eChapters

The role of toxicology in the evaluation of new agrochemicals

The widespread use of agrochemicals for crop protection may expose humans, animals, and the environment to toxic chemicals. Humans may be exposed during manufacture or application, and to a lesser extent as consumers of food products containing trace amounts of residues.

Strategies of toxicity testing

No-Observable-Adverse- Effect-Level (NOAEL)

A level in which the total diet that causes no adverse effect in treated animals when compared to untreated animals maintained under identical conditions. This NOAEL is expressed on a mg/kg of body weight/day basis. Chronic and sub chronic studies are need to be carried out in different species to determine NOAEL values.

It depends on how much of the substance an organism is exposed to, but all chemicals are toxic to some degree. Toxicology studies try to figure out the exact types and amounts of harmful effects, as well as the lowest doses that won’t have any negative effects (called the “No Observed Adverse Effect Level” or NOAEL). Most data come from studies with lab animals and cells that have been grown in a lab. Using these numbers and extrapolation or uncertainty factors, it is possible to figure out an Acceptable Daily Intake (ADI). This is the maximum amount of a substance that a person can take in every day without putting themselves at a very high risk of getting sick. These numbers will be compared to how people might be exposed to a compound in the environment, including through food. This will show if the use of an agrochemical is potentially allowed.

Farmers still rely heavily on insecticides because of their efficiency, portability, and speedy returns on investment. Although synthetic organic insecticides are highly efficient, their widespread usage has led to toxicity to natural enemies, toxic residues in plants and the environment, insect resistance, and a resurgence in insect populations. Ripper (1956) first time recognized the problem of resurgence in plant protection.

Metcalf (1986) distinguished between two types of resurgence: primary insect resurgence, in which populations of target insects that had been suppressed by insecticide application quickly recover to excessive levels (Fig. 1.A), and secondary insect outbreak, in which non-target species develop into serious insect s after insecticide application (Fig. 1.B).

Primary insect resurgence

Primary insect resurgence occurs when the target insect population increases to at least as high (Hajek, 2004) or higher (Hardin et al., 1995) than in an untreated control or higher than before the treatment (Pedigo and Rice, 2006).

Secondary insect resurgence/ secondary insect outbreak (Type II resurgence)

The phenomenon known as replacement of a primary insect with a secondary insect, or secondary insect outbreak, occurs when the population of a non-target insect, which is harmful to the crop, experiences a surge following the application of a insecticide aimed at controlling the population of the primary insect. A second insect outbreak is also called resurgence. A secondary insect outbreak is the rise of a non-target species after an insecticide has been used.

Insecticide resistance is a growing concern for those who need insecticides for medical, veterinary, and agricultural control of insect pests. In response to the introduction of DDT resistance in 1947, the incidence of resistance has increased annually by an alarming amount. Resistance is “the inherited ability of a strain of some organism to survive doses of a toxicant that would kill the majority of individuals in a normal population of the same species” (WHO, 1957).

As a result of continued insecticide application, the proportion of resistant insects increases compared to that of susceptible and the population becomes increasingly difficult to control

Above photograph theoretical example illustrating the increase of insecticide resistance levels in a pest population. Some individuals (black) with genetic traits allowing them to survive insecticide applications can reproduce; if the selection pressure is frequent, they easily become the preponderant part of the population (Fig. 1).

An adaptation to a pesticide leads to reduced susceptibility to it among the pest population targeted by that pesticide. In nature, pests develop resistance to chemicals through natural selection. In the most resistant organisms, their genetic traits are passed on to their descendants. Since 1945, at least 500 species of pests have developed resistance to pesticides (Anonymous, 2007), but other sources estimate as many as 1000 species (Miller, 2004).

Synergists are chemicals that significantly increase the toxicity of an insecticide while being nearly non-toxic on their own. Thus, non-toxic chemicals that contribute to the increased toxicity of insecticides are referred to as synergists. Many chemical substances have the potential to synergize with poisons. Some enzyme inhibitors work well synergistically by inhibiting enzymes that metabolize pesticides, particularly in insecticide-resistant strains. When mixed with other insecticides, some insecticides increase the toxicity of the latter at non-toxic dosages. When a mixture is more harmful than expected based on the sum of its separate efficacies when administered alone, the interaction between substances is known as synergy. Antagonistic interactions among components reduce a mixture’s efficacy.

Synergism can also develop between two or more poisons. Several statistical methods have been proposed to quantify the synergy of both non-toxic and hazardous substances. Toxin interactions are assessed using bioassays, which involve mixing toxicants (two or more) in a specific ratio. The combination is then used to prepare serial dilutions. Alternatively, the poisons can be supplied sequentially in the same amounts as in the mixture. Bioassays are also used to determine the dose-mortality response of each toxin on the same insect strain. The data are subjected to probit analysis, and the toxin interactions can be evaluated using the statistical methods listed below.

