*2.7.2 Bacillus thuringiensis*

*B. thuringiensis* Berliner (known as Bt) is an insecticidal bacterium discovered in early 20th century. There are several dozen recognized sub-species of *B. thuringiensis*.

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

The sub-species commonly used as insecticides include *B. thuringiensis* (Bt) sub-species *kurstaki* (Btk), sub-species *israelensis* (Bti) and sub-species *aizawa,* respectively. During sporulation, many Bt strains produce crystal proteins, called *delta endotoxins*, that have insecticidal action. Commercial Bts are powders containing a mixture of dried spores and crystalline δ-endotoxin, though some contain only the toxin component. Both spores and the toxin crystals are produced within the bacterial vegetative cell of the Bt [3, 40, 41]. Currently, there are 6 strains of Bt, which possess specific activity against different insects species e.g. for control of insects such as lepidopterous on crops, e.g. corn, fruits, tobacco and vegetables, as well as mosquito larvae, but Bt has very low toxicity in mammals (LD50 in mammals is >5000 mg/kg). They are used as biopesticides, in form of sprayable products and currently take about 2% of the global pesticide market.

Bt still finds low use in many countries such as Kenya because of high costs, lower efficiency, poor control of sucking and borers insects (e.g. in maize where there is large need), limited persistence and narrow spectrum; but a significant amount of them are being imported and registered by the PCPB [11], indicating their use in agriculture and vector control in Kenya. The advantages of Bt include their environmentally friendly nature compared with other synthetic pesticides as well as their ability to be adopted in new biotechnology. Bt toxins genes have been inserted into chromosomes in some plants and therefore such plants are resistant to attack by insects as they grow. Such crops are called transgenic crops and are available in the market e.g. Bt-corn (a genetically modified crops/organisms (GMO)). Bt corn produces Cry 1Ab toxin, which is used to control European corn borers; such plants which have been genetically engineered to contain δ-endotoxin are called *plant pesticides*. The major concern with Bt (and GMOs in general) is the potential impacts on non-target insects (e.g. beneficial insects such as bees), and interference with natural processes, such as change in biodiversity, which are still not yet fully known.

The δ-endotoxin is a cytolytic pore-forming toxin with insecticidal action, e.g. Cry 1AB toxin, which is a crystal protein with helical structure. When insects ingest it, it gets activated by proteolytic cleavage and once activated it binds to the mid-gut epithelium cells of targeted pests resulting in their rupture and causing cell death. Other organisms (including humans, other animals and non-targeted insects) that lack the appropriate receptors in their gut cannot be affected by the *cry* protein, and therefore are not affected by Bt [41]. Various types of δ-endotoxin can be found in various hosts e.g. cry 1 protein, cry2 protein, cyt protein, vip 1 protein [41]. To be effective, Bt must be eaten by insects during their feeding stage of development, when they are larvae. It is ineffective against adult insects. More than 150 insects, mostly lepidopterous larvae, are known to be susceptible in some way to Bt. Different strains of Bt have specific toxicities to particular types of insects: Bt *aizawai* (Bta) is used against wax moth larvae in honeycombs; Bt *israelensis* (Bti) is effective against mosquitoes, black flies and some midges; Bt *kurstaki* (Btk) controls various types of lepidopterous insects, including the gypsy moth and cabbage looper; and a newer strain, Bt S*an Diego*, is effective against certain beetle species and the boll weevil [41]. Due to its short biological half-life and its specificity, Bt is less likely than chemical pesticides to cause field resistance in target insects. It is moderately persistent in soil, with a half-life of about 4 months in suitable moderate conditions [6, 41]. Bt spores can be released into the soil from decomposing dead insects but can get rapidly inactivated in soils that have a pH below 5.1 [41].

#### *2.7.3 Abamectin*

A microbial pesticide, is a bacterium containing a mixture of endotoxins*, avermectin B1a* (>80% by wt) and *avermectin B1b* (<20%) as active ingredients [3]. The toxins are macrocyclic lactones derived from the *Actinomycete i.*e. *Streptomyces avermitilis* (a soil microorganism). The lactones are natural fermentation products of this bacterium. Abamectin (LD50 300 mg/kg oral rat) is used against insects and mites on vegetable, fruit, ornamentals and fire ants (at home) and is now being used in horticulture in Kenya. The two components, **B1a** and **B1b** have very similar biological and toxicological properties and act as insecticides by affecting the nervous system of and paralyzing insects, and on exposure to high concentrations in humans, symptoms similar to OP poisoning are shown [3, 33]. It is highly toxic to insects and fish, extremely toxic to aquatic invertebrates, but non-toxic to birds, with LD50 in bobwhite quail being >2000 mg/kg. Abamectin is rapidly degraded in soil, and at the soil surface, if subjected to photodegradation, with half-lives ranging from 8 hours to 1 day [3].

