Pesticides: Uses, Mode of Action and Environmental Impact

Pests are a nuisance to humans. They attack crops and cause physical and economic loss to agriculturists; they are vectors of several diseases such as malaria and tsetse fly and are generally detrimental to humans.

Getting rid of pests – both plants and animals – have historically involved mechanical and biological methods.

The former includes using physical means such as the use of fences and barriers while the later involves introducing natural enemies of said pests.

However, the fastest and most reliable method of pest control involves the use of pesticides. According to the Food and Agriculture Organization (FAO), a pesticide is

“any substance or mixture of substances intended for preventing, destroying, or controlling any pests including vectors of human or animal diseases, unwanted species of plants and 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 for the control of insects, arachnids, or other pests in or on their bodies.”

The World Health Organization (WHO) also defines a pesticide as “chemical compounds that are used to kill pests, including insects, rodents, fungi, and unwanted plant or weeds.”

Pesticides include;

Insecticides and insect repellants for controlling a wide variety of insects:
Nematicides for the control of worms, animal repellants;
Avicides for the control of bird pests;
Rodenticides for control of rodents like rats and grass-cutters;
Antimicrobials and disinfectants for the control of microbes;
Fungicides or fungi control;
Mmolluscicides for controlling snails and slugs;
And herbicides for the control of unwanted plant species (weeds).

Herbicides are the most common type of pesticides and account for 80 percent of pesticide use. Many other types of pesticides fall under these broad categories. They include ovicides, wood preservatives, miticides, bio-pesticides, etc.

History of Pesticides

Pesticides have been utilized for the control of pest since before 2000BC. Ancient civilizations like the Sumerians employed the use of elemental Sulphur to protect their plants from insects.

By the 1600s, the use of toxic chemicals like lead, mercury, and arsenic to control pests was widespread.

The Romans and Greeks used ash, oil, and sulfur to protect themselves, their crops, and their livestock from pests while the Chinese utilized mercury and arsenic compounds to control body lice.

Since the early 20th century, different pesticides have been derived from compounds made of roots of tropical vegetables and chrysanthemums. In 1939, Paul Müller discovered the insecticide properties of Dichloro-Diphenyl-Trichloroethane (DDT), and it proved to be a very effective method of insect control.

However, in the 1960s that DDT was discovered to have biological effects and was deemed a serious threat to biodiversity.

Since then, it has been banned under the Stockholm Convention in about 86 countries. It is, however, still illegally used in developing countries for the control of insect pests.

Mode of Action

Pesticides are prepared in three forms. They are;

Solids: These include particulates, pellets, soluble granules, soluble powders, dusts, granules, and wettable powders.
Liquids: These include aerosols, emulsifiable concentrates, suspensions, and solutions
Gases: These are usually fumigants.

Pesticides are typically grouped in families due to similar chemical characteristics or their mechanism of action.

A pesticide may, however, have an active ingredient from more than one chemical family. Some of the classes of pesticides include:

Organophosphates

These are usually made from phosphoric acid and are mostly insecticides. They kill pests by acting on the nervous system, interfering with the transmission of nerves by disrupting enzymes that regulate neurotransmitters.

Most organophosphates are very toxic but are used because they break down faster (are less persistent). However, many are being phased out or employed only in extreme conditions.

Examples of organophosphates include Dimethoate, Naled, Trichlorfon, Chlorpyrifos, Fenthione, and Malathion.

Carbamates and Thiocarbamates

These are made from carbamic acid. They are similar to organophosphates in their mode of action, being that they act on the nervous system by disrupting the enzyme that regulates neurotransmitters.

Generally, they are less persistent in the environment and break down quickly. They include herbicides, fungicides, and insecticides.

Health hazards of carbamates and thiocarbamates to humans and animals are mild with herbicides and fungicides but greater with insecticides. Examples include;

Herbicides: Barban, propham, triallate, and eptam.
Fungicides: Abam
Insecticide: Carbaryl, methomyl, thiodicarb, carbouran, and propoxur

Organochlorines

These are chlorinated hydrocarbons that control pests by disrupting nerve-impulse transmission. They are persistent in food, soil, and human and animal bodies.

They are typically used for the control of mites and insects but have been phased out due to their ability to persists in the environment for a long time and accumulate in fatty tissues.

Examples include chlordane, endrin, aldrin, dieldrin, and endosulfan. As of December 2016, the use of endosulfan was not permitted.

Pyrethroids

They disrupt nerve-impulse transmission, which stimulates nerve cells and eventually leads to paralysis. They are stable in sunlight and do not degrade quickly. Examples include cypermethrin, permethrin, cyhalothrin, deltamethrin, and esfenvalerate.

Environmental Impacts of Pesticides

On Water

There are four primary ways by which pesticides reach water. It can occur through:

The drift of suspended particulate matter from target area
Surface runoff via wind and water
Leaching through the soil into groundwater
Spillage

The factors that affect pesticides’ ability to contaminate water include distance for the site of application, water solubility, soil type, presence of vegetation, and method of application.

Eutrophication is the enrichment of water bodies with the surface runoff of chemicals from nearby agricultural lands.

This increases in the nutritive factors of the water body affect the ecosystem and biodiversity by reducing organic oxygen necessary for life. This causes the spread of water hyacinth and algae bloom. Thus, altering aquatic life.

Pesticides also contaminate rain and groundwater — the increased concentration of pesticide compromises drinking water.

On Air

Pesticides adversely affect the quality of air and contribute to air pollution. Drift of suspended particulate matter is carried from the site of application by the wind to unaffected areas. This poses a threat to human and animal life.

Factors that determine levels of contamination are the speed of wind, temperature, and relative humidity. Warmer temperatures usually result in an increased concentration of pesticides in the air. Inhalation of contaminated air may cause respiratory problems and irritate the eyes and skin.

Some pesticides react with other elements in the atmosphere in the presence of sunlight to create ozone, a greenhouse gas.

On Soil

The intense use of pesticides in agricultural degrades and damages the microbial community in the soil, especially when misused. The most devastating effect of pesticides on the soil is the loss of biodiversity; the soil becomes less fertile and of low quality.

Also, a large percentage of organic matter is removed. Organic matter helps the soil in retaining water, which helps agriculturists, especially in the period of drought.

Lack of organic matter increases the buildup of pesticides instead of breaking down chemicals. This results in lower crop yield.

On Wildlife

Pesticides affect the quality of both plant and animal life. Pesticides reduce the symbiotic relationship between bacteria and plants necessary for plant growth.

They have also be linked to declining bee pollination. Bees are agents of pollination, and without them, plant fertilization is drastically reduced.

Other wildlife that can be affected by pesticides are birds, amphibians, and reptiles. Many animal species may unintentionally ingest pesticides after it has been contaminated with chemicals.

Due to its ability to spread over long distances, pesticides may affect other ecosystems and cause significant damage. Pesticides also bio-accumulate up the food chain when animals consume contaminated food.

On Humans

Exposure of humans to pesticides occurs in the following ways:

Direct contact by farmers and farm workers when handling pesticides on farmlands.
Local residents getting exposed via drift.
Exposure of forestry and domestic professionals through the treatment of wood with preservatives and treatment of livestock with anti-parasitic preparations.
Exposure via contaminated air, food, and water

Short term effects of pesticide exposure include: