified and adapted in the extraction work along with the use of liquid chromatography- tandem mass spectrometry (LC-MS/MS) for high sensitivity and low detection limits in all the sample matrices studied. The findings from this work suggest that neonicotinoids can accumulate in the soils, particularly after prolonged application. Also, soil types can influence the environmental distribution of neonicotinoids, consequently determining their bioavailability to non-target insects and beneficial soil faunas. The results of the UV photolysis of the neonicotinoids in water showed that neonicotinoids can persist in the environment, especially in temperate regions with lower sun hours. However, the behaviour of neonicotinoids in the aqueous environment can be influenced by their distinct functional groups and their degradation was best modelled with a first-order reaction kinetic with half-life ranging from 11 min to 14 h. The adapted method for the extraction showed good sensitivity and high recovery (72.24 – 102.67 %) with low LODs (0.22 – 1 µg/kg) and LOQs (0.74 – 3.33 µg/kg) when compared to other methods used in neonicotinoid extraction. The results of this study showed that only thiacloprid insecticide was detected in the flower of oilseed rape (OSR) from Essex farm at a 1.42 µg/g concentration and double the maximum residual limit (MRL) established by the European Commission; a potential risk to honeybee during foraging and nectar collection. Also, less than 0.4 % of thiacloprid insecticide was detected in the soil from the same site where OSR was previously sampled and analysed. The result of the rate of photolytic degradation of the insecticides in aqueous environment was ten-fold lower when compared to the rate of disappearance obtained from the field between the mount of neonicotinoid in oilseed rape flowers and those obtained from the soil. The sorption and mobility information obtained from this study will enable risk assessment of these insecticides to be conducted and the outcome will be a contribution towards policy decision making, particularly in their field application. Also, the rate, adsorption and isotherm coefficients obtained in this work are key parameters as inputs in multimedia environmental models in improving our understanding of the behaviour and movement of these pesticides in different environmental compartments.
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