Insecticides are classified into many different groups, but all fit into two large categories: organic and inorganic. Organic insecticides contain carbon, while inorganic insecticides do not. The organic and inorganic distinction made here has to do with the classification and structure of insecticides, not the organic certification created by the U.S. Department of Agriculture. It is important to avoid using these chemicals unless absolutely necessary, and even then to use them sparingly, since they may contribute to Colony Collapse Disorder, which leads to destruction of honeybee hives.
An insecticide containing boric acid, an inorganic insecticide, has to be eaten by an insect in order to kill it. The insecticide is often used in a powdered form. When the insects go through the area with insecticide, it attaches to their legs and antennae. When the insect grooms itself, it ingests the powder and dies. It can be toxic to plants in large amounts.
Silica compounds used in insecticides are also referred to as diatomaceous earth. This inorganic type of insecticide works by drying and destroying the coating of an insect's cuticle. This ultimately causes suffocation to the insect.
Organophosphorous compounds are organic insecticides. They can be taken into the insect through multiple means, including inhalation and ingestion. The chemicals impair the insect's respiratory and nervous systems.
The organochlorine family of insecticides includes DDT, Lindane and Chlordane, and are usually not legal to use because of their damaging effects on humans and the environment. These organic compounds work by damaging the insect's nervous system.
These organic compounds work in the same way that organophosphorous compounds do. They inhibit an enzyme--acetylcholinesterase--that is critical to the functioning of the insect nervous system. These types of insecticides can be damaging to household pets, particularly cats, as well as earthworms, birds and fish. and should be used with caution.
Pyrethrins and Pyrethroids
Pyrethrins are produced naturally by certain species of chrysanthemum. They have low toxicity and work by poisoning an insect's nervous system. Unlike organophosphates and carbamates, they work by disrupting electrical signals sent through the nerve pathways.
Insect Growth Regulators
Insect Growth Regulators (IGRs) are organic insecticides that contain hormones that disrupt an insect's normal growth and prevent it from developing fully. Generally they prevent larvae from becoming adults.
Fumigants are gaseous organic compounds used to poison or suffocate the insects in an area. They can be dangerous and should be used only by those trained to do so.