Chlorpyrifos is a broad-spectrum organophosphate insecticide that is widely used in Minnesota to manage a variety of pests. It is currently registered for use on:
  • Agricultural crops 
  • Ornamentals and golf course turf
  • Greenhouse and nursery production 
  • Cattle
  • Wood structures 
  • Ant mounds, mosquito, and tick habitats 

All homeowner-use product registrations have been canceled, except for ant and roach bait station products. 

Most chlorpyrifos products are classified as a restricted use pesticide (RUP), meaning those selling or using a chlorpyrifos product labeled as a RUP are required by law to be licensed (commercial or non-commercial applicators) or certified (private applicators). 


How it Works

Chlorpyrifos acts as a nerve agent and is classified as an acetylcholinesterase (AChE) inhibitor (Group 1B) by the Insecticide Resistance Action Committee. Insects can be exposed to chlorpyrifos through either direct contact, ingestion, or inhalation.

Chlorpyrifos functions by binding to AChE, thereby preventing the breakdown of acetylcholine (a neural signal carrier). Subsequent accumulation of acetylcholine causes overstimulation of nerves which can result in paralysis, seizures, and eventual death of the insect. Other organophosphate insecticides that share this mode of action include diazinon, malathion, parathion, dichlorvos, and terbufos.  


Use in Minnesota

Minnesota has 46 products registered with chlorpyrifos as an active ingredient (as of July 2020). Products are available in emulsifiable, flowable, soluble concentrates, and granular formulations. The MDA estimates, based on the most recent surveys available, that chlorpyrifos was applied on:

  • 1% of corn acres (in 2015)
  • 13% of soybeans acres (in 2013)
  • 9% of wheat acres (in 2013)
  • 1% of hay acres (in 2015)

Chlorpyrifos is also used in other Minnesota crops such as sugar beets. MDA survey data does not provide a use estimate for sugar beets; however research from the University of North Dakota estimates use on sugar beest at 15% of acres planted (from 1983-2011) [1].

Chlorpyrifos is the highest selling insecticide in the state with consistently high sales from 2010-2018 (see figure). The MDA gathers sales data from pesticide dealers and registrants on the pounds of pesticide products sold each year.

Bar graph showing the pounds of Chlorpyrifos active ingredient sold from 2010-2018 in pounds. Sales data were provided to the MDA by pesticide dealers. Values for each year can be found in the MDA's searchable database. http://www2.mda.state.mn.us/webapp/lis/chemsold_default.jsp

 


Pest Control

Chlorpyrifos manages a wide range of pests in agricultural and non-agricultural settings. For example, potato leafhoppers (Empoasca fabae) on alfalfa, soybean aphid (Aphis glycines) on soybeans, cockroaches in buildings, and mosquitos are a few of the pests that may be managed with chlorpyrifos.

In April 2020, the MDA put on a webinar for applicators, farmers, and agricultural professionals. The webinar provides information on the proper use of chlorpyrifos, label requirements, soybean aphid control, and pyrethroid-resistant soybean aphids. View the "Chlorpyrifos use in Minnesota" webinar (YouTube)

Additional information on aphid monitoring, scouting and treatment is available from the Recommended IPM Approach and Treatment Threshold for Soybean Aphid Control in Soybeans (PDF) fact sheet.


Chlorpyrifos in the Environment

Chlorpyrifos enters the environment from routine use where it can move and degrade in a number of ways. How chlorpyrifos moves is largely determined by its physical and chemical properties, as is the length of time it persists after application. Site-specific conditions, such as weather and soil type, are also key in determining its fate, along with the formulation applied (e.g., liquid vs. granular), method of application (e.g., ground-based or aerial), and other use factors (e.g., rate, timing). Chlorpyrifos has a low solubility in water and binds strongly to soil, which limits its mobility. Being relatively immobile in soil, leaching to groundwater is not expected; however, there is potential for chlorpyrifos to move with runoff into surface water, primarily with eroded sediment. Spray drift and volatilization from ground and aerial spray applications are additional ways chlorpyrifos can enter the environment. Spray drift can occur during the application process as fine spray droplets move away from the treated area. Volatilization of chlorpyrifos predominantly occurs from leaf surfaces shortly after foliar application. The resulting vapors can disperse and move offsite through atmospheric transport.