Insecticide mode of action is divided in to four major chapters 1. Nerve & Muscle 2. Growth & development 3. Respiration & Energy production 4. Metabolic process 5. Unknown or Non-Specific (multisite inhibitors) (Source: IRAC). To understand the mode of action of insecticides, it is necessary to understand the function of nervous system and how it works in insect. The nervous system functions transmits information throughout the body. This system has three components: 1) Peripheral nervous system (PNS) to receive and transmit incoming signals (taste, smell, sight, sound, and touch) and to transmit outgoing signals to the muscles and other organs, effectively communicating them how to respond, and 2) Central nervous system (CNS) to interpret the signals and coordinate the body’s responses and movements. 3) Visceral or sympathetic nervous system (VNS) directly connected with the brain that supplies nerves to the anterior part of the alimentary canal (foregut and midgut), heart, and spiracles. Some additional nerves arise from posterior compound ganglion of ventral nerve cord (VNC) which supply nerves to the posterior part of gut and reproductive system.

Pesticide use in agriculture has evolved at several stages in the last few decades. Pesticides, as defined by the FAO, are any substance or mixture of substances intended to prevent, destroy, or control any pest, including vectors of human or animal disease, unwanted species of plants or animals causing harm during or otherwise interfering with the production, processing, storage, transport, or marketing of food, agricultural commodities, wood and wood products, or animal feedstuffs, or substances that may be administered to animals. A plant growth regulator, defoliant, desiccant, or agent used to thin or prevent premature fruit fall, as well as substances applied to crops before or after harvest to prevent them from deteriorating during storage and transportation, are included in this category.

Today’s agrochemistry has a massive challenge: ensuring the availability of high quality active ingredients to enable long-term control of pest species, weeds, and agricultural diseases. The differential uptake and transfer of the active ingredient by target and non-target organisms can contribute to diverse biological activity.

The way a chemical moves, persists, and behaves in its surroundings is known as its behaviour. A chemical’s behaviour will dictate both its residual properties and field efficacy. The behaviour of insecticides is determined by their chemical and physical characteristics as well as the interaction of environmental elements. Our capacity to comprehend and forecast how insecticides will behave in the environment will determine our ability to keep utilizing insecticides.

Important features of systemic insecticides

1. They are not susceptible to wash off by rain

2. By virtue of their movement inside plants, they can afford to protection to regions where they have not been sprayed

3. They are often been transported towards region of growth

4. Since they have very weak contact action, safe to beneficial organism.

Insecticide classification majorly divided into three categories. A. based on chemical group, B. mode of entry in the insect, C. based on toxicity.

A. Classification of Insecticides based on chemical group

Historians have traced the use of pesticides to the time of Homer around 1000 B.C. earliest records of insecticide use pertain to the burning of “brimstone” (Sulfur) as a fumigant. Pliny the Elder (A.D. 23-79) recorded most of the earlier insecticide uses in his Natural History and included among these was the use of gall from a green lizard to protect apples from worms and rot. Later, a variety of materials used viz., extracts of pepper and tobacco, soapy water, whitewash, vinegar, turpentine, fish oil, brine, lye and many others. At the beginning of World War II (1940), insecticide selection was limited to several inorganic compounds, arsenicals, petroleum oils, nicotine, pyrethrum, rotenone, sulfur, hydrogen cyanide gas, and cryolite. It was during World War II that opened the Chemical Era with the introduction of a totally new concept of insect control.

Introduction

Analysis of insecticide formulations are critical for the availability of quality insecticides in the market for ultimate benefit of farmers and ecosystem health. This chapter highlights the significance of comprehensive insecticide formulation analysis protocols, encompassing methods for determining active ingredient and assessing physical and chemical parameters. Employing advanced analytical techniques such as spectroscopy and chromatography for insecticide formulation analysis will eventually maintain the quality of insecticides for the sustainable management of pests.

