What is Methomyl?

Methomyl is a water-soluble crystalline solid that gives off a sulfurous odor. Highly toxic, it is classified as a carbamate insecticide that is designated as being a Restricted use Pesticide (RUP) by the U.S. Environmental Protection Agency (EPA). Since the late 1960s, the substance has been used as a pesticide on commercial fruit and vegetable crops as well as stored products. Its application as an insecticide is highly effective against a wide variety of pests, particularly those that are resistant to organophosphorus pesticides.

Initially, Dupont registered methomyl with the EPA for use as an insecticide for commercially grown chrysanthemums in 1968. However, its use soon gained favor to help protect livestock and even commercial real estate from pests. Since that time, it has been used to treat almost all commercial production of lettuce, rhubarb, asparagus, artichokes and pomegranates in the U.S.

While there have been very few clinical studies conducted to determine the potential adverse effects of methomyl on human health, it is reasonable to assume that information collected from animal-based models can be extrapolated to people. For this reason, modifications on the regulated usage of methomyl have taken place since the mid-1990s. For example, it is no longer to be used in greenhouses or as an additive to fly bait. In addition, it is now required that a bitter agent be added to preparations to discourage children from accidentally ingesting them.

Methomyl is readily absorbed through the skin and by inhalation of fine particles. Its mechanism of action is to inhibit cholinesterase, an enzyme produced in the liver that regulates nervous system functioning. The telltale symptoms of toxicity by this route are uncontrolled muscle movements, spasms, convulsions, etc. The insecticide is also absorbed through the intestinal tract. In fact, ingestion of this substance equates to a fast-acting poison in both humans and animals. However, if the ingested dose is not too high and action is taken quickly, poisoning may be counterchecked by one or more injections of atropine. Metabolism of methomyl occurs by hydrolysis before it degrades into the byproducts carbon dioxide and acetonitrile.

This substance is also commonly referred to as metomil or mesomile, although it’s proper chemical name is S-methyl N-(methylcarbamoyloxy) thioacetimidate. It is also known by many trade names.

As might be expected, methomyl products carry the crossbones and skull insignia on the label. For workers who come into contact with this insecticide, it is necessary to wear protective clothing, gloves and eye wear, as well as breathe through an organic-vapor respirator.