Malathion

Malathion is a widely used organophosphate insecticide employed in agriculture, public health, and domestic pest control. It is known for its relatively low mammalian toxicity compared to other organophosphates and its broad-spectrum activity against a variety of insect pests. Malathion is particularly valued in programmes aimed at controlling mosquitoes, fruit flies, and agricultural pests, and has also been used in head lice treatments and vector-borne disease control campaigns.

Chemical Composition and Properties

Malathion, chemically designated as diethyl (dimethoxythiophosphorylthio) succinate, has the molecular formula C₁₀H₁₉O₆PS₂ and a molecular weight of approximately 330.35 g/mol. It appears as a yellow to brown oily liquid with a characteristic odour resembling that of mercaptans.
Key physicochemical properties include:

• Boiling point: 156–157°C (at reduced pressure)
• Vapour pressure: 1.78 × 10⁻⁴ mmHg at 30°C
• Solubility: Slightly soluble in water but readily soluble in organic solvents such as acetone, xylene, and ethanol
• Stability: Malathion is unstable in alkaline conditions and undergoes hydrolysis, while it is relatively stable in acidic or neutral environments.

It belongs to the organophosphorus group of insecticides, acting as an acetylcholinesterase inhibitor, which disrupts the normal function of the nervous system in target pests.

Mechanism of Action

Malathion exerts its toxic effect by inhibiting the enzyme acetylcholinesterase (AChE) at nerve synapses. Normally, this enzyme hydrolyses the neurotransmitter acetylcholine, allowing proper nerve impulse transmission. In the presence of malathion, acetylcholine accumulates at nerve endings, causing continuous nerve stimulation, paralysis, and ultimately death in insects.
In mammals and humans, malathion is metabolised more rapidly to non-toxic compounds, which accounts for its comparatively lower toxicity. However, its metabolite malaoxon—formed through oxidation—is significantly more toxic and responsible for many of the adverse effects observed during overexposure.

Historical Background and Development

Malathion was first synthesised in the 1950s by American Cyanamid Company as part of the search for safer alternatives to earlier organophosphate compounds such as parathion and TEPP. Its introduction marked a shift towards less hazardous chemical pest control agents, suitable for both agricultural and public health uses.
Since its registration, malathion has been extensively used under various trade names including Cythion, Fyfanon, and Maldison. It has featured prominently in vector control campaigns by the World Health Organization (WHO), especially for malaria and dengue prevention through mosquito control.

Applications and Uses

Agricultural Use

Malathion is applied to control a wide range of pests on fruits, vegetables, grains, cotton, and ornamental plants. It is effective against aphids, thrips, leafhoppers, fruit flies, and beetles. Application methods include foliar sprays, dust formulations, and aerosol treatments.

Public Health and Vector Control

In public health, malathion is employed in mosquito abatement programmes, especially during outbreaks of diseases such as malaria, dengue, and West Nile virus. It is used for space spraying and ultra-low volume (ULV) applications to control adult mosquito populations.

Domestic and Veterinary Use

Malathion-based formulations are used in head lice and scabies treatments under controlled medical supervision. In veterinary settings, it is occasionally applied in formulations for ectoparasite control in livestock and poultry.

Quarantine and Eradication Programmes

Malathion plays a crucial role in fruit fly eradication campaigns, particularly against the Mediterranean fruit fly (Ceratitis capitata) and Oriental fruit fly (Bactrocera dorsalis). It is applied in bait sprays combining protein hydrolysates to attract and kill adult flies.

Toxicity and Health Effects

Although malathion is classified as moderately hazardous (Class III) by the World Health Organization, exposure can still pose risks to human health, particularly through inhalation, ingestion, or dermal absorption.
Symptoms of acute malathion poisoning may include:

• Headache, dizziness, and nausea
• Muscle twitching and weakness
• Excessive salivation and sweating
• Constricted pupils (miosis)
• Respiratory distress in severe cases

In chronic exposure, effects on the nervous system and liver function have been documented. However, due to its rapid metabolic breakdown, malathion is considered safer than many other organophosphates such as parathion or chlorpyrifos.
Treatment for poisoning involves atropine and oxime reactivators (such as pralidoxime), which counteract cholinergic overstimulation.

Environmental Behaviour and Ecological Impact

Malathion is relatively short-lived in the environment, undergoing degradation through hydrolysis, photolysis, and microbial activity. Its half-life in soil ranges from 1 to 25 days, depending on temperature, pH, and microbial presence.
However, malathion and its metabolites can be highly toxic to aquatic organisms, especially fish and invertebrates. Birds are generally less sensitive, but honeybees and other pollinators are susceptible to acute exposure. Consequently, environmental application guidelines stress the avoidance of contamination of water bodies and blooming crops.

Regulation and Safety Standards

Regulatory authorities worldwide have set maximum residue limits (MRLs) for malathion in food and established acceptable daily intake (ADI) levels for humans.

• The World Health Organization (WHO) and Food and Agriculture Organization (FAO) recommend an ADI of 0.02 mg/kg body weight.
• In the European Union, malathion use is tightly restricted, while in the United States, it remains approved under the Environmental Protection Agency (EPA) with prescribed safety measures.
• In India, malathion is registered under the Insecticides Act, 1968, and extensively used in National Vector Borne Disease Control Programme (NVBDCP) initiatives.

Strict adherence to application rates, pre-harvest intervals, and personal protective equipment (PPE) is required to ensure operator safety and environmental protection.

Degradation, Formulations, and Storage

Commercial malathion is available in several formulations including emulsifiable concentrates (EC), wettable powders (WP), dusts, and ULV sprays. Its efficacy depends heavily on formulation stability and purity.
Degradation occurs through oxidation to malaoxon and hydrolysis of ester bonds, especially in alkaline environments. Hence, it must be stored in cool, dry, and acidic conditions, away from metals and strong oxidisers, to prevent decomposition.

Advantages and Limitations

Advantages:

• Broad-spectrum insecticidal activity.
• Low mammalian toxicity relative to other organophosphates.
• Rapid environmental degradation reduces long-term persistence.
• Cost-effective and compatible with integrated pest management (IPM) strategies.

Limitations:

• Development of insect resistance with prolonged use.
• High toxicity to aquatic life and beneficial insects.
• Odour and chemical instability under alkaline conditions.
• Health hazards from improper handling or overexposure.

Modern Trends and Alternatives

In recent years, the use of malathion has declined in favour of safer and more target-specific insecticides such as neonicotinoids, insect growth regulators (IGRs), and biopesticides. However, it continues to play a vital role in emergency pest eradication and vector control due to its effectiveness, availability, and cost efficiency.
Research into nanoformulations and controlled-release delivery systems seeks to enhance malathion’s efficiency while minimising environmental contamination. Additionally, integrated approaches combining chemical, biological, and environmental management are being promoted for sustainable pest control.