*2.10.1.5 Desulfurization*

Desulfurization is also known as phosphorothioate oxidation, e.g. the OP insecticides with P∙S get oxidatively desulfurized by Cyt P450 monoxygenases to give P∙O analogues, resulting in activation because it gives a metabolite, which binds more strongly to AChE and, therefore, a potent inhibitor of the enzyme AChE. Examples

*Pesticides: Chemistry, Manufacturing, Regulation, Usage and Impacts on Population in Kenya DOI: http://dx.doi.org/10.5772/intechopen.105826*

include parathion and malathion, giving paraoxon and malaoxon, respectively, which are more toxic than the sulfur analogues.

#### *2.10.1.6 Sulfoxidation*

Sulfoxidation of many thioether (R1C∙S∙CR2)-containing insecticides, such as OPs, are oxidized by Cyt P450 monoxygenases to their corresponding sulfoxides (S∙O). Usually, it is an oxidative activation leading to increased anti-AChE activity (i.e. it is an inhibitor), as demonstrated in phorate. Sulfoxides are compounds containing a sulfoxide functional group, with the structure RS(∙O)R′ (R,R′ = alkyl/aryl; S is joined to O (i.e. ∙SO)). Oxidation of certain sulfur and nitrogen-containing insecticides is also performed by another group of microsomal enzymes known as flavin-containingmonoxygenases (FMOs), e.g. sulfoxidation of phorate by FMO was demonstrated in mammalian liver [3]. Like Cyt P450's FMOs also require NADPH and oxygen for their activity, but FMOs are only involved in catalysis of oxygenation reactions.

#### *2.10.1.7 Hydrolysis*

OPs and carbamates and others containing ester linkages are susceptible to hydrolysis. *Esterases* (e.g. carboxylesterases) are *hydrolases* that split ester compounds by addition of water to yield an acid and alcohol (i.e. R′COOR + H2O → R′COOH + ROH; R, R′ = alkyl, phenyl). Carboxyl-esterases have been classified into three categories (A, B and C) on the basis of differential patterns of inhibition by organophosphates, as discussed in detail in other texts [3]. **A-esterases** are typical aromatic esterases, which hydrolyze phenyl acetate and phenyl butyl acetate groups but not aliphatic esters. A-esterase levels of activity in plasma and liver of birds are much lower than those of mammals, the reason why birds are much more susceptible than mammals to OPs such as *pirimiphos-methyl and diazinon*. **B - esterases** are aliphatic and aromatic esterases e.g. *carboxyl esterases and lipases (in lipids)* as well as *acetylcholine esterases(AChE).* B - esterases e.g. *AChE* are *sensitive to OP and carbamate compounds* and hydrolyze both aliphatic and aromatic esters but not choline esters. B esterases are used as non-destructive biomarkers for exposure to anticholinesterase insecticides. Two types of esterases, *carboxylesterases and phosphatases* (or phosphorotriester hydrolases)*,* are involved in metabolism of insecticides, e.g. hydrolysis of *malathion* to yield α and β- monoacids and ethanol by *carboxylesterases*. Carboxylesterase-mediated metabolism is one of the major mechanisms involved in *insecticide resistance*, and *multiple carboxylesterase* genes have been identified which are involved in pyrethroid insecticide resistance in housefly, just like glutathione s-transferase [3]. **C -esterases** preferentially *hydrolyze acetyl esters* and are, therefore, also called *acetyl esterases,* and split *acetylcholine esters* at higher rates than both *aliphatic and aromatic esters*, the latter at lower rate than aliphatic or not at all, typical substrates being 4-nitrophenyl acetate, propyl chloroacetate and fluorescein diacetate. *Phosphatases*, use water to cleave a phosphoric acid monoester into a phosphate ion and alcohol, and detoxify many OP insecticides especially the *phosphate* group, in insects and mammals, e.g. *paraoxon c*an be hydrolyzed to diethyl phosphoric acid and p-nitrophenol in houseflies [3].

#### *2.10.1.8 Reduction*

Insects contain *reductases* that catalyze reduction of xenobiotics. Reduction is less common than oxidation, and there are three types of reduction:

*Nitro reduction* (RNO2 → RNH2), i.e. nitro group reduction to ammine group on the pesticide molecule.

