OFFICE OF

CHEMICAL SAFETY AND

POLLUTION PREVENTION

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON D.C., 20460

PC Code: 059102

DP Code: 426737

April 13, 2015

MEMORANDUM

SUBJECT: Chlorpyrifos-methyl - Preliminary Ecological Risk Assessment and Effects Determination 

in Support of Registration Review

TO: Joel Wolf, Chemical Review Manager

Kevin Costello, Chief

Risk Management and Implementation Branch 2

Pesticide Re-evaluation Division (7508P)

FROM: Rosanna Louie-Juzwiak, Risk Assessment Process Leader

Environmental Risk Branch III

Environmental Fate and Effects Division (7507P)

THROUGH: Dana Spatz, Chief

Environmental Risk Branch III

Environmental Fate and Effects Division (7507P)

REVIEWED BY: James Hetrick, Ph.D., Senior Advisor

Melissa Panger, Ph.D., Senior Scientist

Environmental Risk Branch III

Environmental Fate and Effects Division (7507P)

The Environmental Fate and Effects Division (EFED) has completed the preliminary ecological risk 

assessment and federally listed, threatened, and endangered species (also referred to as “listed” 

species) effects determination in support of the Registration Review of the insecticide, chlorpyrifosmethyl (PC Code 059102). Chlorpyrifos-methyl is registered as an insecticide used to target and kill a 

variety of stored grain insects including beetles, weevils, moths, and grain borers. The registered uses of 

chlorpyrifos-methyl are limited to indoor applications made to the interior of empty grain storage bins 

or warehouses, or, directly to harvested small grains just prior to storage. Virtually no exposure to nontarget organisms is expected to occur when this product is used as labeled; therefore, risk is not 

expected to non-target organisms, including listed species.

ASSESSMENT

Chlorpyrifos-methyl is registered in one end-use product (Storcide II Grain, Bin and Warehouse 

Insecticide end use product; EPA Reg. No. 264-992), and the technical product (Reldan F Insecticidal 

Chemical; EPA Reg. No. 62719-42) for treatment of grains in storage bins and warehouses. Chlorpyrifos-
methyl is applied indoors to empty storage grain bins or warehouses with one application per year by 

sprayer or automatic sprayer at a rate of 0.0253 pound of active ingredient per 1,000 square feet. 

Chlorpyrifos-methyl can also be applied indoors directly to small grains prior to storage, to achieve a 

concentration of up to 3 parts per million. Based on available, but limited recent usage data provided by 

the Biological and Economic Analysis Division, approximately 4,500 pounds of chlorpyrifos-methyl is 

applied annually (EPA, 2015). 

The labeled indoor uses and application methods of chlorpyrifos-methyl significantly limits any potential 

direct or indirect exposure (including drift and runoff) to outdoor terrestrial areas. In addition, these use 

patterns are unlikely to result in exposure to aquatic resources or groundwater. Because of the indoor 

use pattern and very low potential for outdoor exposure to non-target species, only limited data have 

been submitted for this chemical. 

Although chlorpyrifos-methyl is an organophosphate, the available information indicates that when 

compared to its closely-related structural analog chlorpyrifos, chlorpyrifos-methyl is of lower toxicity to 

vertebrates (Beavers, 1978) (Saltatos, et al., 1982). With the exception of potential toxicity to 

invertebrates (as expected for an insecticide), chlorpyrifos-methyl is generally less toxic than other 

organophosphates (e.g., chlorpyrifos). 

Other than exposure to the target pest inside of the storage bins, the only potential exposure to nontarget organisms would be treated seeds that were subsequently used for planting, which would occur 

several months after the initial treatment. A cursory screen was conducted using the lowest reported 

avian acute (100 mg/kg/day) (MRID 00027855), acute oral mammalian (5,000 mg/kg body weight) 

(MRID 48868101), and a chronic mammalian endpoint (body weight gain) from a developmental study 

(50 mg/kg body weight) (MRID 44680603). This analysis showed that even if terrestrial vertebrates were 

exposed to seeds treated at the maximum chlorpyrifos-methyl concentration (i.e., 3 ppm), potential risk

would be low (see Appendix A for the T-REX analysis).

