Drinking Water Exposure Assessment for Proposed New Uses of 

Abamectin on Chives, Dried Beans, and Bulb Onions (Crop 

Subgroup 3-07A)

Abamectin

CAS# 71751-41-2 PC Code 122804

Prepared by:

Greg Orrick, Environmental Scientist

U.S. Environmental Protection Agency

Office of Pesticide Programs

Environmental Fate and Effects Division

Environmental Risk Branch II

1200 Pennsylvania Ave., NW

Mail Code 7507P

Washington, DC 20460

Reviewed by:

Nelson Thurman, Senior Science Advisor

Brian Anderson, Branch Chief
Page 2 of 21

Table of Contents

1. EXECUTIVE SUMMARY .................................................................................................... 3

2. PROBLEM FORMULATION................................................................................................ 4

2.1. Use Characterization ........................................................................................................ 4

2.1.1. Usage Summary........................................................................................................ 7

3. ANALYSIS............................................................................................................................. 8

3.1. Fate and Transport Characterization ................................................................................ 8

3.1.1. Residues of Concern ............................................................................................... 11

3.2. Drinking Water Exposure Modeling.............................................................................. 11

3.2.1. Exposure Modeling Approach ................................................................................ 11

3.2.2. Input Parameters ..................................................................................................... 12

3.2.3. Modeling Results.................................................................................................... 14

3.2.4. Monitoring Data...................................................................................................... 16

3.2.5. Drinking Water Treatment...................................................................................... 16

4. CONCLUSIONS................................................................................................................... 16

5. REFERENCES ..................................................................................................................... 17

Appendix A. SCI-GROW Input/Output Data.............................................................................. 20

Appendix B. PE Input Files. ........................................................................................................ 21
Page 3 of 21

1. EXECUTIVE SUMMARY

This assessment provides Estimated Drinking Water Concentrations (EDWC) in surface 

water and in ground water from the use patterns of maximum abamectin exposure in support of 

the Health Effects Division’s (HED) human health risk assessment (Table 1.1). EDWCs of 

abamectin were modeled for the proposed uses on dried beans, fresh and dried chives, and the 

bulb onions crop subgroup (3-07A) and current uses on cotton and tomatoes. The proposed use 

on dry beans is the use pattern of maximum exposure used to represent potential drinking water 

exposure for the dietary assessment of the proposed uses. Exposure estimates reflect all residues 

of abamectin because the compound is stable to abiotic hydrolysis and acceptable data are not 

available to estimate the formation and decline of residues of concern via other degradation 

routes. For characterization, exposure estimates are also provided that reflect the degradation 

rates used in the previous assessment that are based on studies currently determined to be 

unacceptable. Surface water exposure estimates are adjusted by the national default percent 

cropped area (PCA) adjustment factor (87%). This default PCA does not necessarily mean that 

dry beans will occupy 87% of the watershed, but reflects the likelihood that abamectin may be 

used on multiple crops in the same watershed. Water resource monitoring data for abamectin are 

not available. Therefore, exposure estimates cannot be evaluated with monitoring data.

Table 1.1. Abamectin Total Residue EDWCs in Surface Water and Ground Water for Use 

in Dietary Risk Assessment Based on the Use Pattern of Maximum Exposure, Dry Beans.

A

Use (maximum rate) Source PCA 

(%)

1-in-10-year 

Peak Exposure 

(ppb)

1-in-10-year 

Annual Mean 

Exposure (ppb)

30-year Mean 

Exposure (ppb)

Dry Beans

(0.056 lbs a.i./A/year) Surface water 87 2.3 [1.1] A

1.3 [0.41] A0.94 [0.27] A

Tomatoes, cucurbits

(0.18 lbs a.i./A/year) Ground water N/A 1.6 × 10-3

A

 EDWCs assume persistence in degradation processes. Values based on (currently unacceptable) degradation 

inputs used in previous assessments are in brackets for characterization purposes.

