Imazapyr: Uses, Health Effects, Safety and Regulation

Imazapyr is a herbicide that belongs to the imidazolinone class of chemicals. It is used to control grasses, broadleaf herbs, woody species, trees and shrubs, and aquatic plants. It is not toxic to people at the doses tested in animals. It does not cause cancer in the animal studies reviewed. It also does not affect reproduction or development in guideline studies. Plants are affected most, and it persists in the environment. Agricultural use can contaminate source waters for drinking water. It is regulated in the United States, but it is not approved in the European Union.

What Is Imazapyr?

Imazapyr is a herbicide that belongs to the imidazolinone class of chemicals. Imazapyr works by inhibiting the acetolactate synthase enzyme in plants, like other low-dose, high-potency imidazolinones. Imazamox and imazethapyr are other pesticides in the same class. Registered products contain imazapyr or its isopropylamine or ammonium salt and are intended for professional use. EPA lists Imidazolinones (2014) among its registration review groups of pesticides.

What Is Imazapyr Used to Control?

Imazapyr is used to control grasses, broadleaf herbs, woody species, trees and shrubs, and emergent and floating aquatic plants. Imazapyr is a broad spectrum herbicide. It gives pre- and post-emergence control of certain annual grass and broad-leaf weeds in agricultural crops. It has no activity on submersed plants. The maximum label rate in Florida waters is 1.5 lb acid equivalent per acre per year. That rate applies to aquatic and forestry use and equals 96 fl oz of product.

Where Is Imazapyr Applied?

Imazapyr is applied to crops, including imidazolinone-tolerant field corn, forests and rights-of-way, and noncropland areas and grasslands. Aquatic sites such as lakes, ponds, streams, wetlands and ditches carry applications. So do homes, patios and driveways, parking lots and sidewalks, and athletic fields and golf courses. Imazapyr is currently registered in the United States for use in and around the home. The registration covers noncropland areas, aquatic sites, grasslands and imidazolinone-tolerant field corn. The University of Florida Institute of Food and Agricultural Sciences is an extension service. It reports that application is mostly within the marshes of Lake Okeechobee, a Florida lake. TM Goodwin / Broadmoor Marsh is the other main site. Registered products in Australia are applied with ground boom sprayers.

How Toxic Is Imazapyr to People?

Imazapyr is not toxic to people at the doses tested in animals, since the U.S. Environmental Protection Agency identified no hazard in its toxicity database. Imazapyr is readily and extensively absorbed through the gastrointestinal tract in rats. The majority of the administered dose of radiolabelled imazapyr was excreted within the first 24 hours after dosing. Animals exposed to high doses experienced minimal to no health effects. Acceptable guideline studies showed no toxicity at the highest doses tested, 250-1,000 mg/kg/day. A 13-week dietary study in rats found no treatment-related effects at 879 mg/kg bw/day. Repeated exposure at that dose did not change the result. Human evidence is limited to a 1999 study by H L Lee in the Journal of toxicology. Clinical toxicology. It reported six acute poisonings with an Arsenal formulation containing imazapyr and no deaths.

What Are the Acute Health Effects of Imazapyr?

The acute health effects of imazapyr are low in animal tests. The oral LD50 was 5000 mg/kg bw or greater in rats, rabbits and dogs. Imazapyr has a dermal LD50 above 2000 mg/kg bw in rats and rabbits. Acute oral and dermal toxicity are measured as these LD50 values. Imazapyr appears to be of low acute inhalation toxicity, although some clinical signs were noted in dogs immediately after dosing. A single high exposure in people differed from repeated exposure in animals, because the human cases involved large ingestions. Ingestion of more than 100 mL of Arsenal herbicide was associated with hypotension and pulmonary dysfunction. Transient liver and renal dysfunction followed. Three of the 6 patients (50%) presented with severe symptoms, including impairment of consciousness and respiratory distress requiring intubation. Five cases were suicide attempts and 1 was an act of violence inflicted on a child.

Does Imazapyr Irritate the Eyes or Skin?

It depends. Imazapyr is irritating to the eyes in rabbits yet does not irritate the skin in rabbits. Imazapyr and its isopropylamine salt are not skin sensitisers. Rabbit tests assess eye and skin irritation and give a classification, not a quantity. The eye finding is the exception to the otherwise mild skin result. The label states that the product is harmful if inhaled and advises avoiding contact with skin, eyes or clothing.

What Are the Symptoms of Imazapyr Poisoning?