*Azo reduction—*reductive cleavage of azo linkages on a pesticide molecule (R∙N∙N∙R1), resulting in formation of an ammine, e.g. for aromatic ammine (Ar∙N∙N∙Ar′ → ArNH2 + Ar′NH2). The *Azo* group (RN∙NR) *reduction* is similar to nitro *reduction* in many ways, i.e. it, too, is mediated both by *cytochrome P450* and by *NADPH-cytochrome P450 reductase.*

*Aldehyde or ketone reduction—*Reduction of aldehydes and ketones (hydrogenation) forms various metabolites, including primary alcohols (for aldehydes) and secondary alcohols (for ketones) mainly. Cytosolic a*ldehyde dehydrogenases,* as well as the *NADPHdependent aldehyde reductases* widely distributed in insects and animals and their role in detoxification and insecticide resistance, have been discussed by Jing et al. [66]. Nitro group reduction, azo group reduction and aldehyde/ketonic group reduction, have all been found in insects; e.g. reduction of *parathion* to *amino parathion* and *trifluralin* reduction to *amino trifluralin*, in housefly cytosol, NADPH–cytochrome P450 reductase has mediated the resistance of *Aphis* (*Toxoptera*) *citricidus* (Kircaldy) to Abamectin by Jing et al. [66]. OPs and carbamates have various functional groups, which can be attacked, e.g. malathion can be attacked by two types of enzymes the *carboxylesterases* and the *Cyt P450 monoxygenases* for example demethylation (removal of methyl group from CH3O-P moiety) by *Cyt P450 monoxygenases* to give other polar metabolites, and all carbamates have at least three sites that enzymes can attack; i.e. N-alkyl (methyl) group, the *ester linkage* and the *alcohol* or *pheno*l group, respectively, the most important reaction in all carbamates being *hydrolysis,* which occurs in insects and mammals. Other important reactions in carbamate insecticide metabolism would be *hydroxylation* of both ring and N-methyl and *epoxidation* to give *diols* ultimately, followed by conjugation and excretion. The oxidation, reduction, hydrolysis, epoxidation, hydroxylation, dealkylations, desulfuration, and sulfoxidation, which are involved in changing pesticide molecules to become more polar for excretion, as discussed above, are primary reactions called **Phase I** reactions or Phase I metabolism. Products of Phase I metabolism, if not excreted, can then be subjected to **Phase II** reactions or Phase II metabolism. In Phase II reactions, the phase I products are further metabolized by getting them conjugated to various endogenous molecules, e.g. Phase II conjugation with glucose (sugars), amino acids (AAs), glutathione (GSH), phosphate and sulfate. The metabolism of insecticides involving Glutathione (GSH) binding or conjugation, which is mediated by glutathione-s-transferases (GSTs) is well known as a mechanism for detoxification of pesticides in various insects, demonstrated first in housefly [3].

Conjugations are Phase II reactions and are mediated by various enzymes, leading to products which are more polar, less toxic and more readily excreted, therefore, Phase II metabolism leads to detoxification. There are three types of Phase II metabolism, known as **Type I**, **Type II and Type III**, respectively, depending on the types of functional groups of the metabolites which are involved. The chemical functional groups required for Type I (of Phase II reactions) include ∙OH, ∙NH2, ∙COOH, ∙SH conjugation with *glucose*, *sulfates* and *phosphates*. Type II (of Phase II reactions) involves ∙COOH groups binding with *amino acids*, i.e. amino acid conjugations; and Type III of Phase II conjugation involves halogens, alkene, -NO2, epoxides, ethers functional groups and their conjugation with *Glutathione*, i.e. glutathione conjugation. Glucose conjugation is found in insects and plants but is rare in mammals. Mammals use glucuronic acid instead of glucose for excretion of xenobiotics. Glucose conjugation involves binding of Phase I metabolites to α, D-glucose, mediated by glucosyl transferase:

*Pesticides: Chemistry, Manufacturing, Regulation, Usage and Impacts on Population in Kenya DOI: http://dx.doi.org/10.5772/intechopen.105826*

