What is methomyl?

Methomyl: A Highly Toxic Carbamate Insecticide

Methomyl is a highly toxic oxime carbamate insecticide that inhibits acetylcholinesterase (AChE) activity, resulting in nerve and tissue failure with potentially fatal consequences in cases of human exposure. 1

Chemical Properties and Uses

• Methomyl was first introduced in 1968 as a broad-spectrum insecticide for controlling various insect classes including Lepidoptera, Hemiptera, Homoptera, Diptera, and Coleoptera 1
• It has moderate-to-high water solubility and low-to-moderate soil sorption capacity, making it potentially mobile in the environment 1
• It has low vapor pressure, meaning volatilization is not a major dissipation route from water or soil 1
• Methomyl is classified as a restricted-use insecticide due to its high toxicity to multiple non-target species 1

Mechanism of Action

• Like other carbamates, methomyl acts by inhibiting acetylcholinesterase (AChE) activity in the nervous system 1
• This inhibition leads to accumulation of acetylcholine at nerve synapses, resulting in overstimulation of cholinergic receptors 1
• The inhibition of AChE by carbamates is generally reversible, unlike organophosphate compounds 1

Toxicity Profile

• Methomyl is highly toxic to humans via oral exposure, moderately toxic via inhalation, and slightly toxic via dermal exposure 1
• It is classified as highly toxic to insects, fishes, birds, and mammals 1
• It is very highly toxic to aquatic invertebrates 1
• At high doses, methomyl exposure can be fatal to humans 1

Clinical Manifestations of Poisoning

• Methomyl poisoning can present with severe cholinergic crisis including unconsciousness, hypothermia, and shock 2
• Neurological complications may include:
  • Cortical blindness (documented in case reports) 2, 3
  • Delayed peripheral neuropathy 2
  • Decorticate posture (abnormal posturing indicating severe brain injury) 3
• Cardiovascular effects can include pulseless electrical activity requiring resuscitation 2

Environmental Fate

• Hydrolysis under alkaline conditions represents a major degradation pathway due to methomyl's water solubility 1
• Microbial degradation is significant, with certain bacterial species capable of using methomyl as a carbon and/or nitrogen source 1, 4
• Main degradation products include methomyl oxime, acetonitrile, and CO₂ 1
• Several bacterial genera have been identified as methomyl-degraders, including Paracoccus, Pseudomonas, Aminobacter, Flavobacterium, Alcaligenes, Bacillus, Serratia, Novosphingobium, and Trametes 4

Agricultural Applications

• Methomyl can be applied as both a foliar spray and via soil application through drip chemigation 5
• Soil applications have shown effectiveness against both soil nematodes and foliar insect pests 5
• It has demonstrated control of various pests including aphids (Myzus persicae, Aphis gossypii), thrips (Frankliniella occidentalis), and caterpillars (Spodoptera exigua) 5

Safety Considerations

• Due to its high toxicity, methomyl requires careful handling and application 1
• Despite its widespread agricultural use and high water solubility, it has been infrequently detected as a water contaminant in the USA 1
• Great care should be taken when applying methomyl-containing products to prevent non-target species toxicity or environmental contamination 1