35 Degradation of Insecticides

Meenakshi Nandal

epgp books

 

 

 

 

Objectives:

  • We study briefly about the insecticides and their effect on soil, soil microorganisms and on plants We study the chemical reactions involved on biodegradation of Insecticides
  • We study the techniques involved in bioremediation and reduction of insecticides We study the phenomenon involved in biodegradation of insecticides
  • We understand the Microbial, Chemical and Photodegradation processes involved in insecticide degradation with examples

 

36.1 Introduction

 

 

Insecticides are the chemical formulations that kill insect and pests considering fungi, bacteria, worms, insects, and nematodes etc. that damage the agricultural crops, while herbicides as chemicals kill weeds. Broadly we can define Insecticides as chemicals that are used to control or prevent plant diseases and insect pests. Although application of Insecticides is a wide-scale phenomenon, it is also plays an essential role in augmenting crop yields. The excessive use of these chemicals can cause microbial imbalance, environmental pollution and health hazards. Ideally, an insecticide should have the ability to quickly destroy the target pest and should degrade the non-toxic substances efficiently. In the agricultural and public health scenario, the ultimate “sink” is soil. As we know soil is the storehouse of microbes, in terms of both quantity and quality, it receives these chemicals in multiple ways and acts as a scavenger of harmful substances. But ultimately, there efficiency and the competence to handle these chemicals depends on the physical, chemical and biological characteristics of soil. We have to understand the basics of degradation phenomenon and persistence of insecticides in soil and water keeping in mind to get a maximized insecticide efficacy while a minimize detrimental effects on soil, water, humans and on ecosystem. Now we know that the degradation rate and persistence of insecticides is affected by many factors like physical and chemical properties of the insecticides (water solubility, polarity and volatility etc.), properties of the soil and water (pH, temperature, moisture, organic content soil composition), and resistance power of the pests/insect to degradation (biological, chemical, photochemical).

 

Effects of Insecticides on soil: As the insecticide penetrates the soil in significant quantities it directly affects the microbiological aspects of soil, that indirectly influence the plant growth. Here some important effects have been listed that are caused by insecticides are:

 

(1) Alterations in soil microflora ecological balance,

(2) Continued application causing permanent changes in the soil microflora,

(3) Negative effect on soil fertility and crop yield,

(4) Reduction in Nitrogen fixing soil bacteria such as Rhizobium, Azospirillum

(5) Suppression of the activity of nitrifying bacteria, Nitrosomonas and Nitrobacter by fumigants like ethylene bromide, vapam etc.

(6) Modifications in nitrogen balance of the soil,

(7) Adverse effect on ammonification in soil,

(8) Interference with mycorrhizal symbioses in plants

(9) Modifications in the rhizosphere microflora.

 

 

Persistence of Insecticides in soil: The duration of insecticide persistence in soil is an important factor influencing the pest management and environmental pollution. If the chemicals persistence for longer time period in soil, it may cause certain undesirable changes like: a) accumulation of the chemicals in soil increasing the toxicity levels, b) assimilation and accumulation by plants, c) accumulation in the edible portions of edible plant products and root crops, d) erosion and movement with soil particles entering the water streams, causing soil, water and air pollution. The insecticide persistence in soil may vary from week to several years as depending on moisture, structure and properties of the components of insecticide. For example, Aldrin, may persist for 4-15 year.

 

Considering, agricultural point of view, a longer insecticide persistence could result in increased absorption and accumulation of these toxic chemicals by plants and produce hazardous to living beings. Due to this, a chronic problem of increased accumulation of agricultural chemicals, entering the food chain at highly inadmissible levels has been reported in countries like India, Pakistan, and several other developing countries of the world. Intensive use of chemicals like DDT that is used to control pests, fungicides and diseases in crops can persist for longer period and get accumulated in the food chain causing bioaccumulation, food contamination and health hazards. Therefore, DDT has been, banned to be used in agriculture in concern of public health.

 

Biodegradation of Insecticides in Soil: This process conversion of insecticide into non-toxic compounds aided by microorganisms is known as “biodegradation”. The biodegradation of insecticides depends on biological, physical and chemical forces but microbes play a major role in insecticide degradation. The insecticides or chemicals that are resistant to biodegradation and persist in soil for longer time are called “recalcitrant”.

 

The chemical reactions involved on biodegradation of Insecticides are described below:

 

 

a) Detoxification: In this process the insecticide molecule is converted to a non-toxic compound. In this even a change in side chain of the complex molecule may turn the chemical non-toxic so, this process is different from degradation.

