Chemical control consists of killing the pest with chemical poisons. It appears to be a simple solution to pest problems but there are serious disadvantages – chemicals getting onto plants may damage them or leave residues on a food crop. Harmless, potentially beneficial insects might be killed.
To make chemical control safer and more acceptable, the chemicals nowadays offered for sale are less poisonous and less persistent, meaning it is less likely there will be residues and, if there are, they will be less dangerous. Chemical control is cheap and easy – perhaps too easy.
Because of the disadvantages, chemical control should be used only when there is no other solution. Routine spraying is to be avoided, unless absolutely necessary, because it leaves excess residues, damages the natural balance and encourages the evolution of resistant strains of the pest. Not only is unnecessary spraying a waste of time but it can be very counterproductive.