#### *2.7.4 Spinosad*

Spinosad is a bacterial fermentation product, a natural substance made by a soil bacterium *Saccharopolyspora spinosa* that is toxic to insects. It is a mixture of two chemicals or metabolites called spinosyn A and spinosyn D. Spinosyn A & D have the most insecticidal activity and are used to control a wide variety of pests, including thrips, armyworms, codling moths, cutworms, leafminers, spider mites, mosquitoes, ants, fruit flies and others. Spinosad has been registered for use in pesticide formulations by the US Environmental Protection Agency (EPA) since 1997 [3] and is already being used in fruit and vegetable farming in Kenya [43]. Currently, they are found in over 80 registered pesticide products, many of them being used on agricultural crops and ornamental plants, where they are important in IPM to avoid food residue problems. Other spinosad products are used in and around buildings, in aquatic settings, and as seed treatments. The products are commonly used as sprays, dust, granules, and pellets. They are neuroactive and have same mode of action such as neonicotinoids but affect different binding sites.

#### *2.7.5 Wolbachia*

Wolbachia are obligate endosymbiotic bacteria that infect many insects, living in all orders of insects and other invertebrates, including some species of mosquitoes [44]. Although it is believed that *Wolbachia* does not naturally infect *Anopheles* mosquitoes, which are the species that spread malaria to humans, their prevalence, though sparsely in *Anopheles arabiensis* and *Anopheles funestus*, which are the two main malaria vectors, were reported recently in Tanzania [45]. Factors influencing *Wolbachia* transferring into new species are still being investigated, but the biocontrol technology has already been tried in Brazil [46]. It has not yet been tried in Kenya. Artificial infection with different *Wolbachia* strains can significantly reduce levels of the human malaria parasite, *Plasmodium falciparum*, in the mosquito, *Anopheles gambiae*. In addition, it was found to reduce levels of *Plasmodium falciparum* that could be transferred to humans and, therefore, suppressed malaria infections [47]. The procedure involves infecting or exposing *A. gambiae* mosquitoes, or any disease vector insect, with different *Wolbachia* strains (e.g. wMelPop, wAu, wInn, wMeICS and wAlbB). After infection, *Wolbachia* strains disseminate widely inside the mosquitoes

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

and infect diverse tissues and organs, affecting the host by manipulating its immune response, inhibiting its replication, reducing the parasite (*Plasmodium falciparum*) levels in the mosquito gut or killing the mosquitoes within a day (as was found in *A. gambiae* exposed to wMelPop strain) after the mosquitoes were blood-fed, including other transfers [47–49]. There is a vast diversity of *Wolbachia* strains available in natural populations of insects related to mosquitoes.

#### **2.8 Fungicides**

Fungicides are used against fungi (.e.g. mildews, rusts, smuts, mushrooms), parasitic plants and many allied forms capable of destroying wood, timber, leather, fabrics, glass, industrial products (e.g. paint and adhesives) and higher plants [50, 51]. Fungal attacks can cause problems of very significant importance not only to materials, the environment and aquatic organisms but also to humans. A good example of devastating fungal effects is normally seen in *Aspergillus ssp* fungi, which attack grains producing aflatoxins, and is a common problem in Kenya. Aflatoxins, which belong to the class of mycotoxins, cause acute lethal toxicity problems and, in the long term, carcinogenicity in humans. Fungicides for plant protection act by direct contact and often injure the host as well as the fungus. They can be described as *protective*, *curative* or *eradicative*; where p*rotective fungicides* are applied before appearance of infestation to prevent it, c*urative fungicides are* applied when infestation has already begun to invade the plant, thus they penetrate the plant cuticle, and destroy young fungal mycelium growing in the epidermis of the plant to prevent further development, and e*radicative fungicides* kill and also prevent sporulation, *i.e.* control fungal development following appearance of symptoms usually after sporulation, killing both new spores and the mycelium by penetrating the cuticle of the plant to the subdermal level [50, 51]. These modes of activity are established during product development and are often indicated on the labels. In agriculture, fungicides are used as foliar, soil or seed dressing, respectively.

