Chemical control

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.

Chemical sterilization
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 horticulture.

Active ingredients
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 under formulation.