 

b) Degradation: The process of transformation of a complex chemical to a simpler product causing mineralization is called degradation. It is many times considered synonymous with mineralization, for e.g. Thirum is transformed to dimethlamine by Pseudomonas.

 

c) Conjugation: It is a complex formation or addition reaction in which the complexity of substrate is increased by organism or the insecticide is combined with cell metabolites e.g. sodium dimethyl dithiocarbamate is degraded by microbial metabolism. The microbes combine the insecticide with an amino acid molecule, present in the cell inactivating the insecticides or the chemical.

 

d) Activation: It is the process of conversion of non-toxic substrate into a toxic molecule that affect the pests, for e.g. Phorate insecticide is microbiologically activated in soil and transformed to give metabolites that are lethal to weeds and insects.

 

e) Spectrum changes of toxicity: Generally, only one type of pests/insects is controlled by insecticides but sometimes they produce metabolized products that are inhibitory to an entirely dissimilar group of organisms, for e.g. PCNB fungicide produces chlorinated benzoic acids as metabolites. The factors like soil pH, moisture, temperature, organic matter content, microbial population and solubility of insecticide influence the degradation process. Also, metabolic activities of microbes involved play a significant role in the insecticides degradation. Majorly, all the organic insecticides degrade within a shorter time period (3-6 months) under Indian tropical conditions.

 

Techniques for Bioremediation: Following strategies have been reported to bioremediate insecticides:

 

a) Intrinsic Bioremediation: It is the natural bioremediation process in which the contaminant is degraded by involving indigenous microorganisms but the degradation rate is very slow.

 

b) Bio-stimulation: This phenomenon practices addition of nitrogen and phosphorus as stimulants for indigenous microorganisms in soil.

 

c) Bioventing: In this process the gases like oxygen and methane are added or forced into soil as stimulants to increase microbial activity which degrade the contaminants in soil.

 

d) Bioaugmentation: It involves the inoculation or introduction of indigenous microorganisms into the contaminated site or soil to facilitate the biodegradation process.

 

e) Composting: In this the contaminated soils are piled and treated with introduction of aerobic thermophilic microorganisms that degrade the contaminants with periodic physical mixing and moistening to promote the microbial activity.

 

f) Phytoremediation: Plants are used as hyperaccumulators which hyperaccumulate heavy metals through different plant parts or indirectly by microorganism’s stimulation in the rhizosphere by plants.

 

g) Bioremediation: It is the process of detoxification and removal of toxic or unwanted chemicals and contaminants from the soil and other environment by biological weapons in form of microorganisms.

 

h) Mineralization: This technique involves the complete conversion of an organic contaminant into its inorganic form by a single species or a group of microorganisms.

 

 

36.2 Insecticide Degradation Mechanisms

 

The Insecticides present in soil degrade in the environment through physical, biological and chemical phenomenon. The biodegradation of Insecticides is influenced by soil properties such as pH, structure, moisture, temperature, organic matter content and the soil microbiota microbes. Microbial degradation and microbial detoxification has been reported as the most promising and cost-effective method for the organic pollutants removal, detoxification of Insecticides and remediation of soil in the environment. The biotic transformation of Insecticides is mediated by the microbiota, while abiotic conversion occurs through chemical and photochemical processes and reactions. The structure and the environment conditions by specific degradation reactions for a particular Insecticide. The redox gradients in soils, sediments and aquifers occasionally are used to deduce the type of transformations that can occur. In a similar way, the photochemical transformations require sunlight that is available only on the surface layers of lakes or rivers, plant or sub soil layers. Also, accumulation of biodegradation transformation intermediates occurs when the enzymes reaction rate decreases, affecting the production of the intermediates.

 

Insecticide degradation can occur by various mechanisms; significant role played by physical, chemical, and biological agents in insecticide, herbicide and fungicide transformations. Many soil applied Insecticides are degraded more rapidly following repeated application at the same site. The Transformation processes are oxidation, reduction, conjugation, hydrolysis, isomerization, hydration and cyclization. The molecules of these are degraded to various resultant products that are generally show lesser bioactivity as compared to molecule of parent Insecticide, but with exceptional cases metabolites showing greater bioactivity have also been observed. There are structural changes that alter the physical, chemical properties and significance of the degradation products. Available

