The Ecology Book

246 THE LEGACY OF PESTICIDES

A persistent poison

DDT (dichloro-diphenyl-

trichloroethane) belongs

to a group of pesticides

called organochlorides. It

kills insects on contact by

interfering with their nerve

impulses. The compound is fat

soluble and is deposited in the

tissues of animals exposed to

it, either directly or by eating

contaminated food. Repeated

exposure to DDT results in it

building up in the body fat and

becoming toxic.

DDT also biomagnifies up

the food chain. Humans are

susceptible to poisoning from

regular exposure to DDT and

while the effects of small

amounts in the environment

are unknown, it has been

associated with cancer,

infertility, miscarriage, and

diabetes. It is now banned in

western countries, but studies

carried out by the US Center

for Disease Control in 2003–4

found DDT or its breakdown

product (DDE) in the blood of

99 percent of people tested.

Organisms higher in the food chain suffer from the impacts of DDT the most.

In producers, the poison only represents 0.04 ppm (parts per million), but the

concentration increases with each step up the food chain. By the time tertiary

consumers are involved, levels are high enough to have toxic effects.

DDT biomagnification in the food chain

PRODUCERS 0.04 PPM

PRIMARY

CONSUMERS 0.23 PPM

SECONDARY

CONSUMERS

2.07 PPM

TERTIARY

CONSUMERS

13.8 PPM

intensification of farming. The

chemical age had dawned and by

1952, there were almost 10,000

separate new pesticide products

registered with the US Department

of Agriculture (USDA).

Raising awareness

Carson was not the first person to

notice the harmful effects of DDT.

There were a few early dissenters,

including nature writer Edwin

Way Teale, who warned that a

spray with the indiscriminate

impact of DDT could upset the

balance of nature. In 1945, the

director of the US Fish and Wildlife

Service (FWS), Dr Clarence Cottam,

stated that caution was essential

in the use of DDT because the true

impact of the product was not yet

fully understood. The following

year, Fred Bishop, writing in the

American Journal of Public Health,

stressed that DDT must not be

allowed to get into foods or be

ingested by accident.

Various scientific studies and

reports also raised concerns. For

example, in 1945 the US government

published a study that found traces

of DDT in the milk of cows sprayed

with the chemical. It recommended

that farmers use “safe alternative

substitute insecticides” to control

flies and lice on cattle. Carson’s

A spray as indiscriminate

as DDT can upset the

economy of nature...

Ninety percent of all

insects are good, and if

they are killed, things go

out of kilter right away.

Edwin Way Teale

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