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OFFICE OF

CHEMICAL SAFETY AND

POLLUTION PREVENTION

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON D.C., 20460

PC Code: 129032

DP Barcode: 397089

Date: July 25, 2012

MEMORANDUM

SUBJECT: Tier I Drinking Water Assessment for New Use of Insect Growth Inhibitor 2-[1-methyl2- (4-phenoxyphenoxy)ethoxy]pyridine Pyriproxyfen on Herb Subgroup 19A

TO: Thurston Morton, Risk Assessor

Elissa Reaves, Branch Chief

Risk Assessment Branch IV

Health Effects Division (7509P)

and

Kevin Sweeney, Risk Manager Reviewer 

Richard Gebken, RM 10

Insecticide Branch 

Registration Division (7505P)

FROM: James A. Hetrick, Ph.D., Senior Science Advisor

Environmental Risk Branch 3

Environmental Fate and Effects Division (7507P)

THROUGH: Rosanna Louie-Juzwiak, Risk Assessment Process Leader

Dana Spatz, Branch Chief

Environmental Risk Branch 3

Environmental Fate and Effects Division (7507P)

IR-4, in cooperation with the registrant Valent U.S.A. Corporation, has submitted an application for the 

registration for pyriproxyfen as a new use on herb subgroup 19A. The registrations of concern for these 

crops are: KNACK Insect Growth Regulator (EPA Reg. No. 59639-95); ESTEEM 35 WP Insect Growth 

Regulator (EPA Reg. No. 59639-115), and Pyriproxyfen 0.86 EC (EPA Reg. No. 59639-160). The 

proposed single application rate of pyriproxyfen for use on herbs is 0.067 lb a.i. /A, with a minimum 

application interval of 14 days. Application is allowed twice per season but is not to exceed an annual 

load of greater than 0.134 lb a.i./A per season.
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Tier 1 FIRST and GENEEC modeling was conducted to assess estimated drinking water concentration 

(EDWC) of pyriproxyfen in surface and ground water source drinking water. No monitoring surface or 

ground water monitoring data are available for pyriproxyfen. The drinking water assessment considers 

only parent pyriproxyfen. Tier 1 FIRST modeling predicts that pyriproxyfen concentrations in surface 

source drinking water are not expected to exceed 1.952 μg/L for the daily peak and 0.111 μg/L for the 

annual average (Table 1), based on the proposed application rates for herbs. Tier 1 GENEEC modeling 

predicts that pyriproxyfen concentrations in ground source drinking water are not expected to exceed

0.000804 μg/L for the daily peak and annual average. Based on previous drinking water assessments for 

pyriproxyfen use on watercress, using the Tier 1 Rice Model, the maximum EDWC for pyriproxyfen in 

surface source drinking water are not expected to exceed 2.98 μg/L for peak and annual average 

concentrations. In ground water, the EDWCs for pyriproxyfen are not expected to exceed 0.006 μg/L for 

peak and annual average concentrations. This assessment shows that past drinking water assessments for 

pyriproxyfen provide a higher EDWC. Although the remainder of this memorandum describes the 

modeling and results for EDWCs based on the proposed use on herbs, EFED recommends using 2.98 

μg/L for the EDWC. 

Table 1: Estimated Drinking Water Concentrations of Pyriproxyfen for the Proposed 

Use Compared with the Highest Estimated Drinking Water Concentration from Past 

Assessments

Source Water

Crop Use Peak Annual Average

μg/L

Herb Subgroup 19A 1.952 0.111

Watercress1 2.98

Ground 

Water

Herb Subgroup 19 A 0.0008

Citrus2 0.006 1

Estimates taken from 2009 Drinking Water Assessment D360614

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Estimates taken from Drinking Water Assessment D249524, D249527, D249530

Previous Assessments 

Numerous FQPA drinking water assessments have been conducted to support pyriproxyfen use on 

various terrestrial crops (cotton, pears, walnuts, almonds, fruiting vegetables, blueberries, guava, lychee, 

semi-tropical fruit, citrus, asparagus, leaves of root and tuber vegetables, leafy vegetables, small vines, 

and watercress, as well as sites to control fire ants (D228489, D249530, D249524, D249527, D274381, 

D274384, D360614). Based on past drinking water assessments for terrestrial crops, the pyriproxyfen

concentration in surface source drinking water is not expected to exceed 2.15 μg/L for the annual 1 in 10 

year peak and 0.40 μg/L for the 1 in 10 year annual mean concentration. The EDWC for watercress is not 

expected to exceed 2.98 μg/L, based on estimates using the Tier 1 Rice Model. 