 

strategies include photolysis in water and air, laboratory tests on aqueous hydrolysis, in aerobic and anaerobic conditions testing the biodegradability potential in soils and water-sediment systems and the fate in soil lysimeters. Also, in situ Insecticide transformation reactions including measuring remnant or transformation residue amount and estimating given theoretical transformation potential of environment. The introduction of “environmental activation” concept is done mainly to explain significance of the Insecticide transformation into a degradation product in the environment as a result of its environmental toxicology or chemistry. As the Insecticides begin to synthesize and purify they begin to degrade. The process of formulation can initiate decomposition of active ingredients at a minor rate. The breakdown may also occur due to harsh environmental conditions while the products are stored and shipped. The Insecticide product may also degrade due to chemical interactions among parent molecules, water or other Insecticides when prepared as a tank mix. They are attacked by detoxification enzymes in an organism on target and non-target sites. A part of Insecticide applied always remains in environment as residues in soil, water, and air that are subject to transformation by organism uptake and moving to a different location.

 

 

36.2.1 Microbial degradation of organophosphate Insecticides

 

Methyl parathion (O, O-dimethyl-O- (p-nitro-phenyl phosphorothioate) is one of the most used organophosphorus Insecticides. The hydrolase gene (mpd) and green fluorescent protein gene (e.g. fp) using PET Duet vector in Escherichia coli BL21 (DE3) are efficient in reduction of this Insecticide. The pathway is shown in fig 36.2.1. A high enzymatic degradation activity was shown by recombinant protein MPH on the crops that were applied with the Insecticides. The mineralization process was initiated by hydrolysis forming PNP, dimethyl thiophosphoric acid and PNP degradation proceeding to form hydroquinone. The general path followed for degradation – MP → PNP → hydroquinone → Krebs cycle by the microbial consortium.

36.2.2 Chemical degradation of carbamate Insecticides

 

The carboxyl esterase enzyme aids in hydrolysis of carboxyl esters (CbEs) within the active centre of the protein by reversible acylation of the serine residue (Gupta, 2006). Firstly, the substrate gains access to the active site and then serine causes a nucleophilic attack by acylation on the carboxyl carbamate releasing a stable acylated enzyme and generating a transition state (figure 36.2.2). The factors responsible are complexity of Insecticide and non-extractable residues in soils that are called bound residues and are compounds that persist in form of the parent substance or its metabolite(s) after extraction. (Rosman et al., 2009).

36.2.3 Microbial degradation of carbamate Insecticides

 

The microorganisms metabolize and lead to biodegradation of carbamate Insecticides. The various isolated bacteria; Pseudomonas, Sphingomonas, Flavobacterium, Achromobacterium, Arthrobacter causing the degradation carbofuran have been characterized in an effort to better understand the bacterial role to remove carbofuran from the environment. Carbofuran is one of the Insecticides belonging to the N-methylcarbamate class used extensively in agriculture. It exhibits high mammalian toxicity and has been classified as highly hazardous. Carbofuran was degraded first to carbofuran phenol and the result was degraded to 2-hydroxy-3-(3-methylpropan-2-ol) phenol by Sphingomonas sp. and Arthrobacter sp. (Figure 36.2.3).

 

36.2.4 Microbial degradation of Insecticides of Fenamiphos (FEN)

 

Organophosphate insecticide, Fenamiphos (FEN), ethyl 4-methylthio-m-tolyl isopropyl phosphoramidate, is used for protection of horticultural crops. Pseudomonas putida and Acinetobacter rhizosphaerae are two isolated bacteria that can rapidly degrade the organophosphate fenamiphos. In soil, FEN possesses high nematicidal activity is slowly oxidized to its sulfoxide (FSO) and sulfone (FSO2). It is also and is equally toxic to non-target vertebrates (Figure 36.2.4). Degradation studies of FEN in a range of soils showed half-life values ranging from 10 to 85 days. FEN and its oxidation products FSO and FSO2 depict low to moderate inclination for soil adsorption and its accumulation might result in eventual downward movement towards groundwater. When environmental conditions are favourable FEN could be leached down to groundwater where its persistence could occur. Therefore, certain tools are needed for the decontamination of natural resources by the remnants and oxidation derivatives of chemicals such as FEN (Franzmann et al., 2000).

 

 

36.2.5 Photodegradation and Microbial degradation of Endosulfan (insecticide)

 

Both alpha- and beta-endosulfan can fairly undergo photodegradation but the 2 major disintegrated products, endosulfan sulfate and endosulfan diol, are gullible to photolysis (Fig. 36.2.5) The endosulfan is quite sensitive to alkali, moisture and acids, produces sulfur dioxide and endosulfan alcohol after slow hydrolysis with intermediate product, endosulfan sulfate. The primary product in soil is endosulfan with lower quantity of endosulfan diol and endosulfan lactone being generated. Fungi are the major microbial community causing degradation of endosulfon. In water it is highly soluble when compared with other Insecticides and has low affinity for lipids depicting lower biomagnification and accumulation in food chains. The degradation path is shown in fig 36.5.

