Pesticides are biocides that are designed to prevent, repel or destroy pests. The first use of synthetic pesticides began in 1940. About 2.5 million tonnes of pesticides are applied annually in the world. More than 2 billion pounds of pesticides are sold for use in the US every year. More than 20,000 pesticide products with nearly 900 active ingredients are registered for use as insecticides, miticides, herbicides, rodenticides, nematicides, fungicides, fumigants, wood preservatives and plant growth regulators. Generally pesticides provide significant benefits such as crop protection, preservation of food and materials and prevention of vector-borne diseases (e.g., dengue, encephalitis etc.) Pesticides are used almost everywhere – not only in agricultural fields, but also in homes, parks, schools, buildings, forests and public places. Once use or spilled, pesticides can contaminate nearby ground water or surface water.
When a pesticide is used in the environment, it becomes distributed among 4 major compartments: water, air, soil and biota (living organisms). They are distributed in the environment by physical processes such as sedimentation, adsorption and volatilization. They can be degraded by chemical and/ or biological processes. The chemical processes generally occur in water or the atmosphere and follow one of 4 reactions like oxidation, reduction, hydrolysis and photolysis. Biological mechanisms in soil and living organisms utilize oxidation, reduction, hydrolysis and conjugation to degrade chemicals (biotransformation). Pesticides have significant economic, environmental and public health impacts.
The irrational use of pesticides also causes significant problems. Pesticide exposures may cause approximately 20,000 fatalities worldwide per annum. They contaminate large amounts of food products, destroy beneficial natural biota and destabilize ecosystems.
1. High stability – chemically stable and tend to remain in ecosystems for long periods of time.
2. Non-specificity – generally toxic to a wide range of organisms in addition to ‘target organisms’.
3. High mobility – can be recycled through ecosystems much like nutrients.
4. Biological magnification – concentrations magnified when passed through food chains.
Types of pesticides
Most pesticides are classified according to the pests they kill. The word ending or suffix ‘cide’ means kill. (e.g., herbicide –weeds; fungicide-fungi; rodenticides – rodents).
Pesticides can also classified according to how they work:
· Contact pesticides –kills by contacting pests.
· Systemic pesticides –absorbed and moves to untreated tissues.
· Stomach poisons – kills pests after ingestion.
Common categories of pesticidesThere are 5 principal classes of pesticides.
· Organochlorines – DDT, lindane heptachlor, toxaphene, HCB,PCB
· Organophosphates – parathion, malathion
· Carbamates – carbaryl, aldicarb
· Phenoxy herbicides – 2,4-D, 2,4,5-T
· Pyrethroids – fenvalerate, pyrethrins.
Mode of activity (MoA)
Pesticides act primarily targeting 4 nerve targets: acetyl cholinesterase (AChE), chloride channels, acetylcholine receptors and gamma amino butyric acid receptors. Carbamates and organophosphate insecticides (synaptic poisons) inhibit the enzyme AChE, which serves to interrupt the transmission of nerve impulses. Organochlorines and pyrethroids are axonic poisons which interfere with sodium channels thereby stimulating nervous discharges leading to paralysis. Insect growth regulators (IGRs) act on the hormone system of insects.
Long term health effects
Chronic pesticide exposure causes neurodegenerative disorders (e.g., Parkinson’s disease), reproductive effects (e.g., miscarriages, premature births), birth defects (e.g., cleft palate, spina bifida), variety of cancers (e.g., testicular, prostate, cervical cancers, non-Hodgkin’s lymphoma and multiple myeloma) and susceptibility to infectious diseases (immunotoxicity).
Pesticide exposure can kill beneficial soil bacteria, earth worms, snails, frogs, birds, fish, honeybees and other valuable species. In the 1960s, a decline in the population of bald eagles and raptors was observed. Environmental toxicologists found higher concentration of DDT in birds with a decrease in the thickness of eggshell resulting in the breakdown of eggs during incubation.
A sustainable agriculture must be economically viable, socially responsible and ecologically sound. An agriculture that uses up or degrade its natural resource base or pollutes the natural environment eventually will lose its ability to produce. Such agriculture is not sustainable. Sustainable agriculture integrates three main goals- environmental health, economic profitability and social equity.
Integrated Pest Management (IPM)This technique is an effective and environmentally sensitive approach to pest management that relies on a combination of common-sense practices of using mechanical trapping devices, natural predators (e.g., insects that eat other insects), insect growth regulators, mating disruption substances (pheromones), and if necessary, chemical pesticides.
“For the first time in the history of the world, every human being is now subjected to contact with dangerous chemicals, from the moment of conception until death.” – Rachel Carson, 1963 (‘Mother of modern Environmental Movement’).