Insecticides: A Complete Veterinary Pharmacy and Agrochemicals Guide
01Insects and their significance
Insects are the largest group of living organisms in the animal kingdom. Scientists estimate there are over one million insect species on the planet, living in every conceivable environment — from volcanoes to glaciers.
Insects are very significant as research tools, as vectors for the transmission of many infectious pathogens (parasites, viruses, bacteria) for both man and animals, and as causes of pre- and post-harvest losses in various agricultural farm produce.
02What are insecticides?
Insecticides are products of chemical and biological origins — ovicides, larvicides, adulticides — that are used for the control of insects in agriculture, pharmacy, medicine, industry and household settings.
Classification by chemical composition
Insecticides can be classified on the basis of their chemical composition as follows:
- Carbamates — e.g., carbaryl.
- Organochlorines — e.g., endosulfan.
- Organophosphorus — e.g., monocrotophos.
- Pyrethroids — e.g., permethrin.
- Neonicotinoids — e.g., imidacloprid.
- Miscellaneous pesticides — spinosyns (spinosad), benzoylureas (diflubenzuron), antibiotics (abamectin) and insect repellents such as diethyltoluamide (DEET) and citronella (natural origin).
03Organochlorides
The organochlorides are insecticides that contain carbon (organo-), hydrogen and chlorine. They are also referred to as chlorinated hydrocarbons, chlorinated organics, chlorinated insecticides and chlorinated synthetics. The organochlorines are now primarily of historic interest, since few members of this group survived to date.
In humans, these substances and/or their metabolites act primarily at the level of the CNS, causing changes in its electrophysiological properties and enzymatic neuronal membranes. This results in alterations in the kinetics of Na⁺ and K⁺ through the membrane of the nerve cell, leading to the discharge of multiple action potentials for each stimulus — triggering symptoms such as seizures and acute poisoning, and death from respiratory depression (arrest).
Organochlorines are lipophilic and persist in body fat. Acute poisoning presents with CNS excitation, tremors, convulsions and respiratory failure. There is no specific antidote — treatment is supportive, with emphasis on decontamination, seizure control and airway management.
04Diphenyl aliphatics
Dr. Paul Müller, a Swiss entomologist, was awarded the Nobel Prize in Medicine for his life-saving discovery of dichloro-diphenyl-trichloroethane (DDT) as an insecticide useful in the control of malaria, yellow fever, dengue and many other insect-vectored diseases.
The diphenyl aliphatics are the oldest group of organochlorine insecticides. They include:
- Dichloro-diphenyl-trichloroethane (DDT)
- Dichloro-diphenyl-dichloroethane (DDD)
- Dicofol
- Ethylan
- Chlorobenzilate
- Methoxychlor
Dichloro-diphenyl-trichloroethane (DDT)
DDT is probably the best known and one of the most useful insecticides ever developed, and is still effectively used for malaria control in several third-world countries.
Mechanism of action
The mode of action for DDT has never been clearly established; however, through some complex processes, DDT destroys the delicate balance of sodium and potassium ions within the axons of the neuron in a way that prevents normal transmission of nerve impulses, both in insects and mammals. It apparently acts on the sodium channel to cause "leakage" of sodium ions. Eventually the affected neurons fire impulses spontaneously, causing the muscles to twitch — "DDT jitters" — followed by convulsions and death.
DDT has a negative temperature correlation: the lower the surrounding temperature, the more toxic it becomes to insects.
05Hexachlorocyclohexane (HCH)
Also called benzene hexachloride (BHC), HCH is known to exhibit insecticidal properties attributed to the gamma isomer alone, out of its five isomeric forms — namely alpha, beta, gamma, delta and epsilon. Consequently, the gamma isomer was isolated, manufactured and sold as the odourless insecticide lindane.
In contrast, technical-grade HCH has a strong musty odour and flavour which can be imparted to treated crops and animal products. HCH is cheap and is still used in many developing countries.
Mechanism of action
The effects of HCH superficially resemble those of DDT, but occur much more rapidly, and result in a much higher rate of respiration in insects. The gamma isomer is a neurotoxicant, whose effects are normally seen within hours as increased activity, tremors and convulsions leading to prostration. HCH also exhibits a negative temperature correlation, but not as pronounced as that of DDT.
06Cyclodienes
The cyclodienes include chlordane, aldrin, dieldrin, heptachlor, endrin, mirex, endosulfan and chlordecone. Most of the cyclodienes are persistent insecticides — stable in soil and relatively stable to the ultraviolet light of sunlight.
These properties made the cyclodienes choice soil insecticides (especially chlordane, heptachlor, aldrin and dieldrin) for the control of termites and soil-borne insects whose larval stages feed on the roots of plants.
