Pesticide Profile - Malathion

Quick Facts

• Chemical Name: Diethyl mercaptosuccinate

• Trade Names: Carbophos, Maltox, Celthion, Cythion,Mercaptothion

• Pesticide Type: insecticide; mosquito adulticide

• Class: organophosphate

• Use: Malathion is widely used as a mosquito adulticide. The U.S. Department of Agriculture uses malathion in many special control programs: bollworm eradication efforts in the Southern U.S., Mediterranean fruit fly control in California and Florida, and grasshopper control on Federal Rangelands. Malathion is used for the control of insects on many agricultural crops, and is formulated for use by homeowners on lawns and gardens. Over 7 million pounds of malathion (active ingredient) are used, annually, in the U.S.

Chemical Structure

Background

Malathion has been used in the United States for 45 years. Over 235 products containing malathion are registered in the U.S. The percent of active ingredient in the formulations ranges from 3% in some of the homeowner-use products to over 96% in the Ultra Low Volume (ULV) formulations used as mosquitocides. Malathion is formulated as an emulsfiable concentrate, wettable powder, dustable powder, and liquid ULV formulation.

A relatively low toxicity when compared to other organophosphates, combined with a rather short persistence in the environment has led to extensive use of malathion in many government controlled eradication programs. Most notably, California's efforts to control the Mediterranean fruit fly and efforts by southern U.S. states to control the bollworm, have relied extensively on malathion. Malathion is used extensively in the control of adult mosquitoes, especially in cases where public health concerns about mosquito vectored diseases are a factor.

Malathion, along with other phosphorodithioate insecticides (those containing two sulfur atoms bonded to phosphorous) must be oxidized before they have inhibitory potency and toxicity. Oxidation may occur in the environment or after malathion is absorbed into the body of a live organism. The active cholinesterase inhibiting oxidation product of malathion is called malaoxon.

Malathion gives off an unpleasant odor when sprayed, which sometimes inhibits its use as a mosquito adulticide. In many cases, pyrethrins and pyrethroids are used instead of malathion, however, in some instances, mosquito abatement personnel will choose to spray more environmentally harmful pesticides such as fenthion (in Florida), or naled, because people tend to object to the smell of malathion.

Environmental Effects

Fate

• Persistence: malathion is characterized by low persistence in soils, with half-lives ranging from 1 to 25 days, depending on soil type. Malathion persistence is greatest in sandy soils with few microorganisms.The reported half-life in river water is one week. Malathion is more stable under acidic aquatic conditions.

• Solubility: Soluble in water, 130 mg/L.

• Bioaccumulation: Bioconcentration factors (BCF) for fish are relatively low, when tested under laboratory conditions. The BCF for shrimp is 959. Malathion does not seem to bioconcentrate at high levels.

• Mobility: Malathion is highly mobile in soil, but due to its relatively low persistence values, there does not seem to be much risk of contamination of underground water supplies.

Ecotoxicity 

• Malathion is only slightly toxic to mammals, using EPA standards of determining acute toxicity.

• Malathion can be highly toxic to fish and other aquatic organisms. Acute toxicity ranges from very-highly toxic in walleye and trout, moderately toxic to fathead minnows, to slightly toxic in goldfish. Malathion is highly toxic to larval stages of amphibians and also highly toxic to aquatic invertebrates.

• Malathion is slightly to moderately toxic to birds, with an acute oral LD50 for mallards of 1485 mg/kg, and 100 mg/kg for starlings. Malathion is one of the best-tolerated organophosphates by birds.