The symptoms of imazapyr poisoning in people come from one small series. In it, 3 of 6 patients (50%) had severe symptoms. Imazapyr was in the Arsenal formulation that six patients ingested. A dose-response relationship still needs further observation and studies that include a larger series. The key symptoms include:

• Impaired consciousness and respiratory distress: These severe symptoms required intubation in the most serious cases. They appeared in 3 of 6 patients (50%).
• Hypotension and metabolic acidosis: Metabolic acidosis occurred in 2 patients and hypotension in 2 patients. Leukocytosis occurred in 3 patients and fever in 2.
• Copious vomiting: Vomiting followed ingestion of Arsenal in every case. The risk of severe symptoms rose with ingestion of more than 100 mL.
• Oral ulceration and gastrointestinal irritation: The case series gives no separate detail on ulceration beyond the vomiting. The label warns that the product is harmful if inhaled.
• Transient liver and kidney dysfunction: These effects were transient and no mortality occurred. They were linked to large ingestions of Arsenal.
• Chemical burns of the cornea: Imazapyr is irritating to the eyes in rabbits. The label advises avoiding contact with eyes.

Does Imazapyr Cause Cancer?

No. Imazapyr does not cause cancer in the animal studies reviewed. There was no evidence of carcinogenicity in an 18-month dietary study in mice and a 2-year dietary study in rats. Imazapyr is not considered to be genotoxic, based on in vitro and in vivo short-term studies. The highest dose tested in the mouse study was 1639 mg/kg bw/day, with no treatment-related toxicity. No compound-related tumours were observed in the animal studies. The 2023 EPA tolerance rule gives the current regulatory position. It states that the data show no evidence of neurotoxicity, immunotoxicity, genotoxicity or carcinogenicity.

Does Imazapyr Affect Reproduction or Development?

No. Imazapyr does not affect reproduction or development in well-conducted guideline studies. A 2-generation reproduction study in rats did not produce any evidence of reproductive effects. The 2023 EPA rule gives the regulatory position. No adverse developmental effects or adverse reproductive effects were detected in well-conducted guideline studies. Offspring toxicity was seen only at 10 000 ppm, the highest dose tested. That dose equals about 1471.8 mg/kg bw per day.

How Does Imazapyr Affect Fish, Birds, Bees and Plants?

Imazapyr affects plants most, because levels of concern are exceeded for non-target endangered and non-endangered vascular aquatic plants. Imazapyr risks to honey bees from the technical product are not expected, and precautionary labeling is not required. Waterfowl are not at risk, since imazapyr is non-toxic to them. The mallard duck LD50 is greater than 5,000 ppm. Fish acute toxicity is measured as a 96-hour LC50. The value is 148 mg/L for bluegill and 117 mg/L for rainbow trout. The 96-hour LC50 for Daphnia is 132 mg/L. The label protects desirable plants by directing users not to apply immediately adjacent to desirable emergent plants.

How Long Does Imazapyr Persist in the Environment?

Imazapyr persists in the environment. Its half-life is 1-5 months in terrestrial soils and about 2 days (48 hours) in water. Imazapyr is persistent in water and soil. The potential risk to vascular plants could last a relatively long time, perhaps as long as several years. Soil holds imazapyr longest, and persistence is measured as a half-life. Persistence differs between sites with soil pH. Breakdown in water is faster in spring and summer.

How Long Does Imazapyr Persist in Soil?

Imazapyr persists in soil for as long as 1-5 months, which is the half-life in terrestrial soils. Slow microbial metabolism is the primary degradation pathway for imazapyr in soils. Imazapyr adsorbs to soil at pH below 5. It is more mobile and available to plants in soil above pH 5. Terrestrial soils hold the compound, and the half-life is the measure of its breakdown there.

How Quickly Does Imazapyr Break Down in Water?

Imazapyr breaks down quickly in water, with a half-life of about 2 days (48 hours). Imazapyr rapidly degrades by photolysis in water, and photolysis is what breaks it down. Imazapyr degrades faster in spring and summer, so the season changes the pace. Imazapyr is not sensitive to hydrolysis. The half-life measures the water breakdown.

Does Imazapyr Build Up in Fish or Wildlife?

No. Imazapyr does not bioaccumulate in fish. It is not expected to build up in the tissues of fish or other wildlife. Fish and wildlife could encounter it in waters where it was recently applied. Exposure may occur through drinking contaminated water or eating food that contains pesticide residues. Swimming in waters where imazapyr was recently applied is a third route. Build-up is judged from tissue levels in fish and other wildlife.

Does Imazapyr Contaminate Drinking Water?

It depends. Agricultural use of imazapyr can contaminate source waters for drinking water. Run-off, spray drift or entry into groundwater carries it there. Imazapyr has been detected at low levels in Minnesota waters. The maximum was 0.0376 ppb (parts per billion) in groundwater and 0.0656 ppb in surface water. With good water quality management, exposure of the general population is expected to be well below levels of health concern. Good practice reduces contamination, while run-off and drift increase it. Imazapyr can be measured in water by liquid chromatography-mass spectrometry with an electrospray interface. The limit of detection is 0.004 µg/L.

What Is the Drinking Water Guideline for Imazapyr?