Environmental Fate Assessment

Pyriproxyfen [4-phenoxyphenyl (RS)-2-(2-pyridyloxy)propyl ether] has low volatility, low water 

solubility, and is stable to hydrolysis (Table 3). Pyriproxyfen is not persistent when exposed to light (t1/2 

(aqueous) = 4 to 6 days; t1/2(soil) = 7 to 9 days). Pyriproxyfen is not persistent in aerobic soil (t1/2 = 6 to 9 

days), but is somewhat more persistent under aerobic aquatic conditions (t1/2 = 16 to 21 days). 

Pyriproxyfen is persistent under anaerobic conditions (t1/2 = 750 days). Field dissipation data from 

California (t1/2 = 36 days), Mississippi (t1/2 = 4), and California (t1/2 = 15 days) are consistent with 

submitted laboratory data. 
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Pyriproxyfen is expected to possess low mobility in the open environment, partitioning somewhat 

strongly into suspended and benthic sediment (Koc = 4980 to 34200). Pyriproxyfen shows moderate 

bioconcentration in fish (approximately 1800x), with a rapid depuration rate. The susceptibility of 

pyriproxyfen to aqueous photolysis (t1/2(aqueous) = 4 to 6 days) should contribute to the foliar dissipation 

rate. The overall potential foliar dissipation rate of pyriproxyfen will be partially limited by its stability to 

hydrolysis and relatively low potential for volatilization from foliage. 

Pyriproxyfen can contaminate surface water by spray drift and through runoff and sediment transport 

during rainfall events. The relatively high soil/water partitioning of pyriproxyfen indicates that the overall 

runoff loading potential of pyriproxyfen will be limited, and that its leaching potential is low.

Pyriproxyfen is expected to degrade rather rapidly in clear shallow waters and terrestrial aerobic 

environments. However, the stability to hydrolysis, resistance to anaerobic degradation (t1/2 = 750 days) 

and relatively low potential volatilization from water indicate that pyriproxyfen may be substantially 

more persistent in more turbid and/or deeper in the water columns. 

Major degradates of pyriproxyfen (formed at >10% in the laboratory studies) are 2-(2-

pyridyloxy)propionic acid (PYPAC) and 4-(4’-hydroxyphenoxy)phenyl-2-(2-pyridyloxy)propyl ether (4’-

OH-Pyr). The reported low soil/water partitioning of PYPAC (Kd = 0.11 to 0.34) indicates that it may 

have a substantial leaching potential. The mobility of PYPAC also indicates that the runoff loading 

potential may be high. Available laboratory data (Kd = 5.5 to 32.8; Kocs ranging from 921 to 4,250) 

indicates that 4’-OH-Pyr is immobile compared to PYPAC and that leaching potential for this degradate 

will be low regardless of persistence. 

Table 2. Environmental Fate Properties of Pyriproxyfen

PARAMETER VALUE

Chemical and Physical Properties

Chemical name(s) 2-[1-methyl-2-(4-phenoxyphenoxy)ethoxy]pyridine

Chemical Abstract Service number 95737-68-1

Molecular weight 321.5

Aqueous solubility (at 25°C) 0.37 mg / L

Henry’s Law Constant 1.1 X 10-7 atm m3/mol

Octanol-water partition coefficient (log Kow at 25°C ) 5.3

Environmental Fate Properties

Hydrolysis half life (pH 4, pH 7, pH 9) stable

Aqueous photolysis half life t 1/2 = 4 to 6 days

Soil photolysis half life t 1/2 = 6.8 to 8.5 days

Aerobic soil metabolism half-lives t 1/2 = 6 to 9 days

Aerobic aquatic metabolism half-lives (total system*) t 1/2 = 6.4 to 21 days

Anaerobic aquatic metabolism half-lives (total system*) t 1/2 = 750 days

Soil-water distribution coefficient (Koc) 4,980 to 34,200

* Calculated from combined residues of the test compound measured in both the aqueous and the soil/sediment 

phases of a given test system.
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Surface Water 

Tier 1 FIRST modeling was conducted to assess estimated drinking water concentration (EDWC) of 

pyriproxyfen in surface source drinking water from applications made to herbs. Input parameters for 

aquatic exposure modeling were derived from a previous risk assessment (D310074, D310077, D312392, 

D312390, D312390, D312461, D319446, D319448, D319453, and D313009, 8/8/05). Model input 

parameters are shown in Table 3. Tier 1 FIRST modeling predicts that pyriproxyfen concentrations in 

surface source drinking water are not expected to exceed 1.952 μg/L for the daily peak and 0.111 μg/L for 

the annual average (Table 4; Appendix).