CONCLUSIONS

Based on the use pattern whereby exposure from use of chlorpyrifos-methyl is expected to be 

negligible, and the low potential for risk if exposures occur via planted treated seeds, EPA concludes that 

the currently registered uses of chlorpyrifos-methyl are not reasonably expected to cause direct or 

indirect effects to federally listed threatened and endangered species. A No Effect determination is 

made for all listed species, as well as a No Habitat Modification determination for all designated critical 

habitats for the currently registered uses of chlorpyrifos-methyl.

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ENDOCRINE DISRUPTOR SCREENING PROGRAM

As required by FIFRA and FFDCA, EPA reviews numerous studies to assess potential adverse outcomes 

from exposure to chemicals. Collectively, these studies include acute, subchronic and chronic toxicity, 

including assessments of carcinogenicity, neurotoxicity, developmental, reproductive, and general or 

systemic toxicity. These studies include endpoints which may be susceptible to endocrine influence, 

including effects on endocrine target organ histopathology, organ weights, estrus cyclicity, sexual

maturation, fertility, pregnancy rates, reproductive loss, and sex ratios in offspring. For ecological 

hazard assessments, EPA evaluates acute tests and chronic studies that assess growth, developmental 

and reproductive effects in different taxonomic groups. As part of its reregistration decision for 

chlorpyrifos-methyl, EPA reviewed these data and selected the most sensitive endpoints for relevant risk 

assessment scenarios from the existing hazard database. However, as required by FFDCA section 

408(p), chlorpyrifos-methyl is subject to the endocrine screening part of the Endocrine Disruptor 

Screening Program (EDSP). 

EPA has developed the EDSP to determine whether certain substances (including pesticide active and 

other ingredients) may have an effect in humans or wildlife similar to an effect produced by a “naturally 

occurring estrogen, or other such endocrine effects as the Administrator may designate.” The EDSP 

employs a two-tiered approach to making the statutorily required determinations. Tier 1 consists of a 

battery of 11 screening assays to identify the potential of a chemical substance to interact with the 

estrogen, androgen, or thyroid (E, A, or T) hormonal systems. Chemicals that go through Tier 1 

screening and are found to have the potential to interact with E, A, or T hormonal systems will proceed 

to the next stage of the EDSP where EPA will determine which, if any, of the Tier 2 tests are necessary 

based on the available data. Tier 2 testing is designed to identify any adverse endocrine-related effects 

caused by the substance, and establish a dose-response relationship between the dose and the E, A, or T 

effect. 

Under FFDCA section 408(p), the Agency must screen all pesticide chemicals. Between October 2009 

and February 2010, EPA issued test orders/data call-ins for the first group of 67 chemicals, which 

contains 58 pesticide active ingredients and 9 inert ingredients. A second list of chemicals identified for 

EDSP screening was published on June 14, 20131 and includes some pesticides scheduled for registration 

review and chemicals found in water. Neither of these lists should be construed as a list of known or 

likely endocrine disruptors. 

For further information on the status of the EDSP, the policies and procedures, the lists of chemicals, 

future lists, the test guidelines and the Tier 1 screening battery, please visit our website.2

1 See http://www.regulations.gov/#!documentDetail;D=EPA-HQ-OPPT-2009-0477-0074 for the final second list of 

chemicals. 2 http://www.epa.gov/endo/

3

 
References:

Beavers, J.B.; Fink, R.; Brown, R. (1978) Final Report: Eight-Day Dietary LC50--Bobwhite Quail: Project No. 

103-186. U.S. EPA MRID 00039223.

Bryson, A.M. 1992. A Study of the Effect of Technical Reldan on the Pregnancy of the Rat. Huntingdon 

Research Centre Ltd. Laboratory Study ID: GHE-413. MRID 44680603.