The drinking water guideline for imazapyr is 9 mg/L, based on human health concerns. The health-based guideline for imazapyr rests on long-term effects. The Minnesota Department of Health developed a pesticide rapid assessment of 900 ppb for drinking water. In the USA, the modelled estimated surface water concentration was 0.34-79 µg/L and the groundwater concentration was 39 µg/L.

How Is Imazapyr Regulated in the United States?

Imazapyr is regulated in the United States through the registration of products and through tolerances for residues in food. The U.S. Environmental Protection Agency (EPA) registers pesticides and sets tolerances for residues in food. EPA concluded that dietary, occupational, and residential exposures to imazapyr do not pose a significant human health risk. EPA can establish a tolerance only if it determines that the tolerance is safe. EPA has sufficient data to assess the hazards of and to make a determination on aggregate exposure for imazapyr. The aggregate exposure includes drinking water and residential settings but does not include occupational exposure. EPA prepared a screening level environmental risk assessment for the proposed registration of new aquatic uses for the product Arsenal.

What Are the Food Residue Tolerances for Imazapyr?

Food residue tolerances for imazapyr are 0.15 ppm in or on rice, bran. For rice, grain the tolerance is 0.06 ppm. EPA set these tolerances in the 2023 rule under 40 CFR 180.500 after a petition from BASF Corporation, the registrant. Imazapyr residues in rice bran rest on a highest average field trial residue of 0.031 ppm. The median rice bran processing factor is 3.2x. No additional exposure is expected from the rice tolerances because they are for import only. Compliance is checked with high-performance liquid chromatography with tandem mass spectroscopy (LC/MS/MS), method SOP-PA.0288.

What Is the Acceptable Daily Intake for Imazapyr?

The acceptable daily intake for imazapyr is 2.5 mg per kg body weight per day. An acceptable daily intake estimates what a person can consume daily over a lifetime without appreciable risk. The imazapyr value rests on a no-observed-effect level of 250 mg/kg bw/day from a 1-year dietary study in dogs. The Australian guideline applies a safety factor of 100 and was established in 1998. EPA is not retaining the 10X safety factor for imazapyr.

How Does EPA Review Imazapyr?

EPA reviews imazapyr through registration review in docket EPA-HQ-OPP-2014-0200. The registration review case covers imazapyr alone, and EPA set an anticipated timeline for completing the registration review. There are no anticipated data needs for the imazapyr registration review. Imidazolinones (2014) is the registration review group that includes imazapyr. EPA developed the characterization for imazapyr through a modified version of its public process eligibility and tolerance reassessment. Imazapyr was the only one of the imidazolinones to be subject to reregistration.

Is Imazapyr Approved in the European Union?

No. Imazapyr is not approved in the European Union, because its EC Regulation 1107/2009 status is Not approved. The inclusion of imazapyr under that regulation has expired. Great Britain restricts imazapyr in the same way. Its retained EC Regulation 1107/2009 status is Not approved. Regulation 1107/2009 governs active substance approval in the European Union, and the record shows no remaining approval under it.

How Is Imazapyr Scheduled as a Poison in Australia?

Imazapyr is scheduled as a Schedule 5 poison in Australia. The listing is in the Standard for the Uniform Scheduling of Medicines and Poisons No.1, 2010 (the Poisons Standard). The schedule describes registered imazapyr products, which are intended for professional use. Current versions of the Poisons Standard should be consulted, so the schedule may have changed since 2010. The sources give a schedule only for imazapyr and none for imazamox or imazethapyr.

How Can Imazapyr Be Used More Safely?

To use imazapyr more safely, start with the pesticide label, which must be read and followed for each product used. Observe all cautions and limitations in the package labels of products used in combination with imazapyr. Avoid breathing spray mist during application. Avoid contact with skin, eyes or clothing, because the product is harmful if inhaled. Follow good water quality management practices. Pesticides should then not be detected in source waters used for drinking water supplies. Remember that the Florida considerations do not replace or contradict the EPA approved label.

How Long Must People Stay Out of Imazapyr-Treated Areas?

People must stay out of imazapyr-treated areas for as long as the 12-hour restricted-entry interval lasts. The imazapyr label states that workers must not enter treated areas during that interval. A revision notice on the same 2010 label changes the entry period to 48 hours. The REI must be revised from 12 to 48 hours. The two figures disagree, and the longer period is the revised one.

How Long Must Imazapyr-Treated Water Wait Before Irrigation?

Imazapyr-treated water must wait 120 days after application before irrigation. The wait ends sooner only as long as laboratory analysis shows residue levels below 1.0 ppb. The University of Florida Institute of Food and Agricultural Sciences guidance adds a pause. Do not irrigate for at least 24 hours after application, to allow dissipation. Applications must stay at least 1 mile from an active irrigation water intake. Applications within 1/2 mile of a potable water intake can be made. The intake must be shut down for a minimum of 48 hours.

A 2020 study by Tony M Dugdale from La Trobe University studied imazapyr in canal water. Concentration fell by half for every 4.4 days of ponding when solar exposure exceeded 10 MJ m-2.