The surface water EDWCs in Table 1.1 are 5x to 6x higher than those reported in the 

previous assessment (DP barcode 335118; USEPA, 2007). This increase is due to modeling 

different model scenarios and application dates in this assessment as well as the assumption in 

this assessment that the abamectin is stable to degradation in the absence of acceptable data.

The environmental fate of abamectin is uncertain because submitted environmental fate 

data are not acceptable other than submitted hydrolysis, batch equilibrium, and terrestrial field 

dissipation studies. The compound mixture appears to readily photolyze, biodegrade moderately 

slowly in aerobic soil, and persist in anaerobic soil. However, the studies supporting that 

characterization are considered unacceptable and new data need to be submitted. Abamectin 

does not hydrolyze and data have not been submitted on its biodegradation in water. The 

residues of concern of abamectin are unclear due to uncertainty in the environmental fate data. 

Abamectin is nonvolatile, has a wide range of low mobility in soil, and is expected to be 

transported off of agricultural fields mainly via spray drift and runoff of residues sorbed to 

suspended solids. Field studies indicate that abamectin moderately dissipates with a field 

dissipation half-life of 31 days.
Page 4 of 21

Exposure estimates for the proposed uses of abamectin likely exceed actual values that 

occur in drinking water reservoirs and ground water due to the screening design of the exposure 

models and variability and uncertainty in the submitted environmental fate data.

2. PROBLEM FORMULATION

Abamectin was first registered in 1985 for fire ant control on turf, lawns, and nonagricultural areas, followed by registrations in 1987 for use on ornamentals and in 1989 for use 

on cotton and citrus, the first agricultural uses. A variety of uses have been registered since. The 

Interregional Research Project No. 4 (IR-4) is now proposing use on dry beans, fresh and dried 

chives, and bulb onions (i.e., crop subgroup 3-07A). The bulb onions include “daylily, bulb; 

fritillaria, bulb; garlic, bulb; garlic, great-headed, bulb; garlic, serpent, bulb; lily, bulb; onion, 

bulb; onion, Chinese, bulb; onion, pearl; onion, potato, bulb; shallot, bulb; cultivars, varieties, 

and/or hybrids of these” (40 CFR Part 180.41; Jul. 1, 2010).

This drinking water assessment uses modeling to provide estimates of surface water and 

ground water concentrations of residues in drinking water source water (pre-treatment) resulting 

from abamectin use on vulnerable sites according to current and proposed labels. EDWCs 

reflect drinking water exposure to residues of concern for abamectin, which the Health Effects 

Division’s 2008 dietary risk assessment reported is the parent compound and a mixture of 8-αhydroxy and ring-opened aldehyde derivatives (“Compound 7”; DP 359583; USEPA, 2008). 

Primary routes of transport to source water include runoff, including erosion, and spray drift to 

surface water. Leaching of the residues of concern to ground water is not expected due to the 

compounds’ affinity for soil particles.

The models PRZM and EXAMS, coupled with the PE interface, were used to assess 

exposure in surface water due to runoff, erosion, and spray drift from the proposed and current 

abamectin uses. Exposure in ground water due to leaching was assessed with the screening 

model SCI-GROW.

2.1. Use Characterization

Abamectin is a mixture of avermectins containing at least 80% avermectin B1a (5-0-

demethyl avermectin A1a) and up to 20% avermectin B1b (5-0-demethyl-25-de(1-methylpropyl)-

25-(1-methylethyl) avermectin A1a). The avermectins are natural fermentation products of the 

soil bacterium Streptomyces avermitilis. The active ingredient, abamectin, is used as an 

insecticide (and miticide) and as a seed protectant nematocide. The compound acts as a chlorine 

channel agonist in invertebrates (Fritz, et al., 1979, Mellin et al., 1983 and Arena et al., 1991 in 