α, D-glucose + ROH → (*glucosyl transferase)* → RO. β-D-glucose (in insects/ plants), where R = alkyl or phenyl group and ROH is a metabolite. Glucuronic acid conjugation, which occurs in mammals, involves Phase I metabolite conjugation to UDP-α-D-glucuronic acid (UDPGA); whereby D-glucose is first activated with *uridyl triphosphate* (UTP), mediated by *uridyl diphosphate glucose (UDPG) pyrophosphorylase*, to form *uridyl diphosphate-α-D-glucose (*UDPG), which is then oxidized to *uridyl diphosphate-α-D-glucuronic acid* (UDPGA). It is the UDPGA that binds to the Phase I metabolite, mediated by *UDP-Glucuronyl transferase*, for excretion, summarized as:


 ( ) ( ) <sup>2</sup> 2 UDPG)+2NAD+H O ® oxidation ® UDP - -D- glucuronic acid UDPGA +2NADH α(3)

$$\begin{array}{l}\text{UDPGA}\text{)- ROH} \otimes \text{(UDP - Chunguronyl transformer)}\\\text{\# RO -}\text{\#-glucronic acid} \star \text{UDP + H\_2O}\end{array} \tag{4}$$

In Phase II metabolism, sulfate conjugation requires ATP and *sulfotransferase* and phosphate conjugation, which occurs in insects but is rare in mammals, requires *phosphotransferase*. In amino acid conjugation, *glycine* is most frequently used, and Glutathione conjugation is mediated by a group of enzymes, the *glutathione-stransferases* (GSTs). GSTs are involved in conjugation of various metabolites, e.g. binding *to epoxide, unsaturated compounds, aldehydes, ketones, lactones, nitriles, nitro compounds, phosphorothioates and phosphates* [3]. Phase I metabolism is responsible for decreasing biological activity and toxicity of toxicants and Phase II metabolism is responsible for detoxification or excretion.

## **3. Pesticide importation, regulation and manufacturing in Kenya**

#### **3.1 Pesticide regulation and importation in Kenya**

#### *3.1.1 The role of PCPB*

The Pest Control Products Board (PCPB) was established in 1984 under Cap 346 Laws of Kenya, to regulate the use of pest control products (PCPs) and safeguard human and environmental health from the undesirable risks associated with PCPs. Pesticide regulation includes policy making and changing (with involvement of the PCPB, government agencies, non-governmental organizations (NGOs) and Parliament), adherence to International Conventions that Kenya is a signatory to (such as UNEP), and prosecution, respectively. The importation, registration, distribution and sale, as well as law enforcement against misuse, are implemented by the PCPB. The role of PCPB, i.e. the Board, in pesticide regulation and its mandate are prescribed in PCPB Act Cap 346 Laws of Kenya of 1984 [67, 68] and include issuance of import/export permits, assessment of safety, efficacy and quality of PCPs, assessment of suitability of premises, advising the Cabinet Secretary/Minister, monitoring and adherence to standards in the entire pesticide industry/trade, supervision of

disposal of obsolete PCPs, keeping records of importation and information on specific uses, creating awareness and investigation and prosecution of contravenors of the Act. Since its establishment, the Board has registered many pest control products for use in public health, livestock and agriculture, and provided important information for labeling, and this is all available to the public on the PCPB website. It is an offense under the Pest Control Products Act to import or sell in Kenya any PCP unless it has been registered by the Board.

In undertaking the regulation of PCPs, the PCPB undertakes evaluation and registration of imported pesticide products and those manufactured in the country for safety, efficacy and quality, before registration. In addition, it regulates trade of pest control products through inspection, licensing and product certification. Any other uses of the products outside those specified in the registration are not authorized unless the product is reviewed and given a label extension [67, 68]. The PCPB registration numbers of products are given and continue to be amended as prescribed in the Pest Control Products Act under the labeling, advertising and packaging Amendment Regulations L.N. 127/2006. To carry out its mandate, the PCPB, is thus, composed of three technical departments namely registration, compliance and enforcement and Analytical Departments, respectively, with clearly defined roles available on their website [67–69].

