The insects and mites have three main points of weakness for attack by
pesticides which are as follows:
- their waxy exoskeletons (see Horticultural pests) may be penetrated by waxdissolving
- their abdominal spiracles allow fumigant chemicals to enter tracheae;
- their digestive systems, in coping with the large food quantities
required for growth, may take in stomach poisons.
Four groups of insecticides are described (details of associated hazards to
spraying operators and the general public are discussed later in this chapter):
- An insecticide manufactured from natural plant extracts is
approved for amateur and professional growers. It contains alginates/
polysaccharides and acts by blocking the spiracles of pests such
as aphids, thrips and mites. It has been given clearance for use by
- Pirimicarb is available to professional growers. It belongs to the
carbamate group. It enters the pest as a stomach poison, acting on the
insect’s nervous system. It is slightly systemic in plants, and controls
many aphid species without affecting beneficial ladybirds. Aldicarb,
a related chemical, combines a soil action against nematodes with a
systemic, broad-spectrum activity against foliar pests, such as aphids,
whitefly, leaf miners, mites and nematodes of ornamental plants.
- Bifenthrin is available to amateur and professional growers. It
belongs to the pyrethroid group. It has both cuticle and stomach
action. It is effective against caterpillars, aphids and mites outdoors. It
is residual in soils for a period of up to eight months.
- A fourth group of insecticides contains potassium salts of fatty acids. It is available to amateur and professional growers. It works by
contact action dissolving the cuticle of pests such as aphids, whitefly
spider mites, mealy bugs and scale insects.