Agriculture has many problems and pests:-
Insects or animals can eat or damage the roots, stalks or leaves
of the plant, or even the crop itself
Invasive plants use up the nutrients, or contaminate the harvest
Fungus can spoil the seed when it is sown, or the crop as it
ripens, or later in storage.
Scientists have found many answers to these, using chemicals to
kill the insects, kill the weeds, and to protect against the fungus.
Vast sums are poured in to this research, as the losses can be
enormous, and there has been enormous progress and great successes
relative to the losses a century ago.
Most of these chemicals break down after several weeks, and
should not be used within a certain timespan before the crop
is harvested, so that humans do not ingest the various chemicals.
from Microsoft Encarta 96 Encyclopedia, ref
DDT, colorless chemical pesticide, dichlorodiphenyltrichloroethane,
used to eradicate disease-carrying and crop-eating insects.
It was first isolated in Germany in 1874, but not until 1939
did the Swiss Nobel Prize-winning chemist Paul Müller recognize
it as a potent nerve poison on insects. First used heavily in
World War II for preinvasion spraying, DDT was disseminated
in great quantities thereafter throughout the world to combat
yellow fever, typhus, elephantiasis, and other insect-vectored
diseases. In India, DDT reduced malaria from 75 million cases
to fewer than 5 million cases in a decade. Crops and livestock
sprayed with DDT sometimes as much as doubled their yields.
With the publication of the American marine biologist Rachel
Carson's Silent Spring in 1962, suspicion grew that DDT, by
entering the food chain and eventually concentrating in higher
animals, caused reproductive dysfunctions, such as thin eggshells
in some birds. Some insect pests also gradually developed DDT-resistant
strains whose populations grew unchecked while their natural
predators, such as wasps, were being eradicated by spraying.
In 1973 DDT was banned in the U.S. except for use in extreme
health emergencies. Many other nations have also banned it or
placed it under strict control.
Current Situation. Many environmentalists and ecologists
are concerned that these chemicals interfere with the food chain,
as in the above example of DDT. Current concerns in the UK are
that frogs are dying, and that the population of sparrows is
greatly reduced. Slug pellets have been suggested as a possible
cause of both of these, though these are generally used by gardeners
not farmers !
There are other ideas that modern monocultures are 'unnatural',
and inevitably insects and weeds with fill 'ecological niches'
which are vacant. Nature abhors a vacuum !
Future. Modern techniques of analysis are able to detect
many type of contamination, and at very low levels. However,
the costs of testing are high, and the systems are slow - products
are often marketed and consumed before the results are ready.
Food testing for chemicals and residues will certainly increase
in future ! The use of plants with disease resistance my lead
to future improvements, but there are concerns that if this
result is achieved by genetic modification, then there may be
other consequences on other species which are not predictable.