Chlorpyrifos is a broad-spectrum organophosphate insecticide that is widely used in Minnesota to control a variety of pests. It is currently registered for use on:

  • food and non-food crops,
  • golf course turf,
  • industrial sites,
  • greenhouse and nursery production,
  • sod farms, and
  • wood products.

All registrations for homeowner use of this product have been cancelled, except for roach bait station products. Chlorpyrifos can also be applied by a licensed professional (not by homeowners) to ant mounds.

Chlorpyrifos is classified as a Restricted Use Pesticide (RUP), meaning anyone selling or using a pesticide product containing chlorpyrifos is required by law to be licensed (commercial or non-commercial applicators) or certified (private applicators). It is highly toxic to aquatic organisms (fish, insects, mollusks), pollinators, and other beneficial insects.

Common trade names*: Cobalt; Lorsban; Warhawk; Govern; Hatchet; Dursban

*Reference to trade names is made with the understanding that no discrimination is intended and no endorsement is implied.


How it Works

Chlorpyrifos acts as a nerve agent in insects and is classified as an acetylcholinesterase (ACHE) inhibitor (Group 1B) by the Insecticide Resistance Action Committee. It functions by binding the active sites of the ACHE enzyme, which, in turn, breaks down nerve impulse carriers, causing seizures and eventual death of the insect. Insects can be exposed to chlorpyrifos through either direct contact, ingestion, or inhalation.

The cholinesterase enzyme also occurs naturally in people, which means the insecticide can harm people as well. This is why applicators who apply products containing chlorpyrifos are encouraged to get a blood test to establish their baseline readings of cholinesterase in their blood. With an established baseline, doctors can determine if illness following pesticide application could be due to chlorpyrifos. Proper use of personal protection equipment (PPE), especially respirators, is the best way to prevent accidental exposure when handling this chemical.

Other organophosphate insecticides that share this mode of action include diazinon, malathion, parathion, dichlorvos, and terbufos. Because these insecticides act in the same way, if an insect is resistant to one organophosphate insecticide, other chemicals in this class may not be effective.


Use in Minnesota

Minnesota has 32 restricted use products registered with chlorpyrifos as an active ingredient for agricultural use. The formulations available are emulsifiable concentrate, flowable concentrate, soluble concentrate, and granular. The MDA estimates, based on the most recent agricultural surveys available, that chlorpyrifos is, approximately, applied on:

  • 1% of corn acres (corn borer, earworm and rootworm control),
  • 13% of soybeans acres (soybean aphid control),
  • 9% of wheat acres (aphid control), and
  • 3% of hay acres (lygus control).

It is also used to control some pests on sugar beets in Minnesota, including root maggots, lygus bug, and cutworms although the MDA survey data does not provide a use estimate for sugar beets.

Timing of Chlorpyrifos Application by Crop

Crop Seed Pre-plant At Plant Post Emerge Foliar
Wheat x     x x
Sunflower   x x x x
Soybean     x x x
Corn x x x x x
Sugar Beets   x x x x
Apple   x      
Alfalfa     x   x
 
The MDA gathers data from pesticide dealers on the pounds of pesticide products sold each year. Prior to 2010, sales data were reported by the manufacturer instead of the dealer. The graph below shows the amount of chlorpyrifos sold over the past 20 years.
 
The amount of Chlorpyrifos product sold from 1997-2017 in pounds. Product sold ranges from 200,000 to just over 1,200,000 pounds. In 2017, about 1,200,000 pounds was sold. This is close to double the amount sold in 2016.

Soybean Aphid

In recent years, soybean aphids (Aphis glycines) in parts of Minnesota, Iowa, North Dakota, and South Dakota have developed resistance to the commonly used pyrethroid class of insecticides. The number of reported cases of pyrethroids failing to control soybean aphids has increased since 2015, when the issue was first reported. Due to resistance, growers have turned to chlorpyrifos for effective control, which has led to more frequent use of chlorpyrifos over a longer period of time in the summer.

For growers with documented issues of pyrethroid-resistant aphids in their fields, chlorpyrifos treatment is recommended when aphid populations reach a threshold (250 aphids/plant). For growers that experience failure of pyrethroids for the first time in the growing season, chlorpyrifos is used as a secondary treatment to control the aphids that were not killed by the initial pyrethroid treatment. Additional information on aphid monitoring, scouting and treatment is available in the Recommended IPM Approach and Treatment Threshold for Soybean Aphid Control in Soybeans (PDF) fact sheet.


Chlorpyrifos in the Environment

Chlorpyrifos is generally persistent, depending on soil type and environmental conditions, and is relatively immobile in soil due to its strong adsorption to soil particles. Contamination of water can occur via:

  • direct spray/ overspray,
  • spray drift,
  • runoff,
  • erosion, or
  • vapor drift (volatilization).