Sherma and Cairns, 1993), and may function as a gamma-aminobutyric acid (GABAergic) 

agonist (Kass et al., 1980, 1984 in Sherma and Cairns, 1993). It acts by stimulating the release 

of gamma-aminobutyric acid, an inhibitory neurotransmitter, thus causing paralysis (Tomlin, 

1994). The difference in toxicity between invertebrates and mammals may be partially due to 

different distribution of the GABAergic neurons (Turner and Schaeffer, 1989 in Sherma and 

Cairns, 1993).
Page 5 of 21

The compound is currently registered on dozens of Section 3 end-use labels with uses on 

numerous fruit, vegetable, and other agricultural crops, including seed treatment uses on cotton, 

corn, cucurbit, and tomato seeds, cattle ear-tag uses, and non-agricultural uses including 

ornamental uses, bait uses, and use in residential, commercial, industrial and other non-crop 

areas for fire ant- and related ant-control and for crack and crevice treatments. Proposed uses are 

for control of leafminers and thrips in bulb onions and control of leafminers and spider mites in 

dry beans and fresh and dried chives.

This assessment analyzes the proposed uses as well as currently labeled uses of 

abamectin. Table 2.1 displays the maximum labeled use patterns for each proposed use (dry

beans, bulb onions, and chives) and the current uses of maximum exposure (cotton, tomatoes, 

and cucurbits). Exposure estimates generated from these maximum use patterns were used to 

characterize drinking water exposure from abamectin use, both in surface water and in ground 

water.

Table 2.1. Maximum Use Patterns for Proposed and Selected Current Abamectin Uses. A

Use Pattern Current/

Proposed

Single App. 

Rate (lbs 

a.i./A)

Max. No. of App. B Seasonal App. 

Rate (lbs a.i./A) C

App. 

Interval 

(days) B

Application 

Type

Dry beans Proposed 0.019 Sets of 2 in sequence 0.056 6 Aerial/ground

Bulb onions Proposed 0.019 Sets of 2 in sequence 0.056 7 Aerial/ground

Chives Proposed 0.019 2 per cutting 0.056 7 Ground

Cotton Current 0.019 2 0.038 21 Aerial/ground

Tomatoes, 

cucurbits Current

0.019 Sets of 2 in sequence 0.056 7 Aerial/ground

0.12 1 0.12 N/A Seed treatment

A Current uses selected for this table are those that result in maximum exposure.

B Applications to chives are limited to two per cutting or harvest (which occur approximately six weeks apart, five to 

nine times per year; USEPA, 2010a). Applications to dry beans and bulb onions are limited to two sequential 

applications, with additional sets of two sequential applications allowed if a different insecticide is applied (twice 

in the case of bulb onions) in between sets of abamectin applications.

C One crop season occurs per year for the proposed and current crops.

Seasonal application rates are equivalent to annual application rates for the proposed and 

current uses because only one crop season occurs per year. The current uses on tomatoes and 

cucurbits, which were the uses of maximum exposure in previous drinking water exposure 

modeling (USEPA, 2007), have seed treatment uses as well as foliar uses. When the uses are 

evaluated collectively, this makes them the use patterns of highest ground water exposure. The 

current use on cotton has a seed treatment use as well. However, the application rate per seed 

(0.15 mg) is 25% less than that for tomatoes and cucurbits (0.60 mg), resulting in an 

insubstantial increase in exposure. The proposed use on dry beans (with no seed treatments) is 

the use pattern of highest surface water exposure. The maximum application rate for dry beans 

is 0.019 pounds of active ingredient per acre (lbs a.i./A), limited to sets of two sequential 

applications six days apart that are separated by application of a different insecticide (interval not 

provided) up to a total seasonal (or annual) application rate limit of 0.056 lbs a.i./A.
Page 6 of 21

The proposed uses may be applied by aerial or ground application equipment, with the 

exceptions that 1) use on chives is limited to ground equipment and 2) aerial application is 

prohibited in New York State for the other proposed uses on dry beans and bulb onions.

