Malathion is an insecticide used in three main settings: public health mosquito control, prescription head lice treatment, and home garden pest management. It belongs to a class of chemicals called organophosphates, which kill insects by disrupting their nervous systems. It’s one of the most widely applied insecticides in the United States, used by everyone from municipal mosquito abatement programs to backyard gardeners.
Mosquito Control Programs
Malathion’s highest-profile use is in public health campaigns to control mosquitoes that carry diseases like West Nile virus, Zika virus, and dengue. It works as an “adulticide,” meaning it kills adult mosquitoes rather than targeting larvae in standing water. About 90% of mosquito-control spraying is done from the ground, using fogging equipment mounted on trucks that drive through neighborhoods, typically in the evening when mosquitoes are most active.
When a large outbreak threatens a wide area, malathion can also be sprayed from aircraft. Aerial spraying accounts for less than 1% of all mosquito applications, but it’s the fastest way to knock down mosquito populations across many square miles at once. Both methods use ultra-low volume (ULV) technology, which disperses extremely fine droplets, smaller than the width of a human hair, so that tiny amounts of the chemical can cover large areas.
Prescription Head Lice Treatment
Malathion is available as a 0.5% lotion (brand name Ovide) to treat head lice. It kills live lice and some lice eggs, though not all of them, so a second application is often needed 7 to 9 days later if live lice are still present. It’s approved for use in people six years of age and older.
One important safety note: the lotion is flammable. You should not smoke, use a hair dryer, curling iron, or flat iron while the lotion is on the hair or while the hair is still wet. It can also irritate the skin in some people. Malathion lotion for lice is a prescription product, not something you’d pick up over the counter.
Garden and Landscape Pest Control
Consumer-grade malathion products (commonly sold as 50% concentrate sprays) are registered for use on a wide range of vegetable and ornamental plants. The list of target pests is long, but the most common ones home gardeners encounter include aphids, spider mites, whiteflies, Japanese beetles, cucumber beetles, thrips, mealybugs, and various caterpillars.
In the vegetable garden, malathion is labeled for use on tomatoes, beans, peppers, squash, cucumbers, melons, corn, lettuce, broccoli, cabbage, carrots, potatoes, and many other crops. Each crop has its own list of target insects. Beans, for instance, can be treated for Mexican bean beetles, spider mites, Japanese beetles, aphids, and cucumber beetles. Corn applications target aphids, sap beetles, thrips, grasshoppers, and corn rootworm adults.
For ornamental plants like roses, chrysanthemums, and flowering shrubs, malathion controls aphids, mealybugs, spider mites, whiteflies, bagworms, tent caterpillars, and a variety of scale insects. It’s also used on woody shrubs and vines for pests like birch leaf miner, boxwood leaf miner, and European pine shoot moth.
How Quickly It Breaks Down
Malathion does not persist in the environment for long compared to many other pesticides. In soil, microbes break it down with a half-life of less than 1 to 2.5 days under typical conditions. In water, half-lives range from about 0.3 to 6 days depending on the acidity of the water. Alkaline (high pH) water speeds up breakdown dramatically: at pH 9, malathion’s half-life from chemical breakdown alone is just 12 hours, while in acidic water at pH 5, it can linger for over 100 days.
On hard surfaces, breakdown times vary. On concrete, malathion disappears in as little as a few hours. On sand, it can take 6 to 13 days. On soil surfaces exposed to the elements, it typically lasts 3 to 4 days. This relatively fast breakdown is one reason malathion is favored for residential mosquito spraying: it doesn’t accumulate in the environment the way older pesticides did.
Growing Resistance in Insects
One challenge with malathion is that many insect populations have developed resistance to it over decades of use. Insects evolve resistance primarily by producing higher levels of enzymes that break down the chemical before it can kill them. Grain aphids, green peach aphids, cotton aphids, and brown planthoppers have all developed significant resistance through this mechanism. Field populations of green peach aphids in some areas have shown resistance levels 32 times higher than susceptible populations.
This resistance is one reason malathion is used as part of integrated pest management strategies rather than as a standalone solution. Mosquito control programs, for example, combine malathion spraying with larval control, habitat reduction, and public education to keep resistance from undermining the program’s effectiveness.
Toxicity to Humans and Animals
Malathion is considered relatively low in toxicity compared to other organophosphates. In laboratory studies, the lethal dose in rats ranges from about 1,500 to over 8,000 milligrams per kilogram of body weight when ingested, placing it in a lower toxicity category than many related chemicals. Skin absorption is even less of a concern, with dermal lethal doses above 2,000 mg/kg in rats and nearly 9,000 mg/kg in rabbits.
That said, “low toxicity” is relative. Malathion still works by inhibiting a key enzyme in the nervous system, and overexposure can cause symptoms like headache, nausea, dizziness, muscle twitching, and in severe cases, difficulty breathing. The risk to most people from properly applied malathion, whether from a passing mosquito truck or a garden sprayer, is very low. The concentrations used in ULV mosquito spraying are far below levels that would affect human health.