#### *3.1.2 Laws and regulation of pesticides*

The regulation of pesticides is governed by the Pest Control Products Act Chapter 346 laws of Kenya, which was enacted in 1984 and became operational in 1985 [67]. There are other pieces of legislation in the Pest Control Products Act (Revised Edition 2012) and the Pest Control Products Act (Subsidiary Legislation), available freely on internet. The Pest Control Products Board (PCPB) was established under the Act to oversee its implementation. The Act regulates importation, exportation, manufacture, repackaging, warehousing and distribution. Some important Clauses in the Act include all aspects of manufacturing, storage, distribution, packaging, labeling, sale, importation and exportation, as stated therein, and each piece of legislation is given a number L.N. (L.N. meaning legal notice), e.g. L.N. 45/1984: licensing of premises regulations, L.N. 46 and 109/1984: registration regulations, L.N. 125/2006: the pest control products (importation and exportation) (Amendment) Regulations, etc. The Acts and these pieces of legislation can be retrieved freely from the internet or bought from the Government Printer in Nairobi. The Minister/Cabinet Secretary in charge of the Ministry of Agriculture in consultation with the Board is empowered to make subsidiary legislations (Regulations), which are then printed by the government printer as legal notices (L.N) in the Kenya Gazette.

#### *3.1.3 Pesticide importation*

Kenya is among the largest consumers of pesticides in Africa besides South Africa, Nigeria and Ethiopia [70]. It is an agricultural economy, and therefore farmers use a significant amount of pesticides every year in different parts of the country in order to enhance agricultural productivity. Pesticide imports have increased steadily from about 9.52 thousand metric tons in 2009 to about 14.6 thousand metric tons in 2019. Currently, the PCPB has listed about 1447 formulations and active ingredients registered for use [11]. Most of the products have been insecticides (43%), followed by fungicides (22%) and herbicides (18%), but this changed in 2021, when the volume

#### *Pesticides: Chemistry, Manufacturing, Regulation, Usage and Impacts on Population in Kenya DOI: http://dx.doi.org/10.5772/intechopen.105826*

of total imports further rose to 20.5 thousand metric tons, with a significant increase in fungicides to 6.9 thousand metric tons (34%), herbicides to 7 thousand metric tons (3.14%) and a decline in volume of insecticides to 4.8 thousand metric tons (23.4%). The insecticides include those used in public health and in mosquito nets. The consumption of biopesticides is still very low, with just about 311 metric tons imported in 2020/2021 financial year. In the 2021/2022 financial year, approximately 267 active ingredients of pest control products were imported into the country, either as formulated products or technical grade material (a.i.) for formulation locally, respectively. In some instances, the active ingredients were of mixed form containing more than one active ingredients [11]. On average, 5% of the volume of pesticide imports is technical grade material, therefore, formulation locally is relatively minimal. Approximately 95% of formulated pesticides come mainly from China, India and Germany, and smaller quantities from the USA, the UK, Japan, the Netherlands, and Switzerland, among others [11].

The most recent lists of various registered PCPs have been placed on the PCPB database [11], which are available as an open-source to the public on their website, and provide comprehensive information about all products registered for use in Kenya (www.pcpb.go.ke). The first comprehensive list contains information on names of 1447 various products and active ingredients registered for *use in crops*, their trade names, types of formulations, active ingredients, names of international and local manufacturers, local distributors, specified crops, the maximum residues limits (MRLs), the postharvest intervals (PHI) and the WHO toxicity data and any restrictions (e.g. if the product is restricted), respectively. Fungicides, pyrethroids, neonicotinoids, OPs insecticides and herbicides dominating the list, and much fewer numbers of carbamates (mainly *methomyl and propoxur*), petroleum oil, biopesticides (*e.g. Bt, abamectin and azadirachtin*), biological control products in form of predatory mites (including parasitic wasps) and entomopathogenic fungal spores including *Metarhizium anisopliae,* respectively [11], are given. The list also includes adjuvants and surfactants. Natural pyrethrum extracts are manufactured, formulated and distributed by Pyrethrum Board of Kenya; and other local companies are actively involved in manufacturing and distribution of biological control products including entomopathogenic fungal products for use against thrips and mites in Flowers and French beans. Almost all active ingredients, such as glyphosate, are registered in numerous different formulations, manufactured by different companies (more than 50 international and local companies) and distributed by different local companies; making the list very long.