Figures 2.1 and 2.2 display Census of Agriculture maps of harvested acres of dry beans 

and dry onions, respectively, in 2007 (USDA, 2010). Maps of harvested acres for chives or other 

more specific proposed uses are not available from the Census.

Figure 2.1. Harvested Acres of Dry Edible Beans Other Than Limas in 2007 (USDA, 2010).
Page 7 of 21

Figure 2.2. Harvested Acres of Dry Onions in 2007 (USDA, 2010).

2.1.1. Usage Summary

Figure 2.3 presents the national agricultural usage pattern of abamectin in 2002 (USGS, 

2010). At that time, 21% of the national agricultural usage was on citrus fruits, followed by 19% 

on cotton, 13% on almonds, 9% on pears, and <6% on each of the remaining crops, grapes, 

tomatoes, strawberries, lettuce, avocados, and apples. These data are relatively consistent with 

BEAD’s Screening Level Usage Analysis (SLUA) of abamectin (dated October 6, 2010) based 

on source data from 2001 to 2008 (USEPA, 2010). However, usage on most crops is reportedly 

higher than USGS estimates, with amounts used on cotton and almonds surpassing that on citrus. 

The SLUA reports that cotton (10,000 lbs a.i.), almonds (6,000 lbs a.i.), and citrus (grapefruit, 

lemons, oranges, tangelos, and tangerines; collectively 3,000-4,500 lbs a.i.) account for the 

greatest amount of usage, followed by avocados, grapes, pears, strawberries, and tomatoes 

(accounting for 1,000 lbs a.i. of usage each).
Page 8 of 21

Figure 2.3. National Agricultural Usage of Abamectin in 2002 (USGS, 2010).

3. ANALYSIS

3.1. Fate and Transport Characterization

Abamectin (CAS No. 71751-41-2; PC code 122804; chemical structure in Figure 3.1) is 

a mixture of avermectins containing at least 80% avermectin B1a (5-0-demethyl avermectin A1a; 

CAS No. 65195-55-3) and up to 20% avermectin B1b (5-0-demethyl-25-de(1-methylpropyl)-25-

(1-methylethyl) avermectin A1a; CAS No. 65195-56-4). The environmental fate of this 

compound mixture is uncertain because submitted environmental fate data are not acceptable 

other than the submitted hydrolysis, batch equilibrium, and terrestrial field dissipation studies. 

Based on submitted data, including the currently unacceptable data used in recent assessments, 

abamectin moderately degrades in aerobic soil (with half-lives of 15 to 47 days) and persists in 

anaerobic soil (half-life of 267 days). Data have not been submitted on the biodegradation of 

abamectin in water; however, its close derivative ivermectin has an aerobic aquatic DT50 of 15 

days according to Loffler et al. (2005). The compound mixture is stable to hydrolysis. 

Illuminated, abamectin photolyzes readily (half-life of <1 day) on soil surfaces and in clear, 

shallow waters. However, there is uncertainty in the photolysis data because the studies are 
Page 9 of 21

unacceptable. Photolysis is not likely to be substantial where abamectin is incorporated or under 

canopy. Because the submitted photolysis and biodegradation studies of abamectin are not 

acceptable, the compound mixture will be conservatively assumed stable to these routes of 

degradation, which includes the assumption that any observed degradation results in structurally

similar degradates of similar toxicity and environmental fate to the parent compound mixture, 

until acceptable data are submitted to refine this assumption.

Figure 3.1. Chemical structure of abamectin (mixture of avermectins B1a and B1b; MRID 

46877502).