In soil, chlorpyrifos is primarily broken down through microbial degradation and its half-life can range from a few days to over 100 days. In aquatic environments, chlorpyrifos can undergo abiotic hydrolysis, biodegradation, and photodegradation. Oxidation and photolysis are key degradation processes in air and on foliar surfaces.

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


Human Health Risks

According to the U.S. Environmental Protection Agency (EPA), chlorpyrifos is moderately toxic to humans by oral, skin, and inhalation exposure [2]. Chlorpyrifos is a mild eye irritant and slight skin irritant but is not a skin sensitizer. It is metabolized in the human body to the more toxic and potent AChE inhibitor chlorpyrifos oxon. According to the EPA, chlorpyrifos oxon is more toxic than chlorpyrifos. Adverse health effects can occur as a result of AChE inhibition (neural signals). Chlorpyrifos is categorized by the EPA as “Not likely to carcinogenic to humans.”

Due to the potential risk to human health, the EPA has developed a set of requirements that must be followed when applying chlorpyrifos products. View Chlorpyrifos Application Information for more details. 

Human Health Values for Chlorpyrifos
Data from EPA2 and the Minnesota Department of Health3

Human Health Value Type Value
USEPA Chlorpyrifos Health Advisory (HA)- drinking water 2 µg /L
USEPA Chlorpyrifos Steady-State Population Adjusted Dose5* 0.0000017
mg/kg/day
Chlorpyrifos MDH Health Base Value (HBV) Acute: 2 µg /L
Chronic: 0.6 µg /L
Chorypyrifos oxon MDH Rapid Assessment Advice (RAA) Acute: 0.9 µg /L
Chronic: 0.4 µg /L

*The steady-state population adjusted dose value represents the exposure to chlorpyrifos via food only, with the dose for children listed in the table above.


Ecological Risks

According to the EPA’s Ecological Risk Assessment, chlorpyrifos is very highly toxic to birds and fish, highly toxic to honeybees, and moderately toxic to mammals [4].

Terrestrial organisms such as mammals, birds, and invertebrates may be exposed to chlorpyrifos and its degradates through multiple routes including contact, inhalation, and diet.

Aquatic organisms, including freshwater fish, invertebrates, and plants, can be exposed to chlorpyrifos or its degradates through contaminated surface water bodies such as lakes, ponds, rivers, and streams. Chlorpyrifos may bioaccumulate in organisms, which poses additional risk to organisms higher in the food chain. Due to toxicity to aquatic life, setbacks from surface water are required for many liquid product applications.

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

Aquatic Organism Toxicity Values Toxicity Level
Freshwater fish Acute LC50 a= 0.9 µg/L
Reproductive NOAECb = 0.57 µg/L
Very Highly Toxic
Freshwater invertebrates Acute EC50c = 0.1 µg/L 
Reproductive NOAEC = 0.04 µg/L
Very Highly Toxic

 

Terrestrial Organism Toxicity Values Toxicity Level
Birds Acute LD50 d= 5.6 mg/kg
Chronic NOAEC = 25 mg/kg
Very Highly Toxic
Bees Acute LD50 = 0.059 µg/bee Very Highly Toxic
Mammals Acute contact  LD50 = 97 mg/kg
Chronic NOAEC = 10 ppm
Moderately Toxic

a LC50 (lethal concentration) is the concentration of pesticide in water which kills 50% of a test population.
b NOAEC (no-observed-adverse-effects concentration) is the highest concentration of a toxicant which causes no detectable adverse effects on the test animals.

EC50 (effective concentration) is the concentration of pesticide in water that causes an effect (e.g., immobilization) in 50% of a test population.
d LD50 (lethal dose) is the dose of pesticide which kills 50% of a test population.

References

  1. Khan MFR. Insecticide usage on sugar beet in Minnesota and North Dakota, USA. Int Sugar J. 2014; 424–428.
  2. U.S. Environmental Protection Agency (USEPA). 2016. Chlorpyrifos: Revised human health risk assessment for registration review.  EPA-HQ-OPP-2016-0062-0052.
  3. Minnesota Department of Health. Human Health-Based Water Guidance Table. 
  4. U.S. Environmental Protection Agency (USEPA). 2008. Problem Formulation for the Environmental Fate and Ecological Risk, Endangered Species and Drinking Water Assessments in Support of the Registration Review of Chlorpyrifos.