**2. Pesticides and their groups**

A pesticide is a chemical substance or combination of different chemical substances used to eliminate pests to protect crops. Food and Agriculture Organization (FAO) defined a pesticide as "a substance intended for preventing, destroying, repelling or mitigating any pest in crops either before or after harvest to prevent deterioration during storage and transport." Pesticides are designed to control pests of the standing crop in the crop fields as well as to protect the stored crops after harvest, thus finely ensuring food security. Pesticides are classified according to their chemical nature, their target, modes of action, period of activity, mode of formulation, activity spectrum, toxicity level, etc [6].

#### **2.1 Mode of action**

After application, pesticides either remain on the part of the plant to which those are applied or enter into the vascular system of the plant body and get transported to different organs. According to this principle, pesticides are categorized as systemic and non-systemic ones. In the case of systemic pesticides, the compound penetrates the plant body, gets into the vascular tissue system, and spreads to different parts of the plant showing its effects uniformly. In contrast to this, non-systemic pesticides do not effectively penetrate the plant tissue and remain at the applied area on the plant body.

#### **2.2 Target of the pesticides**

This classification of pesticides is the most common and familiar as the categorization is based on the effectiveness of the pesticide on different types of pests. For an instance,

pesticides those act on insects are called insecticides, those acts on fungi are called fungicides, and those acts on herbs are called herbicides and so on. Likewise, there are rodenticides, molluscicides, nematicides, plant growth regulators, etc., used to protect plants.

#### **2.3 Chemical composition of pesticide compounds**

This type of classification of pesticides is done based on their chemical composition and the active ingredients they contain. This classification of pesticides is actually the most useful one as it helps in studying the occurrence of pesticides in the field, which implies their physical and chemical properties, helps to know their persistency in the environment etc. Based on their chemical nature, pesticides are categorized mainly into seven groups; those are organochlorines (OC), organophosphates (OP), carbamates, pyrethroids, amides, anilines, and azotic heterocyclic compounds. Of these seven classes, organochlorines are highly toxic pesticides. In their chemical structure, they contain five or more chlorine atoms. The chemical structure of this group of pesticides makes them highly persistent in the environment. However, the use of these pesticides is now banned in many countries due to certain problems such as their toxicity toward humans and persistency in ecosystems. Other groups of toxic pesticides are organophosphates and carbamates. Organophosphates have a chemical structure that makes them easily degraded in nature, and hence, these constitute a group of most commonly used pesticides in almost all countries. These pesticides are comparatively less toxic but effective pest controlling chemicals nowadays. However, their widespread use has now become a serious problem for ecosystems due to the occurrence of residues in different environmental compartments including water resources. The groups of pesticides anilines, pyrethroids, amides, azotic heterocyclic compounds—constitute comparatively less toxic groups. Pyrethroid pesticides derive from a plant-based product and are made from flowers of Pyrethrum *(Chrysanthemum cinerariaefolium)*. These are used for their quick action against insect pests, easy biodegradability, and low toxicity toward mammals [7]. However, these pesticides are found to be toxic to aquatic organisms. Amide pesticides are also less persistent, and in many studies, they have been found to be completely degraded after 10 weeks of their field application. Though aniline pesticides are found to be very effective against insect pests, their toxicity toward mammals and aquatic animals made them banned in many European countries.

#### **2.4 Mode of formulation**

Pesticides constitute mainly of two parts—active ingredient (AI) and inert ingredient. The active ingredient is the pure form of the chemical, and this gives a pesticide its actual pesticidal property. However, for improving its activities, long-term storage, safe handling, and enhanced effectiveness, the active ingredients are usually mixed with some inert ingredients. This is called pesticide formulation, and it is of different types such as emulsifiable concentrates (EC), wettable powder (WP), soluble concentrate (SL), soluble powder (SP), suspension concentrate (SC), capsule suspensions (CS), water-dispersible granules (WG), granules (GR), dusts (Dp), etc [8].

#### **2.5 Active spectrum**

Pesticides that are active against a wide range of crop pests are included under broad-spectrum pesticides and those which act only on a selective group of pests are called selective pesticides.

#### **2.6 Toxicity**

Pesticides are categorized into five groups according to the potential risk they exert on humans and based on that pesticides are extremely hazardous, highly hazardous, moderately hazardous, slightly hazardous, products unlikely to present acute hazards in normal use [7].

Organochlorine insecticides were the first group of pesticides that were used successfully in eliminating crop pests. However, due to the reported toxicity toward humans and other mammals and persistency in different ecosystems, the use of organochlorines is now withdrawn. New groups of pesticides developed later, such as organophospahtes in 1960s, carbamates in 1970s, and pyrethroids in 1980s, herbicides in 1970s–1980s brought revolutionary changes in the field of crop pest regulation. Today pesticide production is a large industry with an annual turnover worth USD 35 billion. Currently, about 4.6 million tonnes of chemical pesticides are applied to crop plants, thereby put into the environment each year. In 2004, this amount included 47.5% of herbicides, 29.5% of insecticides, 17.5% of fungicides, and other group of pesticides account for 5.5% [9]. The overall usage of pesticides from 1990–2019 is depicted in **Figure 1**. The trend of use of different groups of pesticides is now changed. For example, the use of herbicides has been increased and the use of insecticides, fungicides, and bactericides has decreased largely in the last few decades [10].

**Figure 1.** *Consumption of pesticides in world from 1990 to 2018. Source: Food and Agriculture Organization (FAO).*

#### **Figure 2.**

*Consumption of pesticides in different countries from 1990 to 2019. Source: Food and Agriculture Organization (FAO).*

China tops the list of the highest amount of pesticide user in the world followed by the United States [11]. India ranks fourth in pesticide manufacturing and 12th in the list of highest pesticide-consuming countries (**Figure 2**) [12].