 In soil, chlorpyrifos is primarily broken down through microbial degradation.

Chemical Properties

Water Solubility 1.4 mg/L (25°C)

Soil

Adsorption Immobile (Koc = 973 - 31,000 mL/g-1)
Soil Metabolism Aerobic half-life = 2-1,576 days
Anaerobic half-life = 15-58 days
Photolysis Stable
Persistence Varies greatly (2- over 100 days)

 Water

Aquatic Metabolism Aerobic half-life = 22-51 days
Anaerobic half-life = 39-51 days
Photolysis Half-life = 29.6 days
Hydrolysis Stable

Air

Volatilization Vapor pressure (25°C) = 1.731 x 10-5 torr
Henry's Law constant = 1.10 x 10-5 atm m3/mol

Degradates

The major degradate of chlorpyrifos is 3,5,6-trichloro-2-pyridinol (TCP). TCP is mobile and persistent (if not exposed to light in the soil). Chlorpyrifos can also undergo transformation (via oxidative desulfonation) to chlorpyrifos-oxon under certain conditions. This degradate is less persistent than the parent, but more mobile in soil (Koc = 146-270 mL/goc). Chlorpyrifos-oxon is of greatest concern as a byproduct of drinking water chlorination. Based on human health studies, chlorpyrifos-oxon is more toxic than the parent.

Toxicity

Human Health

Chlorpyrifos is toxic by oral, dermal, and inhalation exposure routes. It is a mild eye irritant, slight skin irritant, but is not a skin sensitizer. Adverse health effects can be due to acetylcholinesterase inhibition (neural signals) and include potential neurodevelopmental effects.

Human Health Values for Chlorpyrifos
Data from US EPA2 and the Minnesota Department of Health (MDH)3

Human Health Benchmark (HHB)2 Acute = 2 µg/L
Chronic = 0.6 µg/L
Cancer Effect Not likely to be
carcinogenic to humans
Maximum Contaminant Limit (MCL) None
Health Advisory (HA) 0.002 mg/L
MDH Health Based Value (HBV)
(chronic)
0.6 µg/L


Non-target Organisms

Chlorpyrifos is highly toxic to birds, fish, honeybees, and other wildlife. Due to the toxicity to aquatic life, setbacks from surface water are required for all forms that are not soil-incorporated and stated on the label:

The setback distance for each type of application is as follows:

  • ground and chemigation: 25 feet
  • aerial: 150 feet
  • orchard airblast: 50 feet

In addition to setbacks, application must occur:

  • within 4 feet of the crop canopy or soil surface for ground applications
  • within 10 feet of the crop canopy for aerial applications
  • between 3 and 10 mph (wind speed),
  • and after 24 hours following flood irrigation

Chlorpyrifos toxicity values for aquatic and terrestrial organisms (Data from US EPA4)

Aquatic Organism Toxicity Values Toxicity Level
Freshwater fish* Acute LC50 = 1.8 µg/L
Reproductive NOAEC = 0.57 µg/L
Very Highly Toxic
Freshwater invertebrates* Acute EC50 = 0.1 µg/L 
Reproductive NOAEC = 0.04 µg/L
Very Highly Toxic
Most Sensitive Aquatic Life Benchmarck (ALB)*

41 µg/L (invertebrates)

 

 

Terrestrial Organism Toxicity Values Toxicity Level
Birds* Acute LD50 = 10 mg/kg (house sparrow)
Chronic NOAEC = 25 mg/kg (mallard duck)
Very Highly Toxic
Bees* Acute LD50 = 0.059 µg/bee Very Highly Toxic
Mammals

Acute contact LD50 = 97 mg/kg
Chronic NOAEC = 10 mg/kg

Moderately Toxic

*Values indicate toxicity to the technical grade active ingredient.

References

  1. Solomon, K.R., W.M. Williams, D. Mackay, J. Purdy,J.M. Giddings, and J.P. Glesy. 2014 Properties and Uses of Chlorpyrifos in the United States. DOI:10.1007/978-3-319-03865-0_2
  2. U.S. Environmental Protection Agency. Chlorpyrifos: Revised Human Health Risk Assessment for Registration Review.
  3. Minnesota Department of Health. Human Health-Based Water Guidance Table. http://www.health.state.mn.us/divs/eh/risk/guidance/gw/table.html
  4. U.S. Environmental Protection Agency. Problem Formulation for the Environmental Fate and Ecological Risk, Endangered Species and Drinking Water Assessments in Support of the Registration Review of Chlorpyrifos. https://www.epa.gov/endangered-species/biological-evaluation-chapters-chlorpyrifos-esa-assessment#chapter%201
    https://www.regulations.gov/document?D=EPA-HQ-OPP-2008-0850-0007