Pesticide Profile - Fenamiphos

Quick Facts:

• Chemical name: ethyl 4-methylthio-m-tolyl isopropylphosphoramidate

• Trade names: Nemacur, Bay 68138, Phenamiphos

• Pesticide type: nematicide, insecticide; a systemic and contact pesticide

• Class: organophosphate, cholinesterase inhibitor

• Exposure routes: fenamiphos is highly toxic through oral, dermal, and inhalation

• Current use: nearly 1 million pounds of fenamiphos are used annually in the U.S. on 289,000 acres. 60% of the usage is on four crops: tobacco, grapes, oranges, and peanuts. Approximately 25% of all pineapples grown (Hawaii) are treated with fenamiphos. Fenamiphos is also used on commercial and industrial lawns, and golf course, in addition to other food crops.

• In a controlled experiment, fenamiphos was found to be the most toxic of thirteen other commonly used organophosphates to birds.

• Most wildlife mortality has occurred on golfcourses, resulting in fish kills. However, numerous bird deaths have been recorded including an incident where over 1000 birds died after fenamiphos was applied to an orchard.

Chemical Structure

Background

Fenamiphos is an organophosphate pesticide, a class of chemicals that was originally designed to function as a human nerve gas in World War II. It is currently formulated as a 10% or 15% active ingredient granular product or a 35% active ingredient emulsifiable concentrate. The granular products are broadcast over newly planted row crops or turf (golf courses, sod farms, lawns), and incorporated into the soil. The liquid formulations are sprayed directly on crops. Nearly one million pounds of fenamiphos are used in the U.S., annually. While tobacco, grapes, oranges, and peanuts account for 60% of the active ingredient applied, fenamiphos is registered on wide variety of field crops and orchards, commercial/industrial lawns, and golf course turf.

Fenamiphos is a systemic insecticide; the active ingredient is absorbed into the tissues of the plant or taken up by the roots. The toxic effects of the pesticide are expressed in the foliage, seeds, and pollen. A treated plant may accumulate high levels of systemic pesticides such as fenamiphos, especially after irrigation or a rain event. Research on pesticide residues remaining in food after treatment with systemic pesticides shows that decay of absorbed systemic pesticides does not occur before 30 to 40 days after application. Since fenamiphos can be relatively persistent in soil under some conditions (it has a half-life of 88 days in anaerobic soil), the potential for systemic transport to plant tissues is increased and the time in which fenamiphos residues are found in plants is prolonged. The long exposure period to insects and other animals in contact with treated crops increases the risks of acute and chronic effects. Birds nesting on the periphery of treated fields might feed on contaminated insects for one month or more, exposing their chicks to these insects as they are brought to the nest.

Fenamiphos is relatively stable to hydrolysis and moderately persistent in water, although when exposed to light, it degrades rapidly. Fenamiphos has potential to contaminate and persist in groundwater, since sunlight does not penetrate underground water stores. California has listed fenamiphos as a pesticide of concern for groundwater contamination. 

The breakdown products, or degradates, of fenamiphos are relatively persistent in the environment and can inhibit cholinesterase more efficiently than the parent compound. The manufacturer of fenaminphos has been asked to submit laboratory studies that would examine the toxicity of fenamiphos degradates but has continually submitted flawed, and, therefore, unacceptable data. The degradates of fenamiphos are known to be more easily dissolved in water; increasing exposure risks for birds and other non-target species.

Environmental Effects

Fate

• Persistence: Fenamiphos degrades in aerobic soil with a half-life of 15.7 days. One major degradate, fenamiphos sulfoxide, degraded with a half-life of 62 days. Fenamiphos sulfon phenol, a second degradate, was found to degrade with a half-life of 147 days. In anaerobic soil, fenamiphos degrades with a half-life of 89 days.

• Solubility: slightly soluble in water; soluble in organic solvents such as dichloromethane, isopropanol, and toluene.

• Soil mobility: fenamiphos does not bind well to soil particles and has the potential to be mobile in soils. This characteristic increases the risk of ground and surface water contamination.

• Bioaccumulation: An estimated bioconcentration factor of 170 suggests that fenamiphos may bioconcentrate in aquatic organisms; a bioconcentration factor of 468 was measured for earthworms, in one study. These figures indicate that biaoaccumulation might be an important fate of fenamiphos in the environment.

Ecotoxicity 

• Aquatic invertebrates: fenamiphos has been tested in the laboratory and found to be very highly toxic to aquatic invertebrates in freshwater. Estuarine organisms are also at risk.

• Fenamiphos is very highly toxic to fish. Many incidents of massive fish kills have been noted when fenamiphos is used according to label instructions. Usually, these kills occur following a heavy rain event, which washes the compound into ponds or streams.

• Mammals: Fenamiphos is classified by the EPA as very highly toxic to mammals through oral, inhalation, and dermal exposure routes. Studies which would elucidate the fate of fenamiphos in mammalian tissues have not been performed, however, secondary exposure from the ingestion of contaminated small mammals is a risk to predatory birds and mammals.

• Birds: fenamiphos is one of the most acutely toxic compounds known to affect birds. Death may occur within a few minutes of ingesting a small amount of fenamiphos. Prolonged depression of acetylcholinesterase will lead to chronic effects manifested in behavioral, reproductive, and physiological changes that have been shown to increase  birds' susceptibility to predation and decrease its chances of survival during times of stress. Reproduction is adversely affected when cholinesterase levels are depressed in adult and young birds and can be manifested by reduced clutch size, decreased survival of chicks, and an increase in embryo deformities.

  Representative LD 50's:

                     Canary             0.5 to 1.0 mg/kg
                     Pigeon              0.5 to 1.0 mg/kg
                     Pheasant           0.5 to 1.0 mg/kg
                     Bobwhite          0.8 to 0.9 mg/kg

Incidents

Wildlife mortality due to fenamiphos use has been documented in the Ecological Incident Information System (EIIS), compiled by the EPA. Most cases involve deaths of birds and fish when fenamiphos is applied to turf, usually at golf courses. Approximately three incidents of massive (over 200) fish kills are reported each year. Field studies, in which the effects of fenamiphos on wildlife are monitored after application of the compound to particular crops have proven that fenamiphos is acutely toxic to birds.

• Fresno, CA, 1998: 1000 fish and 28 birds were found dead after a kiwi orchard after it was treated with liquid fenamiphos.

• Bay, FL, 1996: At least 28 American coot were found dead on a golf course after fenamiphos application.

• FL, 1995: A family's terrier dog walked across a golf course after the turf had been treated with granular fenamiphos. The dog died within hours of contact; the golf course was closed for the day.

• A controlled field study in a Florida citrus grove documented bird and mammal species living in and around the study site. It documented that one-third of the resident avian species suffered from depressed cholinesterase after exposure to fenamiphos. The depression of cholinesterase lasted for approximately 30 days post-treatment. In the center of the grove, 26 different bird species were recordednumbering from 56 to 265 birds per 100 acres. In the surrounding habitat, 47 species of birds were recorded. Birds were mist-netted before and at time periods after fenamiphos application and were tested for cholinesterase exposure. While there were no carcass searches carried out in this study, it did confirm that adverse effects to birds will occur after one application of fenamiphos at label rates. In addition to increased acute mortality due to organophosphate poisoning, the bird population will be negatively affected by depressed cholinesterase levels for 30 days or more post-treatment. Debilitated birds (and other terrestrial vertebrates) are more susceptible to predation and will have less chance of survival due to fenamiphos exposure.