Pesticide Profile - Methamidophos

Quick Facts

• Chemical name: O,S-Dimethylphosphora-midothiolate

• Trade names: Monitor (only registered product in the US), Nitofol, Tamaron, Swipe, Patrole, Filitox, Acephatemet

• Pesticide type: systemic insecticide, acaricide

• Pesticide class: organophosphate

• Mechanism of action: cholinesterase inhibitor

• Use: registered as a restricted-use product in the U.S. on cotton, potatoes, and tomatoes. Approximately 1.1 million pounds of methamidophos are applied in the U.S., annually.

• Methamidophos is included in the Prior Informed Consent (PIC) procedure. Sponsored by the Food and Agriculture Organization (FAO) of the United Nations Environment Program (UNEP), PIC monitors the sale and use of the most dangerous pesticides on a global scale. Methamidophos was added to the list because of its extreme toxicity to humans through oral, dermal, and inhalation routes of exposure.

• Methamidophos is a major degradate of acephate, another organophosphate pesticide. Toxic effects noted for methamidophos must be considered when using acephate.

• Methamidophos is persistent in many aquatic systems, and among the most toxic of all pesticides to aquatic arthropods, including shrimp and insects.

• Methamidophos has caused chromosome aberrations in mice.

Chemical Structure

Background

Methamidophos is a broad-spectrum systemic and contact insecticide registered to control a variety of plant and soil insects in cotton, tomatoes, and potatoes. It is registered as a restricted use pesticide and can only be applied by a licensed professional. Formulated as an emulsifiable concentrate, methamidophos is applied as a foliar treatment during the growing season. The systemic effects of methamidophos may extend to two weeks after initial application.

Environmental Effects

Fate

• Persistence: The data describing the degradation and fate processes of methamidophos are incomplete. In soil, methamidophos has been observed to degrade rapidly with a half-life of one day or less in aerobic soils. However, methamidophos has been detected in groundwater, which suggests that the degradation time is at least long enough for the pesticide to leach into underground water supplies. In aerobic soils, the half-life of methamidophos is as follows: 1.9days in silt, 4.8 days in loam, 6.1 days in sand, and 10-12 days in sandy loam.  Persistence in aquatic systems varies with conditions. The half-life of Methamidophos under acidic conditions is up to 309 days. Hydrolysis at pH 7 occurs with a half-life of 27 days. Methamidophos is resistant to photodegradation in water. The data presented to the EPA by the registrant for methamidophos was insufficient to assess the real potential for persistence in the environment. No field studies were submitted. Very limited metabolism studies were presented. Allusions as to the "long lasting biological effect" were made in EPA documents, yet no studies documenting the effects were cited. No studies examining degradates of methamidophos have been submitted to the EPA.

• Mobility: Methamidophos is highly mobile in soil.

• Solubility: highly soluble in water.

• Bioaccumulation: The potential for methamidophos to bioaccumulate is extremely low.

Ecotoxicity 

• Methamidophos is very highly toxic to birds on an acute, oral basis. Risk Quotients estimated for birds utilizing agricultural areas after methamidophos use range from 5.6 to 12.2 times the level of concern for acute risk to small, insectivorous birds. Representative acute oral LD 50's:

  red-winged blackbird                1.78 mg/kg
         common grackle                        6.7 mg/kg
         starling                                   10.0 mg/kg
         mallard:                                  8.48 mg/kg

  Chronic effects have been noted when sub-lethal doses are administered to birds. Laboratory studies received by the EPA have found that methamidophos has been found to cause thinning of eggshells at very low concentrations when fed to caged birds.

Incidents

• Extensive die-offs of sage grouse from 1983 to present, have been documented in the potato-growing region of the Northwest. An investigation revealed that bird kills are common among farmers in the Northwest when using methamidophos, but the incidents are rarely reported.

• An incident where 200 European starlings were found dead in a cauliflower field sprayed with methamidophos was reported to the EPA in 1985. The starlings were killed when they ingested contaminated invertebrates.