Abamectin is soluble in water only up to 0.010 mg/L (at 21°C) and is non-volatile under 

field conditions (vapor pressure of 1.5 × 10-9torr at 25°C). In soil, abamectin displays a wide 

range of low mobility (Kd range of 18 to 132 L/kg), with no correlation between absorption to 

soils and their organic carbon content. Therefore, the compound mixture is not expected to be a 

ground water concern. Important means of transport off of agricultural fields following 

application will include spray drift and runoff of residues sorbed to suspended solids. Field 

dissipation data appear to be consistent with the laboratory data, indicating that abamectin 

dissipation on the field is moderate (dissipation half-life of 31 days). However, leaching of 

abamectin and its delta-8,9-isomers to the 12-18-inch depth was observed by 28 days after 

application, indicating that abamectin residues may have increased mobility in some soils. A 

summary of the environmental fate characteristics of abamectin from registrant-submitted studies 

is presented in Table 3.1.
Page 10 of 21

Table 3.1. General Physicochemical Properties and Environmental Fate Parameters of 

Abamectin.

Parameter Value Reference

Physical/Chemical Parameters

Molecular mass avermectin B1a: 873.1 g/mol

avermectin B1b: 859.1 g/mol

Calculated

Vapor pressure (25°C) 1.5 x 10-9torr MRID 47436204

Henry’s Law constant (25°C) 1.7 x 10-7atm·m3/mol Calculated

Water solubility (21°C) 0.010 mg/L MRID 47436204

Octanol-water partition coefficient 

(KOW); (log KOW) (25°C)

avermectin B1a: 300,000; (5.5)

avermectin B1b: 100,000; (5.0)

MRID 46877502

Dissociation constant (pKa) 7.22 MRID 46877502

Persistence in Water

Hydrolysis half-life (pH 5, 7, and 9) No evidence of degradation MRID 131083

Aqueous photolysis DT50 [3.5, 12 hrs (0.29, 1.0 solar day)] A[MRID 159163]

A

Aerobic and anaerobic aquatic 

metabolism half-lives

(No submitted data) --

Persistence in Soil

Soil photolysis DT50 [21 hrs (1.7 solar days)]

A

[MRID 159163]

A

Aerobic soil metabolism half-lives [15, 20, 20, 28, 40, 47 d]

A

[MRID 159163]

A

Anaerobic soil metabolism half-life [267 d]

A

[MRID 159163]

A

Mobility

Soil-water partitioning coefficient 

(Kd)

77 ± 58 L/kg (range: 9.7-161 L/kg; 3 soils) MRID 40856301

Field Dissipation

Field dissipation half-life 31±6 d (leaching to 12-18” depth at 28 days) MRID 42627601; 

41191501

A Data in brackets are unacceptable and provided for characterization.

The major abamectin degradates in soil included a mixture of 8-α-hydroxy and ringopened aldehyde derivatives of the parent compound mixture (“Compound 7”; maximum of 20% 

of the applied 56 days after treatment) and two other unidentified compounds or mixtures 

(“Compound 4”, maximum of 12% of the applied at 84 days, and “Compound 12”, maximum of 

12% of the applied at 28, 56, 112 days). These major degradates appeared to be more persistent 

in soil than the parent compound mixture. Their mobility in soil is not determined.

The major abamectin photodegradates were not identified. However, three 

chromatographic fractions representing degradates were described, including a major non-polar 

fraction consistent with the delta-8,9-isomers of abamectin, a major “moderately polar” fraction, 

and a major polar fraction that consisted of multiple components.
Page 11 of 21

3.1.1. Residues of Concern

The Health Effects Division’s 2008 dietary risk assessment included abamectin and its 

soil degradate mixture “Compound 7” as residues of concern in drinking water (DP barcode 

359583; USEPA, 2008). Compound 7 was considered as toxic as the parent compound due to 

structural similarity and because its toxicity is unknown. Other major environmental degradates 

may meet these criteria as well pending their identification. Because submitted photolysis and 

biodegradation studies are not acceptable due to failure to identify major degradates among other 

deficiencies, abamectin is conservatively assumed stable to these routes of degradation, as stated 

above, which includes the assumption that any observed degradation results in structurally

similar degradates of similar toxicity and environmental fate to the parent compound mixture.