Table 3. Pyriproxyfen Environmental Fate Data Used in the Drinking Water Exposure Assessment

Fate Property Value Source

Solubility in Water 0.367 mg/L Product Chemistry

Photolysis in Water 6.4 days MRID 43795501

Aerobic Soil Metabolism Half-life 27 days (3X aerobic soil metabolism half-life)

 

MRID 43795502

Hydrolysis Stable @ pH = 7 MRID 43073601

Aerobic Aquatic Metabolism 62.4 days MRID 42902001

Kads 185.3 MRID 43795504

Spray Drift 0.05 - aerial; 0.01 - ground EFED Input 

Parameter 

Guidance

Application Efficiency 0.95 -aerial 0.99 - ground EFED Input 

Parameter 

Guidance

Table 4. FIRST Estimated Pyriproxyfen Concentrations in Index Reservoir

Scenario PCA Peak EDWC Annual Average EDWC 

μg/L

Tier I 0.87 1.952 0.111
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Ground Water

SCI-GROW modeling was conducted to predict EDWC in ground source drinking water. The input 

parameters for SCI-GROW and EDWC are shown in Table 5. 

Table 5. Pyriproxyfen Environmental Fata Data Used in SCI-GROW

Fate Property Value Source

Aerobic Soil Metabolism Half-Life 27 days (3X aerobic soil metabolism 

half life) MRID 43795502

Kads 185.3 MRID 43795504

Koc 18530 Calculated for Soil Organic Carbon 

Fraction (0.01)

Monitoring Data

There are no surface or ground water monitoring data for pyriproxyfen and its degradation products.
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Appendix

FIRST Output

RUN No. 1 FOR Pyriproxyfen ON Herb Subgr * INPUT VALUES *

--------------------------------------------------------------------

RATE (#/AC) No.APPS & SOIL SOLUBIL APPL TYPE %CROPPED INCORP

ONE(MULT) INTERVAL Kd (PPB ) (%DRIFT) AREA (IN)

--------------------------------------------------------------------

0.067( 0.114) 2 14 185.3 367.0 GROUND( 6.4) 87.0 0.0

FIELD AND RESERVOIR HALFLIFE VALUES (DAYS)

--------------------------------------------------------------------

METABOLIC DAYS UNTIL HYDROLYSIS PHOTOLYSIS METABOLIC COMBINED

(FIELD) RAIN/RUNOFF (RESERVOIR) (RES.-EFF) (RESER.) (RESER.)

--------------------------------------------------------------------

27.00 2 0.00 6.40- 793.60 62.40 57.85

UNTREATED WATER CONC (MICROGRAMS/LITER (PPB)) Ver 1.1.1 MAR 26, 2008

--------------------------------------------------------------------

PEAK DAY (ACUTE) ANNUAL AVERAGE (CHRONIC)

CONCENTRATION CONCENTRATION

--------------------------------------------------------------------

1.952 0.111
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SCI-GROW Output

SCIGROW

VERSION 2.3

ENVIRONMENTAL FATE AND EFFECTS DIVISION

OFFICE OF PESTICIDE PROGRAMS

U.S. ENVIRONMENTAL PROTECTION AGENCY

SCREENING MODEL

FOR AQUATIC PESTICIDE EXPOSURE

SciGrow version 2.3

chemical:Pyriproxyfen

time is 4/12/2012 9:42:14

------------------------------------------------------------------------

 Application Number of Total Use Koc Soil Aerobic

 rate (lb/acre) applications (lb/acre/yr) (ml/g) metabolism (days)

------------------------------------------------------------------------

 0.067 2.0 0.134 1.85E+04 27.0

------------------------------------------------------------------------

groundwater screening cond (ppb) = 8.04E-04*

*Estimated concentrations of chemicals with Koc values greater than 9995 ml/g

are beyond the scope of the regression data used in SCI-GROW development.

If there are concerns for such chemicals, a higher tier groundwater exposure

assessment should be considered, regardless of the concentration returned

by SCI-GROW.

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