Toxicologic Information About Insecticides
Used for Eradicating Mosquitoes
(West Nile Virus Control)
April 2005
The organophosphate insecticides (fenthion, malathion, naled, and temephos) summarized in this document are used as adulticides for agricultural and domestic control of insects. They are all effective in eradicating mosquitoes by inhibiting acetylcholinesterase in the targeted insects Temephos is used as a larvicide for mosquito control. The persistence of organophosphate insecticides varies, by individual chemical and physical properties. Aerial application of these insecticides may result in oxidation in air to their more potent oxon analogs. The organophosphate insecticides also inhibit acetylcholinesterase in humans and animals, which can lead to cholinergic poisoning. Depending on the level and duration of exposure, symptoms can range from mild (headache, drowsiness) to moderate (difficulty breathing, cardiac arrhythmias, confusion) to lifethreatening (coma, seizures, paralysis). The organophosphate insecticides have varying degrees of toxicity, with temephos and malathion appearing to be the least toxic, followed by naled, and fenthion. For example, although no MRLs or RfDs are available for temephos, humans tolerated doses as high as 256 mg/day for 5 days or 64 mg/day for 4 weeks with no symptoms and no effects on plasma or erythrocyte cholinesterase. Malathion has relatively low toxicity to humans, other mammals, and birds compared with other organophosphate insecticides The chronic oral MRL and RfD for malathion are 0.02 mg/kg/day, whereas the RfD for fenthion is 0.00007 mg/kg/day.
Resmethrin, phenothrin, and permethrin are synthetic analogues (pyrethroids) of original pyrethrins, which are natural extracts in the flowers of the chrysanthemum plant. They typically are used as insecticides for both home and commercial use and are the most common pyrethroids used for mosquito control. They are not persistent in the environment. They act by rapidly paralyzing flying insects. Almost all systemic effects are related to the action of pyrethrins and pyrethroids on the nervous system. These chemicals exert their profound effect by prolonging the open phase of the voltage-gated sodium channels when a nerve cell is excited. They have relatively low mammalian toxicity, a rapid rate of degradation in the environment, and a typical use as insecticides for both home and commercial use. The RfDs for resmethrin and permethrin have the same order of magnitude as malathion: 0.03 mg/kg/day for resmethrin and 0.05 mg/kg/day for permethrin. There is not RfD for phenothrin, but WHO established an ADI of 0–0.07 mg/kg/day.
Methroprene is an insect growth regulator used as a larvicide. It is a synthetic analogue of the insect juvenile hormone that has very low toxicity for animals and humans. Unlike conventional insecticides that act as direct poisons, methoprene acts by disrupting the morphologic development of insects. It rapidly degrades in the environment, but it may accumulate in aquatic organisms. People may be exposed to small amounts of methoprene through the food supply, but the amount of methoprene in the U.S. consumer’s diet is well below the level at which any adverse health effects could occur.
More than one of these or other insecticides may be used simultaneously, and exposure to several insecticides can occur simultaneously. Thus, joint toxic action, either by additivity or synergism may be a concern.
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