Due to this assumption of no degradation, only abamectin is modeled in this assessment as the 

residue of concern.

Once acceptable environmental fate data for abamectin are submitted, a total residues 

approach will be used to estimate exposure to the abamectin residues of concern. This approach 

involves the use of total-residues degradation half-lives based on the sum of residues of concern 

at each sampling interval in submitted laboratory degradation studies. Also, the chemical 

properties and soil mobility of the residue of concern most vulnerable to environmental transport 

are typically used to represent the chemical properties and soil mobility of all residues.

3.2. Drinking Water Exposure Modeling

3.2.1. Exposure Modeling Approach

The Tier II models, Pesticide Root Zone Model (PRZM v3.12.2, May 12, 2005) linked 

with EXposure Analysis Modeling System (EXAMS v2.98.04.06, Apr. 25, 2005) via the PE 

model shell (PE v5.0, Nov. 15, 2006, i.e., PRZM/EXAMS) were run to estimate screening-level 

exposure of surface water drinking water sources to abamectin following the proposed and 

current uses. Ground water drinking water sources were assessed with the Tier I model, 

Screening Concentration in Ground Water (SCI-GROW v2.3, Jul. 29, 2003).

The PE model generates estimated drinking water concentrations (EDWC) of abamectin 

residues that may occur in surface water used as drinking water down gradient from applications 

on terrestrial crops. The PRZM model simulates pesticide movement and transformation from 

crop application through soil residue processes. The EXAMS model simulates pesticide loading 

via runoff, erosion, and spray drift assuming a standard watershed of 172.8 ha that drains into an 

adjacent standard drinking water index reservoir of 5.26 ha and an average depth of 2.74 m. A 

more detailed description of the index reservoir watershed can be found in Jones et al., 2010.

SCI-GROW estimates a single screening-level ground water concentration that might be 

expected in shallow unconfined aquifers under highly permeable soils, for use as both an acute 

and chronic drinking water endpoint. SCI-GROW was developed by fitting a linear model to 

ground water concentrations with the Relative Index of Leaching Potential (RILP) as the 

independent variable. Ground water concentrations were taken from 90-day average high 

concentrations from Prospective Ground Water studies. The RILP is a function of aerobic soil 
Page 12 of 21

metabolism and the soil-water partition coefficient. The output of SCI-GROW represents the 

concentrations of abamectin residues that might be expected in shallow unconfined aquifers 

under sandy soils, which is representative of the ground water most vulnerable to pesticide 

contamination and likely to serve as a drinking water source.

The coupled PRZM/EXAMS (PE) model, SCI-GROW model, and associated users 

manuals may be downloaded from the U.S. Environmental Protection Agency (EPA) Water 

Models web-page (USEPA, 2010a).

3.2.2. Input Parameters

Ground Water Exposure

Input parameters for the SCI-GROW model appear in Table 3.2. The selected 

application inputs represent the maximum proposed or current use pattern, which is for the 

current uses on tomatoes and cucurbits (one seed treatment per year at 0.12 lbs a.i./A and three 

foliar applications per year at 0.019 lbs a.i./A). The seed treatment application rate of 0.12 lbs 

a.i./A was multiplied by 6%, producing 0.0072 lbs a.i./A, to account for the availability of 

abamectin on treated seeds (MRID 47000503). Because SCI-GROW cannot accept multiple 

applications at different rates (and abamectin is persistent below the soil surface), all applications 

were summed and input as one total application of 0.064 lbs a.i./A.

Table 3.2. SCI-GROW Input Parameters for Abamectin Residues. A

Input Parameter Value Source

Application Rate 0.064 lbs a.i./A B Current labels

Applications per Year 1

B Current labels

Organic Carbon Partition 

Coefficient (KOC)

4700 L/kgOC MRID 40856301

Aerobic Soil Metabolism

Half-life (from previously 

assessments) C

10,000 d (101 d) No acceptable data available 

(USEPA, 2007)

A

Source data are in Tables 2.1 and 3.1.