#### *2.8.1 Inorganic fungicides*

Inorganic fungicides include elemental sulfur and alkyl/aryl compounds of heavy metals e.g. copper (Cu), mercury (Hg), tin (Sn) (e.g. organo mercury Hg(CH3)2 and organotin tin Sn(CH3)4). Heavy metal fungicides are not popular anymore due to their environmental persistence, ability to biomagnify in food chain and toxicity, and have been banned in the EU, where organotin compounds additives are no longer allowed in paints used in ships, where they were widely used [52, 53]. Methyl mercury fungicides were used in storage of cereal grain storage but were banned following two accidents of severe poisoning reported in Iraq and Minamata in Japan, respectively [53]. These heavy metal fungicides are not used in Kenya. Three inorganic fungicides, *Bordeaux mixture, lime sulfur and copper oxychloride*, respectively, are registered by the PCPB and are used to control molds and mildews in fruit and vegetable farms in Kenya [54].

#### *2.8.2 Organic fungicides*

Organic fungicides are commercially produced by chemical synthesis and are commonly used for control of vegetable blights, especially in potatoes and tomatoes, as wood preservatives. Examples of organic fungicides include *dithiocarbamates*, *chlorinated phenols* (e.g. *pentachlorophenol* 5%), *formalin* (40% formaldehyde) and *coal-tar creosote* (which is used to preserve fencing posts and wooden rail truck ties).

Fungicides are also used widely in large quantities in agriculture and domestic sectors in Kenya due to frequent damp weather conditions which encourage microbial growth. Maize, fruits and vegetable farmers in Kenya, use a number of fungicides, with main active ingredients including *carbendazim, tebuconazole, metalaxyl, mancozeb, azoxystrobin, difenoconazole, fludioxonil, epoxiconazole, trifloxystrobin and mefenoxam* [50, 51, 55, 56].

#### **2.9 Other pesticides available in Kenya for specific uses**

#### *2.9.1 Petroleum products*

The use of emulsions of certain petroleum oils with water for use as fruit tree sprays against insects such as scale insects, red spider mites, aphids and mosquito larvae has been known [57], and kerosene products are still being imported and are registered for by the PCPB [11].

#### *2.9.2 Rodenticides*

Rodenticides are used to control certain pest animals e.g. mice, rats, groundhogs, bats, squirrels and field rodents, which can cause extensive damage to crops property or spread disease [58, 59]. In food storage, cereal farming, food handling and distribution and rodenticides are important e.g. *thallium sulfate, zinc phosphide (Zn*2*PH*3*), strychnine, and red squill*, *fluoroacetate* (CH2FCOONa), *fluoracetamide* (CH2FCONH2) and *ANTU* (Alpha-naphthylthiourea) are used in Kenya. Other rodenticides include *fluoroacetate* (CH2FCOONa), *warfarin*, *fluoracetamide* (CH2FCONH2) and *ANTU* (Alpha-naphthylthiourea). Organic rodenticides, such as *difenacoum, brodifacoum, difethialone, flocoumafen and bromadiolone*, are toxic to mammals and extremely toxic to birds (e.g. *brodifacoum* LD50 values of 0.31, 0.72, and 19 mg kg−1 in ducks, gull and quails, respectively) are not registered in the PCPB database.

#### *2.9.3 Fumigants*

Fumigants act on insects through respiratory system by emitting vapors, but also kill nematodes, weed seeds, fungi, in soil, silos for stored grains, and fruits and vegetables. Often treatment is carried out in enclosures since they are volatile. Fumigants, such as carbon tetrachloride, ethylene dichloride (CH2ClCH2Cl), ethylene dibromide (CH3CH2CH2Br), methyl bromide and carbon disulfide, have been used as liquid fumigants in commodities e.g. grain storage but have been banned due to human toxicity and ozone depletion properties. They have been replaced with others such as CO2, phosphine (PH3; a liquid, storage of grains) and sulfuryl fluoride (SO2F2, termite control), which are not listed by the PCPB. However, *malathion* dust (2%) and *pirimiphos-methyl* (actellic) dust formulations, respectively, are registered and are used in bulk grain storage in silos [60, 61].

