16.4. Chemical control
Despite the hazards of conventional insecticides, some use is unavoidable. However, careful chemical choice and application can reduce ecological damage. Carefully timed suppressant doses can be delivered at vulnerable stages of the pest’s life cycle or when a pest population is about to explode in numbers. Appropriate and efficient use requires a thorough knowledge of the pest’s field biology and an appreciation of the differences among available insecticides.
An array of chemicals has been developed for the purposes of killing insects. These enter the insect body either by penetrating the cuticle, called contact action or dermal entry, by inhalation into the tracheal system, or by oral ingestion into the digestive system. Most contact poisons also act as stomach poisons if ingested by the insect, and toxic chemicals that are ingested by the insect after translocation through a host are referred to as systemic insecticides. Fumigants used for controlling insects are inhalation poisons. Some chemicals may act simultaneously as inhalation, contact, and stomach poisons. Chemical insecticides generally have an acute effect and their mode of action (i.e. method of causing death) is via the nervous system, either by inhibiting acetylcholinesterase (an essential enzyme for transmission of nerve impulses at synapses) or by acting directly on the nerve cells. Most synthetic insecticides (including pyrethroids) are nerve poisons. Other insecticidal chemicals affect the developmental or metabolic processes of insects, either by mimicking or interfering with the action of hormones, or by affecting the biochemistry of cuticle production.