B

Input is annual sum of applications [(0.019 lbs a.i./A × 3) + 0.0072 lbs a.i./A = 0.064 lbs a.i./A] because SCIGROW does not accept different application rates per year.

C

 For characterization (in parentheses), the total residue half-life used in previous assessments (USEPA, 2007) was

also modeled.

The lowest measured mean organic carbon partition coefficient (4700 L/kgOC) from the 

submitted batch equilibrium study of abamectin (MRID 40856301) was input into SCI-GROW, 

in accordance with current input parameter guidance (USEPA, 2009), because of the greater than 

three-fold variation in measured values. Because acceptable aerobic soil metabolism data have 

not been submitted, the compound was assumed stable for the aerobic soil metabolism half-life 

input (represented as 10,000 days). For characterization, the total residues half-life value (101

days) used in previous assessments (USEPA, 2007) was modeled as well, although the data 

represented by the half-life (MRID 159163) have since been reclassified as unacceptable. Input 

and output data for SCI-GROW are attached in Appendix A.
Page 13 of 21

Surface Water Exposure

The model input parameters used in PE to simulate the scenarios and application 

practices for the proposed and current uses of abamectin are provided in Table 3.3. Input files 

used for modeling results are listed in Appendix B.

Table 3.3. PRZM Scenarios and Input Parameters Describing Proposed and Current 

Maximum Use Patterns of Abamectin.

Use Scenario

Date of 

Initial 

App.

App. Rate in lbs 

a.i./A

(kg a.i./ha)

App. 

per 

Year

App. 

Interval 

(days)

CAM 

Input

IPSCND 

Input

Application 

Efficiency/ 

Spray Drift

Dry beans MI beans Aug. 1 0.019 (0.021) 3 6, 12 A2 1 0.95/0.16

Bulb onions GA onion Apr. 24 0.019 (0.021) 3 7, 21 B2 1 0.95/0.16

Chives OR mint Jul. 1 0.019 (0.021) 3 7 2 1 0.99/0.064

Cotton MS cotton Jun. 15 0.019 (0.021) 2 21 2 1 0.95/0.16

Tomatoes FL tomato

Mar. 5 0.019 (0.021) 3 14, 7 C2 1 0.95/0.16

Jan. 18 0.0072 (0.0081) D1 -- 8 E1 1.0/0.0

A

 The second application occurs after 6 days. The third application was assumed to occur after 12 days, allowing 

for an application of an alternative insecticide after the second application.

B

 The second application occurs after 7 days. The third application was assumed to occur after 21 days, allowing 

for two applications of an alternative insecticide after the second application.

C

 The second application was assumed to occur after 14 days, following an application of an alternative insecticide. 

The third application occurs 7 days after the second application.

D The application rate of 0.12 lbs a.i./A was multiplied by 6%, producing 0.0072 lbs a.i./A, to account for the 

availability of abamectin on treated seeds (MRID 47000503).

E

 Treated seeds are planted at a 0.5-inch (1.3-cm) depth. However, a 2.0-cm incorporation depth was modeled due 

to limitations with PRZM.

Applicable scenarios were modeled for each proposed use. The Oregon mint scenario 

was used as a surrogate for chives in the absence of a scenario specific to chives and because 

mint is an herb as well. Scenarios listed in Table 3.3 are those that are most vulnerable to 

exposure for each use. Dates of initial application were selected that were within the application 

window and characterized by vulnerability to runoff. Runoff vulnerability was explored by

modeling a range of initial application dates approximately 14 days apart within the labeled 

application timing and scenario crop season, and selecting the date for which exposure estimates 

were highest. This was expected to produce high-end exposure estimates that are conservative 

but that may not represent the highest possible exposure. Use on cucurbits was not modeled, as 

was done in previous assessments, because use on tomatoes produces higher exposure. All foliar 

uses were modeled as aerially applied, with the exception that use on chives was modeled as a 

ground application per labeled directions.