In India, over 339 pesticides are registered that includes insecticides, fungicides, herbicides (DPPQS, 2024). The current market of non-genuine insecticide is INR 3,200 crores (USD 525 Million) which constitutes 25 per cent by value and 30 per cent by volume of the total domestic market of agrochemicals in India as per Industry reports, primary interviews, news articles and Tata Strategic analysis (FICCI, 2015). Monitoring registration and regulation of insecticides is governed by the Insecticide Management Bill 2020, which replaced the Insecticide Act 1968 (DPPQS, 2023). The Insecticide Act of 1968 established the Central Insecticide Laboratory, Faridabad, under section 16 with two regional insecticide testing laboratories at Chandigarh and Kanpur (DPPQS, 2023). Current Quality Control of Insecticides in States/UTs during the last five years shows that 68078 insecticide samples were analysed (DPPQS, 2023). In the beginning, these laboratories involved in analysing insecticide samples drawn by any officer or the body authorized by the Central or State Governments and submit certificates of analysis to the concerned authority. Regional Pesticide Testing Laboratories (RPTLs) had a huge target of analysis of 1550 samples per annum that necessitated the establishment of 71 State Pesticide Testing Laboratories (SPTL) spread across India of which 14 are NABL accredited (DPPQS, 2023).

Formulation is a word that can be used in a lot of different ways, but at its core, it means putting things together in the right way, usually by following a formula. Insecticides come in different forms, which are called formulations. A formulation is just the way a product is used. The chemical or biological makeup of a pesticide and its active ingredient (also called the active substance) determine how it works on living things. Most of the time, the active ingredient is mixed with other things, and the product as it is sold will need to be weakened even more before it can be used. Formulation makes a drug easier to handle, store, and use. It can also have a big effect on how well it works and how safe it is.

Insecticides come in many different forms because the active ingredients vary in how easily they dissolve, how well they kill pests, and how easy they are to handle and move. The pesticide is made up of both active and non-active (used to be called “inert”) chemicals. The manufacturer will need to know the identity of the pest, as well as details about its feeding behavior, reproduction, and life cycle, in order to create an effective formulation. Manufacturers also take into account factors including surface and equipment type, runoff and drift rates, pest habits, and safety when formulating insecticides. Inactive materials (adjuvants/ additives) are often added to insecticides to make them stick to the application surface or spread out over leaves. There are solvents (liquids) that dissolve the active ingredient, carriers (liquids or solids) that aid in the delivery of the active ingredient, etc.. Boosting the efficacy of active ingredients requires additives, such as spreaders, stickers, wetting agents, compatibility agents, foaming agents, etc.

A bioassay, also known as a biological assay, is a scientific experiment used to quantify the impact of a substance on a living organism. According to Finney (1952), bioassay is defined as measurement of potency of any stimulus (physical, chemical, biological, physiological) by means of reactions which it produces in living matter. It is an essential procedure in insecticide development and the assessment of environmental pollutants, as well as the evaluation of the effects of interventions viz. pesticides and herbicides on the environment. In the field of biostatistics, a bioassay refers to a set of techniques used to determine the quantity or intensity of an agent or stimulus based on the response of the subject. Typically, many methods are employed to assess the strength or effectiveness of an insecticide by examining its impact on living organisms.

An insecticide is a pesticide used to kill or eliminate insect pests in agriculture, households, and industries. Judicious use of insecticides may be a factor in the increase of agricultural productivity, but by their nature of having high toxicity to non target organisms and capability to develop resistance through widespread use, most insecticides have high potential to significantly affect and alter ecosystems. Many are toxic to humans and animals (both domestic and wildlife), and can accumulate as concentrates in the food chain and water resources, giving rise to serious environmental contamination and pollution.

History of Chemical Control, Insecticide Toxicology and Properties

History of pest control probably started when the first person swatted a mosquito or pulled off a tick, the battle for the control of our planet didn’t start until organized agriculture, when pests attacked the plants, and we grew to eat and threatened our very survival.

A pesticide is any substance or mixture of substances used to stop, kill, avoid, or lessen the effects of a pest (insects, mites, nematodes, weeds, rats, etc.). This includes insecticide, herbicide, fungicide, and many other substances used to control pests. Pesticide was defined in different ways at different times and in different places. But pesticide is still generally the same: it is a (mixed) substance that is poisonous and effective on target organisms but safe for non-target organisms and environments.

Chemical control, commonly known as pest control, is the use of chemicals to reduce insect populations. Herbicides, fungicides, insecticides, rodenticides, acaricides, nematodes, and fungicides are all examples of pesticides. Insecticides are a class of chemicals used to combat insect pests. A substance or combination of compounds used to kill, repel, or otherwise prevent insects is called an insecticide. When it comes to controlling pests, insecticides are most powerful tools available for management. They have a wide range of positive effects, including high efficiency, speed of healing, general applicability, adaptability to different settings, adaptability to shifting agronomic and ecological conditions, and low cost. When insect pest numbers approach or surpass the economic threshold, insecticides are the sole viable instrument for emergency action in pest management. The usage of pesticides, a widely used method, can be central to such a system. The use of chemical pesticides will remain pivotal in these initiatives. In many cases, chemical control of pests is the best option available. Some people’s view of pesticide use for pest management as an ecological sin is incorrect. Chemical pesticides are a reliable and useful tool for the biologist, so long as they are used in accordance with good ecological principles.

