5.7 Effects of pesticides on biodiversity
The continuing intensive chemicalization of the world's
agriculture introduces large amounts of pesticides into the planet's
biosphere-the habitat of all living beings, including humans.
As environmental contaminants, pesticides are different from
other types of chemicals:
· It is nearly
impossible to prevent their circulation in the biosphere.
· They are biologically active.
This creates potential dangers to nature and people.
· A large proportion of the
human population comes into contact with pesticides.
· Many pesticides persist in
natural conditions and are transferred along the food chain.
· Many pesticides can be
accumulated in the bodies of organisms that come into contact with even low
Pesticides can have both lethal and sub-lethal effects on
organisms they come into contact with.
· The average crop loss
resulting from insects, diseases and weeds has been calculated as high as 35% of
the potential production of crops.
· Pesticide consumption in India
for agriculture and public health has risen from 2000 tonnes a year in the
fifties to over 80,000 tonnes.
· Annually in the world, there
are about 750,000 reported pesticides poisonings with about 13,800 deaths. India
accounts for 1/3 of pesticide poisoning cases in the world.
· In nine weeks, earthworms can
accumulate 18 ppm of DDT from soil containing only 1 ppm of DDT. This
demonstrates the serious big-accumulation effects of pesticides in the
· Per average of 76 mg/kg of
pesticide residues have been found in samples of cow's milk obtained from local
vendors in Bombay. This level is more than 500 times higher than the maximum
intake level of 0.15 mg/kg recommended by WHO.
· The "Handigodu Syndrome" in
the population of many rural areas in Kamataka was traced to the victim's diet
of crabs found in ponds and rice fields which were contaminated with pesticides
(endrin and parathion). Victims were crippled; their limbs, lips and shoulders
· The major source of dietary
intake of pesticide residues is human milk and milk products, followed by oils
Effects of pesticides on the environment
Presence of residual amounts in soil, water and air.
Presence of residual amounts. Damage due to phytotoxicity.
Changes in vegetative development.
Presence of residual amounts in domestic and wild animals.
Physiological actions (non-vitality of birds eggs). Extermination of some wild
species. Development of second-generation pests.
Presence of residual amounts in tissues and organs. Occupational
Presence of residual amounts.
Organism being controlled
Development of resistance.
Biological transfer of pesticides
Pesticides enter a biological system by three main routes:
aerial, terrestrial and aquatic.
Biological transfer of pesticides
Biomagnification of pesticides
The accumulation of pesticides in various biological systems is
called "biomagnification". Some persistent pesticides accumulate in various
biological system at levels much higher than those in their surroundings. For
example, 1 kg of soil may contain only 1/1000 of a milligram of organochlorine
pesticides-but a kilogram of carrots grown in the same soil may contain as much
as 6 mg of pesticides.
Pesticides in the air
Pesticides enter the atmosphere mainly through the treatment of
agricultural crops, seeds, forests, and water basins. They get into the air
together with soil dust' via wind erosion, during soi! cultivation and crop
harvesting. They are also evaporated from moist surfaces such as soil, water and
plants. From the atmosphere, the pesticides and their metabolites get into water
and soil and continue to circulate in the environment.
Pollutants, including pesticides, accumulate in sediments which
serve as a habitat for various organisms in the aquatic food chain, which
ultimately involves fish. Even in insignificant concentrations, some pesticides
may change the taste and odour of water, have a negative effect on the process
of oxygen formation by phytoplankton, and affect the vital activities of the
inhabitants of the water ecosystems.
Pesticides are dispersed in water and are picked up by living
creatures. The chemicals enter the food chain and accumulate faster in living
organisms through the aquatic route than through the other routes.
Pesticides can have both lethal and non-lethal effects on fish.
Non-lethal effects include disturbed population dynamics and changed food habits
and reproductive behaviour. Pesticides such as DDT reduce the ability of fish to
adapt to changing temperatures. They also result in vertebral fractures and can
deform fish backbones and fertilized eggs. Fish can be used to indicate the
level of pesticide contamination in water.
Sub-lethal dosages of pesticides can reduce the thickness of
eggshells, making them fragile and reducing the number of eggs that hatch.
Pesticides also cause hormonal changes in birds, influencing their courtship and
When pesticide residues get into the hydrosphere, some are
volatilized and lost to the atmosphere, some are degraded, some are incorporated
in the biota, and some move into the sediment.
Animals, plants and micro-organisms are responsible for
degradation and detoxification of pesticidal residues. Pesticides are modified
or completely decomposed in the soil as a result of physicochemical processes,
microbiological decomposition and absorption by higher plants and the soil
fauna. The soils are rich in micro-organisms, which mostly include
actinomycetes, fungi and bacteria. These micro-organisms play an important role
in the degradation of pesticide residues from the soil. Many pesticides become
detoxified by their adsorption by humus and other colloids or the formation of
stable complexes in the soil. Poisonous chemicals are removed from the soil as a
result of volatilization, evaporation with water vapour, migration beyond the
root-habitat layer, washing out by rain water, melted snow, irrigation, ground
and soil water.
Biomagnification in the food chain
When a pesticide enters the food chain, it can be deposited in
the bodies of organisms. Predators that prey on large numbers of these organisms
can accumulate large quantities of the pesticide. For this reason, the maximum
accumulations of the toxicant are found at the top of the food chain.
The species affected depend on the predator-prey relationships
in the food chain. For instance, a pesticide in the soil that is picked up by
earthworms may end up in snakes:
A pesticide picked up by soil insects may enter a different food
chain that also ends with snakes:
Soil insects-predacious insects-toad-snake
Some types of pesticides persist in the environment because they
are not broken down easily into harmless substances. The use of persistent
organochlorine pesticides like DDT and HCH for agricultural and nonagricultural
purposes should be discouraged. They should be replaced with easily degradable
"soft" pesticides of organophosphates, carbamates and synthetic pyrethroid
group. Better still, integrated pest management approaches that avoid pesticide
uses should be used to control pests.
Detoxification of pesticides in soil
Sources of pesticides
Pesticides can reach rivers, lakes, ponds and oceans from
· Industrial wastes
and factory effluents
· Spray drift at the time of field
· Direct application to the soil
for control of crop pests
· Agricultural wastes
· Sewage effluents.
Pesticide cycle in the environment
Alternatives to chemical pesticides
The dangerous side-effects of a number of conventional
pesticides on wildlife, their human health hazards and their pollution of the
environment have forced the discontinuation of their use or manufacture. In
addition, many harmful insects have become resistant to synthetic insecticides.
It has become imperative to find other ways of controlling pests.
Various approaches are used to control pests. Those using
natural predators, parasites and pathogens (biological control), sexual
sterilization, sex pheromones and insect growth regulators (third generation
pesticides) appear to offer the greatest opportunity for success- particularly
if used in integrated pest management programs.
Micro-organisms that affect insects, or "entomo-pathogens",
induce diseases that often suppress and, in some cases, completely eliminate
natural populations of insect pests. Over 1000 such pathogens have been isolated
from insects. Many are associated with major pests and could be developed into
Plants are the richest source of renewable bioactive organic
chemicals. Plant-based pesticides are the oldest pesticides used by man.
Recently there has been renewed interest in botanical pesticides.
Prepared by Dr. Vijayendra P.