The second list comprises of 157 pesticidal products registered for use in *public health* [11], consisting mainly of various active ingredients of pyrethroids, OPs (*temephos, pirimiphos methyl, chlorpyrifos and fenthion*), carbamates (*propoxur and carbaryl*), rodenticides *(zinc phosphide, brodifacoum, bromadiolone and flocoumafen*), neem oil, boric acid (specified for cockroach control), plant extracts, and neonicotinoids (*imidacloprid*), respectively, in various formulations (liquids, solids, vaporizing liquids with electrical heaters, baits, sticky tapes); and sold by various companies. The registrations are for specified uses, including *pyriproxyfen* as a mosquito larvicide, *deltamethrin* for indoor residual spraying (IRS), *alpha cypermethrin* for Long Lasting Insecticide-treated Nets (LLIN) and *bifenthrin* as a grain storage dust. The third list contains products, which are registered as technical grade materials for formulation purposes only, where information on technical mixtures (a.i.), mostly >95% pure, are given and the formulations for which they are imported are stated. The active ingredients of pyrethroids, OPs, carbamates, fungicides, rodenticides and neonicotinoids, as

well as adjuvants such as PPO and plant oils, are given. The last two lists (**4th and 5th**) in the database include the 4th one containing information on *temporarily registered products* with their specified uses; and 5th one for *banned pesticides* including *monocrotophos, alachlor and endosulfan*, which the farmers in Kenya are still using illegally [4], as well as *restricted products* such as DDT for malaria vector control only.

#### **3.2 Pesticide manufacturing in Kenya**

The PCPB also regulates the manufacture, distribution and sale of PCPs. According to the information in the database, pesticide manufacturing/formulation and trade, respectively, in Kenya involve several multinational companies (e.g. Bayer, BASF, Monsanto, Syngenta and DuPont) with branches in Kenya, as well as numerous local companies. The world's six largest pesticide manufacturers including Syngenta (and ChemChina), Bayer Crop Science, BASF, Dow Agrosciences, FMC and Adama, control nearly 75% of the global pesticide market, with products ranging from insecticides such as DDT, organophosphates, carbamates, herbicides, fungicides, neonicotinoids, and biopesticides [70]. Weed killers (herbicides) account for about one-third of the global pesticide market.

Manufacturing of pesticides involves formulation, packaging and labeling of the product to make it ready for sale. A pesticide formulation is defined as a combination of active ingredients with compatible inert ingredients of chemicals, which ultimately control a pest. Formulating a pesticide involves processing it to improve its storage, handling, safety, application or effectiveness [71, 72]. A pesticide product, which is ready for use, therefore, contains two parts, the active and inert ingredients. Active ingredients (or technical mixtures >95% purity, usually) are chemicals, which actually control the pest. Inert ingredients are solvents, solids and other adjuvants that help present the active ingredients to the target pest. *Adjuvants* assist in the mixing of some formulations during formulation and dilution just before field application and include surfactants, thickeners, baits, buffers, abrasives and *synergists*, which lack any direct pesticidal activity, but they are added to pesticide formulations to optimize product performance while using the minimum amount of it. The inert ingredients serve to enhance the utility of the product by diluting and reducing costs and field effectiveness [73], because an active ingredient in a fairly pure form is not suitable for field application. The formulation process also improves pesticide safety features and enhances handling qualities.