In the earlier chapter the major soil vulnerability namely, water and wind erosion, depletion of soil organic matter, loss of plant nutrients, salinity, alkalinity, acidity, pollution and contamination, compaction, sealing, crusting, water-logging, aridity, landslide and abandoned mines were described. In this chapter two more parts, Importance of remote sensing (RS) and geographical information system (GIS) in soils and Watershed management are also included after seeing their importance in agriculture and soils. Restoration means that by various actions or processes to return something to its original owner, site, place, or condition. In this context our aim is to restore our mother soil from various vulnerabilities by different actions and processes to its natural and original condition up to the maximum possible extent. Management is defined as the scientific methods to attain targeted production through planning, organising, training and fixing targets for employees. Therefore, after the restoration, soils should be managed according to scientific method of action by scientifically selected and trained staff to perform the assigned responsibilities, effectively, efficiently and rapidly within stipulated period. On these guidelines restoration and management of vulnerabled soils is described here,

Vulnerability of Soils

Soil vulnerability, Soil degradation, soil vulnerability index

Water and wind erosion, depletion of soil organic matter, loss of plant nutrients, salinity, alkalinity, acidity, pollution and contamination, compaction, sealing, crusting, water-logging, aridity, lowering of water table, over grazing, deforestation, landslide, loss and change in soil biodiversity, Abandoned mines etc.

Soil vulnerability, Soil Degradation, Soil Vulnerability Index

The terms vulnerable and vulnerability are now bites of every one and used for everything. Vulnerable means to the possibility of attack or harm by any mean. Human beings are vulnerable to Covid-19, computers are vulnerable to virus, softwares are vulnerable to hackers and attackers to control system, environment is vulnerable to pollution. Vulnerabilities refer to the lack of ability to resist the effects of the aggressive environment. Soils are vulnerable to numerous threats like water and wind erosion, depletion of soil organic matter, loss of plant nutrients, salinity, alkalinity, acidity, pollution and contamination, compaction, sealing, crusting, water-logging, aridity, lowering of water table, over grazing, deforestation, landslide, loss and change in soil biodiversity etc. However, this book will describe some of the most pertinent vulnerabilities for soils.

Physical Properties of Soil

Physical properties of soil have many important roles in the crop production. The basic requirements for germination, seedling emergence and root development are adequate moisture and aeration, optimum temperature and absence of mechanical hindrance. After germination the further development of plant is also governed by physical conditions of soil. Soil texture, soil structure, soil density, soil consistence, soil water, soil air, soil temperature and soil colour are main physical properties of soils also affecting mechanical behaviour of soils. These are described here.

1. Soil Texture

The weathering divides larger particles of rocks and minerals in to the smaller ones. This process goes continuously. Thus, particles of different sizes ranging from large fragments of rocks to the very fine clay minerals are found in soil. However, for texture particles less than 2mm diameter is considered. The relative amount of these particles indicates the soil type also. Such as sandy soils contain more amounts of coarse particles i.e., sand and clayey soils are rich in fine particles. The rock and mineral particles having diameter less than 2.0 mm and come between specifies size are called as soil separates. International society of soil science (ISSS) and United States Department of Agriculture (USDA) systems are Generally, followed to classify the soil separates. This classification is given in table 3.1.

Genesis of Soil

World’s geography is very widely diversified in relief showing various features like mountains and hills covered with snow and trees, glaciers, higher altitudes to plain to lowest valleys, forests to barren lands, fertile fields to waste lands, plateaus, deltas, swamps, lowlands, deserts etc. Temperature varies from arctic cold to scorching hot of equator, Rainfall from aridity to per humidity. All these situations and conditions under different sets of combinations generate and develop numerous types of soil from a single type of parent material. Therefore, many types of soils may develop at a smaller area.

Physical, chemical and biological weathering processes disintegrate and decompose various rocks and minerals to make asymmetrical soil parent material. This parent material is converted in to the soil profile by soil forming processes in the presence of soil forming factors. Soil genesis is defined as mode of formation of the soil from the parent material due to the action of soil forming factors and processes. Parent material, Factors of soil formation and Processes of soil formation, are three main part of soil genesis. These are described here:

1. Parent Material,

2. Classification of parent material,

3. Factors of soil formation,

4. Processes of soil formation,

5. Types of soil forming processes.

This chapter presents a birds’ eye view of the soil to develop the interest among the readers to know more about the fundamental concepts of soil.

