Geoscience Reference
In-Depth Information
carried Dutch elm disease. A Punch cartoon of the day described this food chain in
a clever paraphrase of the well-known nursery rhyme: 'This is the bird that ate the
worm that fed on the leaf that Jack sprayed!' The example illustrates the point that a
worm cannot afford to take too narrow a view of pesticides, for the soil is often the
ultimate repository of sprays even when they are applied to crops or trees.
The controversy over pesticides in America was brought to a focus in Rachel
Carson's topic
Silent Spring.
At the same time, the tension produced within the inter-
national scientific community initiated a very productive period of analytical, ecolo-
gical and toxicological research during the 1960s. Wild life aspects of this story have
been well captured in John Sheail's topic
Pesticides and Nature Conservation: the
British Experience
, while a general account of pesticides, their value as well as their
undesirable effects, has been admirably given in the New Naturalist series by Ken-
neth Mellanby.
DDT was discovered in Switzerland by P. Müller of the Geigy Chemical Com-
pany just before the start of World War II. It was in large scale use in the 1940s for
the control of medical pests and saved millions of lives. It was subsequently used
so widely on crops and fruit that for some years it was the most common pesti-
cide residue found in agricultural soils. DDT and HCH (formerly known as BHC or
Lindane - the active gamma isomer of hexachlorocyclohexane) were the first of a line
of insecticides known as chlorinated hydrocarbons or organochlorine compounds;
they were followed by compounds such as aldrin, dieldrin, chlordane, endrin, hep-
tachlor and endosulphan. The chlorine atoms in these molecules, as well as their in-
solubility, renders them peculiarly resistant to microbial breakdown, so they remain
effective in soil for many weeks or months. Adequate persistence is an important re-
quirement for ensuring protection against pests which cannot be sprayed directly but
which might attack a seed or plant at a later date. However, lack of selectivity results
in the killing of beneficial and harmless species, and the danger of uncovering new
pests hitherto kept under control by predators. Moreover, lingering sublethal persist-
ence is a disadvantage as it may produce resistant pests.
The persistence of pesticides in soil varies widely depending on conditions, but
is most marked when the chemical is mixed into the top few centimetres of soil.
Around 80 per cent of DDT thus applied can be detected in the soil a year later, with
50 per cent after three years and 5-35 per cent after ten years. Compounds such as
HCH and aldrin disappear more quickly but, whereas HCH breaks down into non-in-
secticidal substances, aldrin is oxidized to dieldrin which is equally insecticidal and
more persistent. DDT has the added complication of being broken down, or partly
metabolized, within the bodies of beetles, worms and slugs into DDE and sometimes
TDE (rhothane) which produce sublethal effects in them or their predators.
