During the seventeen years she worked in the US Fish and Wildlife Service, Rachel Carson learned about the problems of pesticides. Undaunted by the chemical companies' hostility and by the public's high enthusiasm for pesticides, she wrote a book called Silent Spring, which caused a major shift in public consciousness about the environment.
In 1942 Paul Hermann Muller of Switzerland discovered the
insecticidal properties of a synthetic chlorinated organic
chemical, dichlorodiphenyltrichloroethane (DDT) which had first
been synthesized in 1874. In 1948 Muller received the Nobel
Prize for Physiology or Medicine for his discovery.
According to Fundamentals of Pesticides, DDT
belongs to the chemical class of diphenyl aliphatics, which
means that it consists of an aliphatic, or straight carbon
chain, with two (di-) phenyl rings attached.
Pure DDT is made by the reaction of chloral with
chlorobenzene in the presence of sulfuric acid. A colorless,
crystalline solid, it melts at 109 degrees Celsius. The
commercial product is 65% to 80% active compound. An amorphous
powder, it has a lower melting point than the pure compound.
DDT is applied as a dust or by spraying its aqueous
A persistent pesticide, DDT has many properties that are
desireable in an insecticidal point of view. It is stable and
so remains in soil and aquatic environments, and in animal and
plant tissues. Microorganisms, enzymes, heat, or ultraviolet
light do not readily break it down.
DDT is an organochlorine, one of four types of synthetic
organic insecticides. Usually containing carbon, chlorine, and
hydrogen, organochlorines are also known as "chlorinated
organics," "chlorinated insecticides," or "chlorinated
synthetics." Included are methoxychlor, lindane, toxaphane, and
the closely related group of aldrin, dieldrin, chlordane,
heptachlor, and endrin.
DDT affects the central nervous system. Both in insects and
in mammals, DDT affects the normal transmission of nerve
impulses. The neurons eventually send spontaneous impulses that
cause the muscles to twitch; this may lead to death.
Large amounts of insecticide repeatedly sprayed on plants
will eventually enter the soil, killing living matter there.
These organisms include invisible bacteria, fungi, and algae
which break down plant residues to release minerals, carbon,
and nitrogen, insects which break down plant matter into new
soil, and earthworms which dig tunnels that aerate the soil.
Pesticides can cause the soil to become useless for
Chemicals sprayed on land can seep into rivers and streams.
Some come in contact with groundwater, a major source of
The major effect of environmental pollution is a process
known as bioconcentration. Widely used insecticides affect the
food chain because they degrade very slowly and are highly
soluble in fat. After these chemicals are applied on land, they
are washed by rains into streams, rivers, and lakes. There they
are intaken by microscopic life forms, which are food for fish.
The fish are then eaten by larger fish and aquatic birds. After
animals ingest the pesticide through their prey, the compound
is stored and concentrated in body fat. Repeated feeding causes
high concentrations of the pesticide to build up. Thus, the
higher up on the food chain an organism is, the more
concentrated the intake of pesticide.
The accumulation of DDT or its relatives in birds affects
their ability to reproduce. DDT disrupts the formation of
eggshells in the body of the female. Consequently, some species
lay soft-shelled or shell-less eggs. Ospreys and bald eagles
have been especially threatened as a result.
The effects of pesticides on mammals are not as obvious and
are still being investigated. Some studies suggest that DDT can
inhibit the productivity of plant plankton, upon which all
other marine life depends.
How harmful a pesticide is to humans depends on the amount
and susceptibility of the person. People with heavy exposure to
chemicals during manufacture or use are most seriously
affected. In the short term, there are severe reactions to
touching or inhaling large quantities of pesticides. In the
long term, the exposure to pesticides can cause adverse health
effects such as cancers, birth defects, genetic damage,
respiratory ailments, liver and kidney damage, neurological
disorders, and reproductive problems.
Some insects are naturally more resistant to certain
chemicals. As their susceptible members die, these survivors
multiply, passing their resistance to the next generation. The
pesticides no longer control at normally recommended rates.
Some insects might develop cross-resistance, where resistance
to one insecticide means resistance to a second with a similar
mode of action as the first, or multiple resistance, where
there is resistance to several classes. Depending on the type
of resistance and the species of pest, resistance tends to last
in the absence of the pesticide.
Pest resurgence occurs when pests killed by insecticides
return in larger numbers. This happens because the pesticides
remove target insects and their natural enemies. Either the
natural enemies are killed or they leave the area since their
food is no longer available. There is an opportunity for the
temporarily removed pests to reproduce before their natural
A new species may become serious pests when their natural
predators are killed. Spider mites, for example, caused havoc
when DDT and other insecticides killed their predators.
Because of the discovery of insect resistance, persistent
residues in soil and in living tissue, accumulation in food
chains, and harmful effects to wildlife, there has been a
decline in the usage of organochlorines. They have been replace
by the other three synthetic organics
insecticides--organophosphates, carbamates, and synthetic
Current insect control practice is done with an approach
called Integrated Pest Management ( IPM). This controls pests
by combining methods such as chemicals, natural enemies,
resistant plants, and cultural and mechanical controls.
Biological controls include releasing predators, parasites, or
pathogens of the pest species, simulating the scent of female
insects to lure males into traps, and releasing sterilized
pests to disrupt the reproductive cycle. Farming techniques
incorporate the planting of pest-resistant crops and frequent
rotation of crops. The goal of IPM is to reduce the number of
pests to a level not damaging to the economy. It also is used
to maximize the good from controlling insects and to minimize
the harm to the environment.