The methods of destroying injurious insects may be roughly classified for convenience into mechanical and chemical. The former includes all those means by which the insect is crushed, or forcibly removed from the plant that it is attacking, or is prevented by some form of barrier from gaining access to its food plant; the latter comprises the use of chemical substances, known as insecticides which poison or otherwise destroy the insect by being eaten, breathed, or by coming into contact with its body.