Examples of specific inert materials include *diatomaceous earth, petrolatum, crop oil, biodiesel, surfactants*, etc. Carrier materials can allow the pesticide to be dispersed effectively, e.g. *a talc in a dust formulation*, the *water for mixing a wettable powder* before a spray application, or the *aerosol that disperses the pesticide* in an air blast application. Inert means the carrier or diluent cannot interfere in the toxicity of the active compound. However, inert does not imply that the chemical, say a surfactant, is nontoxic, as some of the inert diluents or carriers can be toxic e.g. to the plant weeds or other non-target plants, and need to be tested alongside the formulation in a performance field trial [56, 74] as well as in a non-target active ingredient toxicity test. Therefore, pest control products exist in different formulations which are manufactured bearing in mind the nature of a.i.'s (solids and liquids), their solubility, ability to control the pest, storage, ease of application and transportation. A review of materials used as carriers in the pesticide industry can be found in other texts [75–77]. The principles involved in formulation are determined by end-use and behavior of the pesticide, and important factors to consider include, chemical and physical properties

*Pesticides: Chemistry, Manufacturing, Regulation, Usage and Impacts on Population in Kenya DOI: http://dx.doi.org/10.5772/intechopen.105826*

of a.i. (e.g. bp, mp, specific gravity, vapor pressure, water solubility, rate of hydrolysis, toxicity (LD50 or EC50), biodegradability and UV-degradability) and inert materials, type of application equipment, nature of target surfaces, containers, marketing and transportation needs. For inert ingredients, there is need to know compatibility with containers, and therefore their physico-chemical properties, as well as the physical properties of the ultimate mixture. The formulation must then be tested to document various characteristics including homogeneity, particle size, storage stability, retention on target surfaces, wetting properties, penetration and translocation in plants, residual nature on target or in soil, nature of deposit, efficiency and potential hazards to users.

#### *3.2.1 Manufacturing of synthetic pesticides*

In general, there are approximately twelve (12) types of formulations, which are commonly used and these types of formulations which are discussed briefly below, are included in the manufacturing of various PCPs in Kenya and are listed in the PCPB database.

#### *3.2.1.1 Dusts*

Dusts contain 2 ingredients, i.e. an *inert diluent* and a *toxicant*, with a toxicant accounting for 1–10% by weight of the mixture. Inert diluents here must be relatively non-adsorptive material to avoid inactivating the pesticides, e.g. *talc*, *pyrophyllite* or other clay. The diluents are finely ground for ease of application and coverage. The advantages of dust formulations include simplicity to manufacture and application. However, dust is least effective and least economical since it tends to drift during application resulting in poor deposition on target surfaces. To reduce importation costs, dust can be formulated as dust concentrates, containing say 90% of a.i. by weight for further dilution with local diluents in Kenya, or by mixing or blending at the farm before application.

#### *3.2.1.2 Wettable powders (WPs)*

Wettable powders (WPs) are the most widely used in agriculture, and consist of a toxicant + inert diluent + wetting agent. Inert diluents are usually adsorptive clay, e.g. *attapulgite* (Mg, Al, Si clay), and the wetting agents may be a blend of 2 or more *surfactants*, with the *toxicant* in the range of 25–75% (wt/wt) of the mixture; therefore, highly effective due to high concentrations of the a.i.'s. WPs can be prepared by (1) (i) first spraying the *toxican*t (if liquid) onto the clay material at a controlled temperature or, (ii) mixing the clay with a solution containing the *toxicant* (if solid) and (iii) then allowing the solvent to evaporate or, (2) by direct grinding of *crystalline toxicant* mixed with diluents, to get a homogeneous mixture, which can be ground to powder. Packaged WPs are bought and diluted at the farm by mixing a specified quantity (as on the label) with water, before spraying.

#### *3.2.1.3 Emulsifiable concentrates (ECs)*

Emulsifiable concentrates (ECs) are formulations which consist of a toxicant + a solvent (e.g. water or other types such as *petroleum distillates, kerosene (C*9*-C*10 *fraction), Aromax, Solvesso and biodiesel* (e.g. vegetable oil, Neem oil and xylene)) for the toxicant + emulsifier (usually a *surfactant* e.g. calcium alkyl dodecyl benzene sulfonate, or alkyl

phenolic polyexthoxylates), which are also imported. The toxicant content of ECs is expressed as weight/volume and not as wt/wt as in dust or WPs. ECs, which are very common in Kenya, typically contain approximately 25–50% by weight of a.i. On mixing at the farm (usually with water) before spraying, the product gives a stable milky emulsion, which can remain stable for up to 24 hours. ECs are more easily absorbed by the skin and plants than WPs and dust and are, therefore, more hazardous, but more effective than WPs since there is no masking effect of diluents.