To appreciate the effectiveness of these cyclodienes as termiticides, evidence has shown that wood and wooden structures treated with chlordane, aldrin and dieldrin in the year of their development are still protected from damage after more than 60 years. The cyclodienes were the most effective, long-lasting and cost-effective termiticides ever developed.
However, because of their persistence in the environment (non-biodegradable), resistance that developed in several soil insects and pests, and in some instances biomagnification in wildlife food chains, most agricultural uses of cyclodienes as termiticides were banned.
Mechanism of action
Unlike DDT and HCH, the cyclodienes have a positive temperature correlation — their toxicity increases with increasing ambient temperature.
Cyclodiene modes of action are also not clearly understood; however, it is known that these groups of compounds act on the inhibitory mechanism called the GABA (γ-aminobutyric acid) receptors. Pharmacologically, these GABA receptors are activated by increasing chloride ion permeability of neurons. Cyclodienes block chloride ion influx into the neurons, thereby antagonising the "calming" effects of GABA. Cyclodienes appear to affect all animals similarly, first with the nervous activity, followed by tremors, convulsions and prostration.
07Organophosphates (OPs)
Organophosphates (OPs) are the broad term for all insecticides containing phosphorus. All organophosphates are esters derived from phosphoric acid, and as a class are generally the most toxic of all pesticides to vertebrates. Because of the similarity of OP chemical structures to the "nerve gases," their modes of action are also similar. Their insecticidal qualities were first observed in Germany during World War II in the study of the extremely toxic nerve gases — sarin, soman and tabun.
Two distinctive features
- They are generally more toxic to vertebrates than other classes of insecticides.
- Most OPs are chemically unstable (non-persistent) and effective through ingestion and contact. It is this latter characteristic that brought them into agricultural use as substitutes for the persistent organochlorines.
Action in humans
In humans, OPs act on the CNS by inhibiting acetylcholinesterase, an enzyme that modulates the amount and levels of the neurotransmitter acetylcholine, disrupting the nerve impulse by serine phosphorylation of the hydroxyl group in the active site of the enzyme. Common symptoms are loss of reflexes, headache, dizziness, nausea, convulsions, coma and even death.
Mechanism of action
The OPs cause inhibition or blockade of cholinesterase (ChE). This enzyme is phosphorylated when it becomes bonded irreversibly to the phosphorus moiety of the OP insecticide. This inhibition results in the accumulation of acetylcholine (ACh) at the neuron/neuron and neuron/muscle (neuromuscular) junctions or synapses, causing rapid twitching of voluntary muscles and finally paralysis.
Acute OP poisoning presents with a cholinergic crisis: miosis, salivation, lacrimation, urination, diarrhoea, bronchospasm and bradycardia (the "SLUD" syndrome), progressing to muscle fasciculations, convulsions and respiratory failure. Atropine reverses muscarinic effects; pralidoxime (2-PAM) reactivates cholinesterase if given early.
08OP classification
All OPs are esters of phosphorus having varying combinations of oxygen, carbon, sulfur and nitrogen attached, thus creating six different sub-classes:
- Phosphates
- Phosphonates
- Phosphorothioates
- Phosphorodithioates
- Phosphorothiolates
- Phosphoramidates
These sub-classes are easily identified by their chemical names. The OPs are generally divided into three groups:
- Aliphatic derivatives
- Phenyl derivatives
- Heterocyclic derivatives
Aliphatic derivatives
The aliphatic OPs are carbon chain-like in structure. The first OP brought into agricultural practice was tetraethyl pyrophosphate (TEPP). Other examples are malathion, trichlorfon, monocrotophos, dimethoate, oxydemetonmethyl, dicrotophos, disulfoton, dichlorvos, mevinphos, methamidophos and acephate.
Phenyl derivatives
The phenyl OPs contain a phenyl ring with one of the ring hydrogens displaced by attachment to the phosphorus moiety and other hydrogens frequently displaced by Cl, NO₂, CH₃, CN or S. The phenyl OPs are generally more stable than the aliphatics, thus their residues are longer lasting. The first phenyl OP brought into agriculture was parathion (ethyl parathion) in 1947. Examples of other phenyl OPs are methyl parathion, profenofos, isofenphos, fenitrothion, fenthion and famphur.
Heterocyclic derivatives
These OPs contain ring structures that are composed of different atoms — oxygen, nitrogen or sulfur. Some examples include diazinon, azinphos-methyl, azinphos-ethyl, chlorpyrifos, methidathion, phosmet, isazophos and chlorpyrifos-methyl.
09Pyrethrum
Pyrethrum is extracted from the flowers of a chrysanthemum grown in Kenya and Ecuador. It is one of the oldest and safest insecticides available. The ground, dried flowers were used in the early 19th century as the original louse powder to control body lice in the Napoleonic Wars.