U.S. EPA, 2015. Email communication from D. Atwood, Biological and Economic Analysis Division. March 

26, 2015.

Cunningham and Schafer, 1968. Starling and Redwinged Blackbird. 1968. EPA Ascension No. 242149, 

Report #3a. U.S. EPA MRID 00028755.

Saltatos, et al. 1982. Factors Involved in the Differential Acute Toxicity of the Insecticides Chlorpyrifos 

and Methyl Chlorpyrifos in Mice. Toxicology and Applied Pharmacology, 65, pp. 144 – 152.

Verma, et al. 2012. Acute Oral Toxicity Study of Chlorpyrifos Methyl TGAI in Rats. HAI Research 

Foundation. Laboratory Project Study Number 401-1-01-4229. MRID 48868101.

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Appendix A. TREX Modeling for Exposure to Eating Treated Seed

A cursory analysis was conducted using the lowest reported avian acute (100 mg/kg/day) (MRID 

00027855), acute oral mammalian (5,000 mg/kg body weight) (MRID 48868101), and a chronic 

mammalian endpoint (body weight gain) from a developmental study (50 mg/kg body weight) 

(MRID 44680603). 

The application rate for the end use product (EPA Reg. No. 264-992) for wheat is 12.4 fluid 

ounces of product per 1,000 bushels, to achieve a 3 ppm concentration on seed.

Chemical Name: chlorpyrifos methyl

Seed Treatment? (Check if yes) Yes, Seeding rate of 156.0 lb/acre

% A.I. (leading zero must be entered for 

formulations <1% a.i.): 21.60%

Application rate (fl oz/cwt) 0.02

Half-life (days): 35

Application Interval (days): 0

Number of Applications: 1

Are you assessing applications with variable 

rates or intervals? no

Reported Endpoints Tested Body Adjusted LD50

Size class for 

Weight (g) adjusted LD50

Avian LD50: 100.00 29 94.58 Small (20g)

Avian repro. 

NOAEC: 0.00 120.40 Medium (100g)

170.07 Large (1000g)

Mammalian 

LD50: 2000.00 350 4395.66 Small (15g)

Mammalian 

NOAEL: 6.60 3556.56 Medium (35g)

1538.32 Large (1000g)

Adjusted NOAEL for Mammals

Small (15g) 109.89

Medium (35g) 88.91

Large (1000g) 38.46

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Animal Size Crop

Maximum 

Application 

Rate

Maximum 

Seed 

Application 

Rate

Avian Nagy 

Dose

Mammalian 

Nagy Dose Available AI

(lb a.i./A)

(mg a.i./kg 

seed)

(mg a.i./kgbw/day)(mg a.i./kgbw/day) (mg ai. ft-2)

Small wheat, all 

or 

unspecified

<0.01 2.81

0.71 0.60

Medium 0.41 0.41 0.00

Large 0.18 0.10

Crop

Risk Quotients†

Avian (20 g) Mammalian (15 g)

Acute (# 1) Acute (# 2) Chronic Acute (# 1) Acute (# 2) Chronic

wheat, all or 

unspecified 0.01 <0.01 N/A <0.01 <0.01 0.01

Avian (100 g) Mammalian (35 g)

Acute (# 1) Acute (# 2) Chronic Acute (# 1) Acute (# 2) Chronic

wheat, all or 

unspecified <0.01 <0.01 N/A <0.01 <0.01 <0.01

Avian (1000 g) Mammalian (1000 g)

Acute (# 1) Acute (# 2) Chronic Acute (# 1) Acute (# 2) Chronic

wheat, all or 

unspecified <0.01 <0.01 N/A <0.01 <0.01 <0.01

† Acute RQ #1 = (mg a.i. /kg-bw/day) / LD50 N/A = not assessed

Acute RQ #2 = mg a.i. ft-2 /(LD50*bw)

Avian Chronic RQ = mg kg-1 seed / NOAEL

Mammalian Chronic RQ = mg a.i./kg-bw/day / adjusted NOAEL

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