36.2.6 Photodegradation of Imidacloprid (Insecticide)

 

Imidacloprid, 1-(6-chloro-3-pyridyl) methyl-2-nitroiminoimidazolidine (Figure 36.6) is a highly efficient insecticide belongs to a novel class of insecticides called “chloro-nicotinoids”. It possesses acetylcholine receptors causing almost complete and irreversible blocking of the postsynaptic nicotinergic. It is highly effective on sucking pests such as aphids, leaf- and planthoppers and other pest species. The process of photo transformation in xenobiotics can occur by two ways

 

 

 

  • (1) direct photo transformation through an excited state
  • (2) indirect photo transformation by photosensitized processes, with presence of sensitizers like TiO2and quenchers.

36.2.7 Microbial degradation of organochloride Insecticides

 

The degradation of insecticides depends on biotic and abiotic factors with different degradation rates. Insecticides like DDT and dieldrin are recalcitrant and when applied to soil persist in the environment for a long duration with accumulation in food chains for decades (Liu et al., 2003). Co-metabolism is the major process causing DDT to Biodegrade with requirement of a substitute carbon source with microorganism growth on substrate and transforming DDT residue without any nutrient or energy derivation. In reducing conditions, is the major mechanism for the microbial transformation is reductive dechlorination in which both the o,p’-DDT and p,p’-DDT isomers of DDT are converted to

 

  • The reaction initiates with the substitution of hydrogen atom on an aliphatic chlorine. The anaerobic pathway for transformation of DDT by bacteria is shown in Figure 36.7. As the degradation proceeds there are continuous reactions involving reductive dechlorination of DDT to yield 2,2-bis(p-chlorophenyl) ethylene (DDNU) with further oxidation to 2,2-bis(p-chlorophenyl) ethanol (DDOH) which oxidizes to yield bis (p chlorophenyl) acetic acid (DDA) which then undergoes decarboxylation to produce bis (p-chlorophenyl) methane(DDM). The DDM produced is metabolized to  4,4’dichlorobenzophenone (DBP) or one of the aromatic rings can undergo cleavage to form p-chlorophenyl acetic acid (PCPA). DDT metabolites can also undergo co-metabolism by bacteria belonging to genera Bacillus, Pseudomonas, Arthrobacter and Micrococcus and convert DDE to 1-chloro-2,2-bis(p-chlorophenyl) ethylene – DDMU. (Lal et al., 1982: Johnsen et al., 1986; Menone et al., 2001; Patnaik, 2003).

Summary:

 

  • Knowledge about the various pathways followed by insecticide while degradation We studied the insecticide effect on soil based on their use and structure.
  • Breakdown of insecticide in the soil can take place by physical, chemical, biological processes and various bioremediation techniques have been reported for reduction of insecticides
  • We studied the mechanism involved in pesticide degradation
  • We studied the agents affecting the pesticide degradation mechanisms. We studied the major degradation strategies exhibited by microbes
  • Various examples of insecticides and their detailed degradation pathway
  • Risk assessment of insecticides requires information on the toxicological and ecotoxicological properties as they directly or indirectly affect the food chain, humans and ecosystem
you can view video on Degradation of Insecticides

References:

 

  • Lal, R and Saxena, D.M. Accumulation, Metabolism, and Effects of Organochlorine Insecticides on Microorganisms, Microbiological reviews, 46(1), 95-127, 1982, ISSN 0146-0749.
  • Liu, Z., Hong, Q., Xu, J.H., Wu, J., Zhang, X.Z., Zhang, X.H., Ma, A.Z., Zhu, J., Li, S.P. Cloning, Analysis and Fusion Expression of Methyl Parathion Hydrolase, Acta Genetica Sinica, 30(11), 1020-1026, 2003, ISSN 1671-4083.
  • Menone, M.L., Bortolus, A., Aizpún de Moreno, J.E., Moreno, V.J., Lanfranchi, A. L., Metcalfe, T.L. and Metcalfe, C.D. Organochlorine Insecticides and PCBs in a Southern Atlantic Coastal Lagoon Watershed, Argentina, Archives of Environmental Contamination and Toxicology, 40(3), 355-362, 2001.