Soil is a major determinant of terrestrial ecosystems and helps in the distribution of land flora and fauna. Soils are sources, transformers and stores of plant nutrients. Soils are the recycling places of plant and animal residues and wastes. Soils are also the screen and filter of pollutants in many ways, forms and degrees. Soils are the main link and recharger of hydrological cycle. Soils influence radiant heat exchange, temperature change and land surface reflection characteristics. Soils are important sources and sinks of biogeochemical cycles of carbon dioxide, methane, nitrogen oxides and sulphur dioxide. Soil is used in buildings, roads, brick kiln, naturopathy etc. Soils are good record of past climatic conditions and human activities, as they have some stable features originated from the interactions of climate, organisms, relief, parent material and time. Soil provides nutrients, water, air, heat and mechanical support to arable plants to produce the food, fodder, fibre, fuel and other plant products for human and animals. Mainly the soil is studied as a natural body and as a medium for plant growth.

Introduction

Conventional agriculture is highly dependent on fossil fuel based mineral fertilizers to compensate for removal of plant nutrients by crops. Chemical fertilizers are not only in short supply but also are very costly. The preparation and use of organic manures are not being practiced in a scientific manner to harness the plant nutrients removed by plants, animals and humans.

The application of chemical fertilizers to soil alone and in an imbalanced way has resulted in deterioration of soil health and its quality. The use of chemical fertilizers has to be reduced to check the soil and water pollution and simultaneously the role of soil microorganisms has to be augmented by different ways such as greater use of biofertilizers. Moreover, the demand for organic food particularly both in developed and developing countries is on rise which opens up immense possibility to use organic manure and biofertilizers for production of foods grains, fruits, vegetables, spices, tea and coffee. Therefore, the production and the use of biofertilizers assumes greater significance and could be an excellent biological alternative to chemical fertilizers.

The term ‘mycorrhizae’ was coined more than a century ago (Frank 1885) describing that the roots of most plants are colonized by fungi and transformed into fungus roots organ. Mycorrhizae result from a mutualistic symbiosis between plant roots and certain fungi. Roots of most flowering plants live in mutual symbiosis with mycorrhizae which bio-tropically colonise the root cortex and extra-metricial mycelia help the plant to obtain plant nutrients from soil (Barea, 1991).

These fungi are ubiquitous in soil and are found in the roots of many Angiosperms, Gymnosperms, Pteridophytes and Thallophytes (Mosse et al., 1981). The mycorrhizal fungi perform the function of root hairs. The fungus takes carbohydrates from the plant and in turn supplies the plants with nutrients, hormones and protects it from root pathogens. The dependency of fungal species on host plant ranges from obligate to facultative (Brundrett, 2002; Read and Perez-Moreno, 2003). The mycorrhizal plants have greater tolerance to toxic heavy metal, nigh soil temperature, soil salinity, unfavourable soil pH and to transplantation shocks.

They play an important role in increasing plant growth and nutrients uptake (Bagyaraj, 1992, VasanthaKrishna and Bagyaraj 1993). These fungi can be used to promote biotechnologically developed plant in soil (Varma and Schuepp 1994).

Phosphorus is one of the essential plant nutrients, which is required in optimum amounts for proper growth of plants and soil biota. Being a constituent of ATP it is involved in various processes such as cell division, energy generation and transfer of genetic characteristics and regulation of metabolic pathways (Theodorou and Plaxton, 1993). Phosphorus constitutes about 0.2% plant dry weight (Schachtman et al., 1998). The average soil contains 0.05% phosphorus but only one tenth of this is available to plants due to its poor solubility and chemical fixation in soil (Narayansamy et al., 1981, Barber, 1984). About 98 percent of Indian soils have inadequate supply of available phosphorus (Ghosh and Hasan 1979, Hasan, 1994). A survey of Indian soils showed that out of 363 districts, only 2,2% were high, 51.5% medium and 46.3% were low in phosphorus content (Ghosh 1982).

The problem of P fertilization may become serious in coming years because of the fact that manufacture of phosphatic fertilizers requires the use of non renewable resources such as high grade rock phosphate and sulphur which are becoming costlier. The situation is further aggravated by the fact that P is readily fixed in the soil and the average utilization efficiency of added fertilizer P by plants ranges from 15-25% only (Gaur 1982, 1990). The recent report showed that 42 per cent of soil samples fell in low category, 38 per cent in medium and 20 per cent in high category (Motsara, 2002). It has been observed that the vast alluvial tracts of the country are either low or medium in available P. Hence they need P application at recommended rates.