#### *2.9.4 Avicides*

Avicides are used against certain birds when they become pests, such as quail birds on rice farms. The red-billed quelea (*Quelea quelea* Linnaeus) is the most important avian pest of small grain crops in Africa, causing damage up to the equivalent of US\$ 88.6 million per annum [62]. It is controlled by *fenitrothion*, *fenthion* (Queletox) and

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

*cyanophos,* which are both highly toxic to non-target and costly [62] and have been used in Kenya by aerial or ground spraying. An avicide can be used as a repellant e.g. Avitrol (4-aminopyridine) or reproductive control e.g. Ornitrol, a derivative of cholesterol, which produces temporary sterility in pigeons but has no effect on mammals. *Fenitrothion* and *fenthion* are listed in the database confirming their use in Kenya.

#### *2.9.5 Nematicides*

Nematicides are used against nematodes, which can infest plant root systems and damage roots and/or encourage other microorganisms e.g. fungus to attack plants. Fumigation with *1,3-dichloropropene* can control these, although conventional pesticides such as some OPs have both insecticidal and nematicidal properties.

#### *2.9.6 Molluscicides*

Molluscicides also known as snail baits, snail pellets or slug pellets, are pesticides against gastropods such as mollusks, which are usually used in agriculture or gardening. These organisms can damage crops or other valued plants by feeding on them or exposing disease pathogens, which they carry on their bodies to humans (e.g. in vegetables, or *bilharzia* in freshwater) [63]. Synthetic *niclosamide* is mostly used although others such as *metaldehyde* have also been used against mollusks [63].

#### **2.10 Metabolism, detoxification and excretion of pesticides**

Insecticides are toxic to target insects as well as humans. However, like other xenobiotics, there are mechanisms of degradation and metabolism in both species, which are mediated by various enzymes and are responsible for reducing their toxicity and excreting them. Apart from killing the target pests such as insects, pesticides are just like any other chemical, which the human is inevitably exposed to through air, food and water, and exposure to them can lead to acute toxicity, long term-diseases or excretion. The ability of pesticides and other xenobiotics to cause long-term diseases or endocrine disruption is statistical and dependent on many factors, but the human body has inherent mechanisms to detoxify them or reduce their toxicity. Studies on pesticide metabolism, detoxification and excretion by insects and mammals, with reference to OCs, OPs and carbamates, which have been most studied, have made us understand how organisms naturally deal with toxic pesticides [3].

#### *2.10.1 Metabolism, detoxification and excretion of pesticides: OCs, OPs, CBs*

In insects and mammals, hydrophobic compounds such as OCs undergo various metabolic reactions, which make them more water-soluble and ready for excretion through urine or other matter. These reactions include hydrolysis, oxidation and reduction, followed by conjugation to more polar metabolites or biomolecules, such as sugars, amino acids, glutathione, phosphates and sulfates, which make them even more hydrophilic and, therefore, excretable. The reactions are mediated by various enzymes. *Cytochrome P450 monoxygenases* (a group of enzymes) located in the microsomes in the mitochondria are responsible for oxidation reactions in mammals, birds, fish, mollusks and insects and can transform various functional groups or moieties of the pesticide molecules, through various chemical changes such as *epoxidation, demethylation, hydroxylation, oxidation and reduction*. All other pesticides including

OPs, carbamates and other xenobiotics also undergo similar biochemical changes that make them more water-soluble for excretion. From the onset, the chemical structures of various pesticides, as shown by examples in **Figure 1a-e**, determine the kind of biochemical reactions, which are expected to occur in the environment and organisms. The metabolic pathways for these biochemical reactions have been elucidated and can be found in Hodgson and Levi [64], Usmani et al. [65], Yu [3] and Jing et al. [66]. The OPs, carbamates, neonicotinoids, pyrethroids, herbicides and fungicides are not as bioaccumulative as the OCs in the organisms and in the environment because of the nature of their chemical structures [3]. They undergo more rapid metabolism and get excreted more [3]. These descriptions of metabolic pathways can be understood by making references to the specific chemical structures (**Figure 1a-e**).