Pyrethrum acts on insects with phenomenal speed, causing immediate paralysis — hence its popularity in fast-knockdown household aerosols. However, unless it is formulated with one of the synergists, most of the paralysed insects recover to once again become pests. Pyrethrum is a mixture of four compounds: pyrethrins I and II, and cinerins I and II.
10Pyrethroids
Pyrethroids are generally effective against most agricultural insect pests when used at the very low rates of 0.01 to 0.1 pound per acre. The pyrethroids have an interesting evolution, conveniently divided into four generations.
The first generation
The first generation contains only one pyrethroid — allethrin. Its synthesis was very complex, involving 22 chemical reactions to reach the final product.
The second generation
The second generation includes tetramethrin, resmethrin (20× as effective as pyrethrum), bioresmethrin (50× as effective as pyrethrum), bioallethrin and phonothrin.
The third generation
The third generation includes fenvalerate and permethrin. These became the first agricultural pyrethroids because of their exceptional insecticidal activity and photostability. They were virtually unaffected by the ultraviolet rays of sunlight, lasting 4–7 days as efficacious residues on crop foliage.
The fourth and current generation
The fourth and current generation is truly exciting because of their effectiveness. These include bifenthrin, lambda-cyhalothrin, cypermethrin, cyfluthrin, esfenvalerate, fenpropathrin, flucythrinate, fluvalinate, prallethrin, tau-fluvalinate, tralomethrin and zeta-cypermethrin.
All of these agents are photostable and may never undergo photolysis (splitting) in sunlight. And because they have minimal volatility, they provide extended residual effectiveness — up to 10 days under optimum conditions.
Mechanism of action
The pyrethroids share similar modes of action, resembling that of DDT, and are considered axonic poisons. These agents apparently act by enhancing the continuous opening of the sodium channels in neuronal membranes.
Types of pyrethroids
- Type I — among other physiological responses, has a negative temperature coefficient, resembling that of DDT.
- Type II — in contrast, has a positive temperature coefficient, showing increased kill with increase in ambient temperature.
Pyrethroids affect both the peripheral and central nervous system of the insect. They initially stimulate nerve cells to produce repetitive discharges and eventually cause paralysis. The stimulating effect of pyrethroids is much more pronounced than that of DDT.
11Quick-reference comparison table
The table below summarises the key characteristics of the major insecticide classes covered in this guide.
| Class | Examples | Mechanism of action | Temperature correlation |
|---|---|---|---|
| Diphenyl aliphatics | DDT, DDD, dicofol, methoxychlor | Na⁺ channel "leakage" → spontaneous neuron firing | Negative |
| HCH / Lindane | Gamma isomer of benzene hexachloride | Neurotoxicant — rapid tremors and convulsions | Negative (mild) |
| Cyclodienes | Chlordane, aldrin, dieldrin, endosulfan | GABA receptor antagonism — blocks Cl⁻ influx | Positive |
| Organophosphates | Parathion, malathion, chlorpyrifos, diazinon | Irreversible cholinesterase inhibition → ACh accumulation | — |
| Pyrethrum (natural) | Pyrethrins I & II, cinerins I & II | Axonic poison — fast knockdown | — |
| Pyrethroids — Type I | Allethrin, tetramethrin, resmethrin | Na⁺ channel enhancement | Negative |
| Pyrethroids — Type II | Permethrin, cypermethrin, deltamethrin | Na⁺ channel enhancement (with CNS effects) | Positive |
12Key takeaways
Insecticides remain essential tools in agriculture, public health, veterinary medicine and household pest control. Yet their toxicology is unforgiving — a fact that the history of organochlorines and organophosphates makes clear.
- Organochlorines were the first major class. Their environmental persistence and biomagnification led to widespread bans, though DDT remains in limited use for malaria vector control.
- Organophosphates are the most acutely toxic class to vertebrates. Their mechanism — cholinesterase inhibition — is shared with nerve agents, making them a both valuable and dangerous group.
- Pyrethroids are today's workhorse class. Low application rates, photostability and low mammalian toxicity have made them the preferred choice for most agricultural and public-health uses.
- Mechanism matters. Understanding whether an insecticide targets sodium channels, GABA receptors or cholinesterase determines both its clinical toxicology and its antidotal treatment.
Insecticides must be handled with strict adherence to safety protocols. Always read the label, use appropriate personal protective equipment, observe pre-harvest intervals, and never exceed recommended application rates. When poisoning is suspected, seek immediate medical attention.
13Authoritative resources
For regulatory guidance, safety data sheets and poisoning management protocols, consult authoritative sources.
14About this guide
This guide is an educational compilation based on lecture material from the Department of Pharmacology and Toxicology, Faculty of Pharmacy, Niger Delta University, Wilberforce Island, Amasoma, Bayelsa State. It is intended for pharmacy students, veterinary learners and agrochemical practitioners.
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