Pesticides

Introduction

A numerous and diverse group of chemical compounds with an extensive range of action,¹'¹ pesticides are very widely used, though primarily to eliminate insect pests and weeds and to limit their negative effects in agriculture and in the household.^12-41 They enable the quantities and quality of crops and food to be controlled and help to limit the many human diseases transmitted by insect or rodent vectors. Despite their many merits, however, pesticides are some of the most toxic, environmentally stable and mobile substances in the environment, which are able to bioaccumulate. They can also participate in various physical, chemical, and biological reactions, as a result of which even more toxic substances may be produced; by accumulating in living organisms, these can lead to irreversible, deleterious changes. The non-rational application of pesticides also adversely affects the environment and humans, increasing susceptibility to diseases and poisoning.¹'¹

Characteristics of Pesticides

The following are the current uses of pesticides:

• • To remove, control the number of, or attract various kinds of pests destroying plants and plant products
• • To kill weeds
• • To destroy foliage and excess numbers of flowers
• • To increase the production of animal and plant biomass
• • To combat human, animal, and plant pathogens
• • To control the growth of plants or their parts
• • To combat microorganisms causing farm produce to rot and decay
• • To combat insects and other animals occurring in homes, cellars, stores, etc.
• • To protect fabrics in textile mills and dry-cleaning establishments
• • To prevent the growth of algae in swimming pools
• • To combat fungi in paints and paper products
• • To protect museum exhibits against the action of pests
• • To counteract growths on boats and ships¹'¹

FIGURE 1 The effects of using pesticides.

Source: Biziuk et al.,M Moreno et al.,^|s| and Hajslova and Zrostlikova.¹⁶¹

Ideally, pesticides should be applied only in accordance with their purpose, so that they do not have a negative effect on humans, flora, or fauna. In practice, however, such complete selectivity is unattainable.^11,41 The factor determining whether a particular agent can be used is its rapid biodegradability and its minimal toxicity vis-a-vis the environment.

Chemical pesticides are of enormous importance in increasing the efficiency and quality of agricultural produce. They enter the environment in various forms: powders, moistened powders, powders for preparing aqueous solutions, and concentrates for making up emulsions or sprays. The ubiquitous application of pesticides not only contaminates water, soil, and air but also causes them to accumulate in farm produce like fruit and vegetables. Figure 1 summarizes the effects of pesticide application.^11,5,61

According to Their Chemical Structure

TABLE 1 Classification of Pesticides according to Their Chemical Structure

Source: Biziuk et al.¹¹¹

According to Their Chemical Class

TABLE 2 Classification of Pesticides according to Their Chemical Class

Source: Biziuk et al.¹'¹

The diversity of their chemical structure, action, and application makes any classification of pesticides difficult.¹'¹ There are a number of criteria according to which they can be categorized: toxicity, purpose of application, chemical structure, environmental stability, and the pathways by which they penetrate target organisms. Tables 1-4 outline some of these classifications:¹'¹

According to Their Application

TABLE 3 Classification of Pesticides according to Their Application

(Continued)

TABLE 3 (Continued) Classification of Pesticides according to Their Application

Source: Biziuk et al.¹¹¹

According to Their Toxicity

TABLE 4 Classification of Pesticides according to Their Toxicity

Source: Biziuk et al.¹¹¹

Circulation of Pesticides in the Environment

Nowadays, pesticides are very widely applied, not just in agriculture. Their ubiquity and ever-increasing consumption pose a greater hazard to the environment to which they are transported. Pesticides can enter the environment in their primary form or as decomposition products. When in the environment, they are subject to various transformations. They may be borne by wind or rain from their points of application to neighboring areas and crops, where they are not required or may be harmful. The quantity of pesticides circulating in a given region depends to a large extent on their intensity of application or the type of crop being grown there. Both modern pesticides and older ones are present in the environment. Figure 2 illustrates the circulation of pesticides in nature.

Pesticides in the Aquatic Environment

Pesticides are the most common contaminants in surface water and groundwater. There are also many reports of pesticides being found in drinking water, well water in farming areas, rainwater, subterranean water, and ice from the polar regions. Once in the aquatic environment, pesticides have a deleterious effect on the quality of waters used as sources of drinking water for the large majority of the Earth’s

FIGURE 2 The circulation of pesticides in nature. Source: Biziuk et al.M

human population. Concentrations are highest during the spring snow melt period and when pesticides are being applied to crops.¹¹’²¹ There are many sources from which pesticides get into the aquatic environment. Usually, these are area sources (e.g., precipitation, farming areas), but they may also come from point sources such as effluents of various kinds or leaking waste disposal sites. They can also be transported in the air for great distances.^11,71 When in the aquatic environment, pesticides are subject to a variety of transformations and processes:

• • Physical (accumulation, deposition, dilution, diffusion)
• • Chemical (hydrolysis, oxidation)
• • Photochemical (photolysis, photodegradation)
• • Biochemical (biodegradation, biotransformation, bioaccumulation)¹³’⁸¹

Substances of greater toxicity may be formed as a result of these processes, and when they accumulate in aquatic organisms, they can cause much irreversible damage to them.

Pesticides in the Air

The widespread application of pesticides contaminates not only waters and soils; the air is also affected. This is because pesticides rise into the air as they are being sprayed and thereafter (post-application emission).¹¹¹ An estimated 30%-50% of pesticides sprayed onto crops get into the atmosphere. By “postapplication emission,” we understand the evaporation of pesticides from soil or plant surfaces and due to the wind erosion of the soil. Depending on their stability, pesticides in the air may be degraded, or transported over long distances before being deposited. After their application, pesticides are present in the atmosphere in gaseous form, adsorbed onto solid particles, or dissolved in water vapor.

Pesticides in Crops

The presence of pesticides in the environment causes them to accumulate in crops like fruit and vegetables. The contamination of plant foods by pesticides is particularly dangerous because these compounds can reach every part of the plant, regardless of how they are applied. The degree of contamination of crops depends, among other things, on the dose and number of applications, the form of the pesticide preparation, the weather conditions, and the time elapsing between pesticide application and crop harvesting. Contamination is reduced by such factors as rainfall, wind, and chemical changes caused by oxygen, moisture, light, and plant enzymes. Plants with a large surface area relative to their mass retain larger amounts of pesticides.¹¹¹ Fruit and vegetables are capable of retaining larger quantities of pesticides. Pesticides can accumulate in fruit skins. The crops most exposed to the presence of pesticides are grapes, citrus fruits, and potatoes. Adsorbed pesticides can reduce the nutritious value of crops or alter their organoleptic properties. The contamination of crops with pesticides is due to their application in contradiction to good agricultural practice and the insufficient monitoring of their application. Their total elimination is, of course, often not possible, but the amounts applied can be limited to those that are harmless to human health.

Pesticides in Soil

Pesticides are transported to soil because of deliberate human activity. It is estimated that about 50% of pesticides come from washing processes of soils’ surface. Transport of pesticides to plant material depends on their stability, and the main ways of absorption are via roofs and leaves. They can be also adsorbed on the molecules of clay. In this case, they do not penetrate the soil. Stability and fate of pesticides in soil depend on chemical structure, type of plant material, type of soil and their pH/temperature, weather conditions, etc.¹¹¹