Benefits: Produce a rapid control. Are easily accessible.
Limitations: Can be dangerous to humans, animals and plants. Can
cause resistant strains of pests and diseases to develop.
In past centuries pests, such as apple woolly aphid, were sprayed
with natural products, such as turpentine and soap, while weeds were
removed by hand. In the nineteenth century, the chance development
of Bordeaux mixture from inorganic copper sulphate and slaked lime,
and in the early twentieth century the expansion of the organic chemical
industry, enabled a change of emphasis in crop protection from cultural
to chemical control.
The word ‘pesticide’ is used in this book to cover all crop protection
chemicals, which include herbicides (for weeds), insecticides (for
insects), acaricides (for mites), nematicides (for nematodes) and fungicides (for fungi). About 2.5 million tonnes of crop protection
chemicals are used worldwide each year, about 40 per cent being
herbicides, about 40 per cent insecticides and about 20 per cent
fungicides. Health and Safety aspects of chemical control are described
at the end of the ‘chemical control’ section.
This involves the use of substances toxic to most living organisms
and must be used only by specialist operators and professional
horticulturists. The chemical’s toxicity to plants also means that they can
only be applied to soil or compost that has no crops.
With the recent discontinuation of methyl bromide, two remaining soil
applied ingredients are dazomet (applied outdoors and in protected
crops as a granule against soil-borne insects, fungi, and weed seeds) and dichloropropene (applied outdoors as a vapour-releasing liquid
by an injection apparatus against nematodes). The fumigant action
of these substances is prolonged by rolling the soil after application.
Precautions such as rotavating the soil need to be taken several weeks
after application, to release any chemical residues before succeeding
crops can be planted.
Professional horticulturists sterilize greenhouse structures using toxic
compounds, such as formaldehyde and burning sulphur. Common
pests and diseases, such as whitefly, red spider mite and grey mould,
may be greatly reduced by this intercrop method of control.
European legislation saw the withdrawal of methyl bromide in 2005
on the grounds of human and environmental safety. Whilst being very
effective as a sterilant of growing media, methyl bromide had three
serious drawbacks. It was very poisonous to man and animals. As a
gas, it found its way into the atmosphere. Lastly, its chemical similarity
to the chlorinated hydrocarbons used in refrigerator coolants meant that
it was held partly responsible for the ‘ global warming ’ phenomenon.
These three factors combined to rule out its continued use in
Each container of commercial pesticide contains several ingredients.
The active ingredient’s role is to kill the weed, pest or disease.
More detailed lists of the range of active ingredients can be found in
government literature. The other constituents of pesticides are described