16.4.1. Insecticides (chemical poisons)
Chemical insecticides may be synthetic or natural products. Natural plant-derived products, usually called botanical insecticides, include:
- alkaloids, including nicotine from tobacco;
- rotenone and other rotenoids from roots of legumes;
- pyrethrins, derived from flowers of Tanacetum cinerariifolium (formerly in Pyrethrum and then Chrysanthemum);
- neem, i.e. extracts of the tree Azadirachta indica, have a long history of use as insecticides (Box 16.3).
Insecticidal alkaloids have been used since the 1600s and pyrethrum since at least the early 1800s. Although nicotine-based insecticides have been phased out for reasons including high mammalian toxicity and limited insecticidal activity, the new generation nicotinoids or neonicotinoids, which are modeled on natural nicotine, have a large market, in particular the systemic insecticide imidacloprid, which is used especially against sucking insects. Rotenoids are mitochondrial poisons that kill insects by respiratory failure, but they also poison fish, and must be kept out of water-ways. Neem derivatives act as feeding poisons for most nymphs and larvae as well as altering behavior and disrupting normal development; they are dealt with in section 16.4.2 and in Box 16.3. Pyrethrins (and the structurally related synthetic pyrethroids) are especially effective against lepidopteran larvae, kill on con- tact even at low doses, and have low environmental persistence. An advantage of most pyrethrins and pyrethroids, and also neem derivatives, is their much lower mammalian and avian toxicity compared with synthetic insecticides, although pyrethroids are highly toxic to fish. A number of insect pests already have developed resistance to pyrethroids.
The other major classes of insecticides have no natural analogs. These are the synthetic carbamates (e.g. aldicarb, carbaryl, carbofuran, methiocarb, methomyl, propoxur), organophosphates (e.g. chlorpyrifos, dichlorvos, dimethoate, malathion, parathion, phorate), and organochlorines (also called chlorinated hydrocarbons, e.g. aldrin, chlordane, DDT, dieldrin, endosulfan, gamma-benzene hexachloride (BHC) (lindane), heptachlor). Certain organochlorines (e.g. aldrin, chlordane, dieldrin, endosulfan, and heptachlor) are known as cyclodienes because of their chemical structure. A new class of insecticides, the phenylpyrazoles (or fiproles, e.g. fipronil), has similarities to DDT.
Most synthetic insecticides are broad spectrum in action, i.e. they have non-specific killing action, and most act on the insect (and incidentally on the mammalian) nervous system. Organochlorines are stable chemicals and persistent in the environment, have a low solubility in water but a moderate solubility in organic solvents, and accumulate in mammalian body fat. Their use is banned in many countries and they are unsuitable for use in IPM. Organophosphates may be highly toxic to mammals but are not stored in fat and, being less environmentally damaging and non-persistent, are suitable for IPM. They usually kill insects by contact or upon ingestion, although some are systemic in action, being absorbed into the vascular system of plants so that they kill most phloem-feeding insects. Non-persistence means that their application must be timed carefully to ensure efficient kill of pests. Carbamates usually act by contact or stomach action, more rarely by systemic action, and have short to medium persistence. Neonicotinoids such as imidacloprid are highly toxic to insects due to their blockage of nicotinic acetylcholine receptors, less toxic to mammals, and relatively non-persistent. Fipronil is a contact and stomach poison that acts as a potent inhibitor of gamma-aminobutyric acid (GABA) regulated chloride channels in neurons of insects, but is less potent in vertebrates. However, the poison and its degradates are moderately persistent and one photo-degradate appears to have an acute toxicity to mammals that is about 10 times that of fipronil itself. Although human and environmental health concerns are associated with its use, it is very effective in controlling many soil and foliar insects, for treating seed, and as a bait formulation to kill ants, vespid wasps, termites, and cockroaches.
In addition to the chemical and physical properties of insecticides, their toxicity, persistence in the field, and method of application are influenced by how they are formulated. Formulation refers to what and how other substances are mixed with the active ingredient, and largely constrains the mode of application. Insecticides may be formulated in various ways, including as solutions or emulsions, as unwettable powders that can be dispersed in water, as dusts or granules (i.e. mixed with an inert carrier), or as gaseous fumigants. Formulation may include abrasives that damage the cuticle and/or baits that attract the insects (e.g. fipronil often is mixed with fishmeal bait to attract and poison pest ants and wasps). The same insecticide can be formulated in different ways according to the application requirements, such as aerial spraying of a crop versus domestic use.