For seed treatment of tomatoes, the application rate of 0.12 lbs a.i./A was multiplied by

6% (producing 0.0072 lbs a.i./A) to account for the availability of abamectin on treated seeds 

(MRID 47000503). The entire application (100% efficiency) was modeled with a 2.0-cm 

incorporation depth (chemical application method 8) due to limitations with PRZM, although 

treated seed may be planted at a 0.5-inch (1.3-cm) depth (DP 335118; USEPA, 2007).
Page 14 of 21

The chemical and environmental fate data for abamectin listed in Table 3.1 were used for 

generating model input parameters for PE (listed in Table 3.4). The chemical inputs were based 

on abamectin parent and reflect stability to degradation due to a lack of acceptable degradation 

data, with the exception that hydrolysis data indicate that abamectin is stable to abiotic 

hydrolysis. This results in exposure estimates that are conservative unless abamectin degrades 

into persistent residues of similar or greater toxicity and mobility. The soil-water partition 

coefficient (Kd) input (77 L/kg) is the mean of three mean values for abamectin.

Table 3.4. PE Chemical Input Parameters for Abamectin.

A

Input Parameter Value Justification Source

Molecular Mass (g/mol) 873.1 MW of ≥80% of avermectins in abamectin (Calculated)

Henry’s Law Constant 

(atm m3/mol) 1.7 x 10-7 Ratio of vapor pressure to water solubility (Calculated from MRID 47436204)

Solubility in Water 

(mg/L) 0.010 Study value MRID 47436204

Soil-water Partition 

Coefficient (Kd) (L/kg) 77 Mean of three mean values MRID 40856301

Aerobic Soil Metabolism 

Half-life (days) 0 [150] Assumed stable in absence of acceptable data [value from previous assessments]

No acceptable data 

[USEPA, 2007]

Aerobic Aquatic 

Metabolism Half-life 

(days)

0 [300] Assumed stable in absence of acceptable 

data [value from previous assessments]

No acceptable data 

[USEPA, 2007]

Anaerobic Aquatic 

Metabolism Half-life 

(days)

0

Assumed stable in absence of acceptable 

data No acceptable data

Hydrolysis Half-life at pH 

7 (days) 0 Study value MRID 131083

Aqueous Photolysis

Half-life (days) 0 [0.5] Assumed stable in absence of acceptable data [value from previous assessments]

No acceptable data 

[USEPA, 2007]

A

 For characterization, values in brackets represent total residue half-lives used in previous assessments (USEPA, 

2007).

3.2.3. Modeling Results

Ground Water Results

The single exposure estimate from the SCI-GROW model is used as a conservative peak 

and annual mean value in ground water leachate. The exposure estimate for the maximum use 

patterns (tomatoes and cucurbits) is 1.6 × 10-3μg/L (1.6 ng/L) assuming residues do not degrade. 

Using the mean total residue aerobic soil degradation half-life value used in previous 

assessments, for characterization, the maximum exposure estimate is 9.1 × 10-4μg/L (0.91 ng/L); 

however, the degradation data used to calculate this value have been downgraded to 

unacceptable. These values support the expectation that abamectin residues will not produce 

appreciable exposure in ground water due to low mobility in soil and low application rates.
Page 15 of 21

Surface Water Results

Proposed and current use patterns were modeled to estimate exposure in surface water, as 

described above. Maximum abamectin EDWCs in surface water per use are listed in Table 3.5. 

Exposure estimates were adjusted by the default national percent cropped area (PCA) value 

(87%) that accounts for the maximum area within any watershed that may be in agriculture 

(Jones et al., 2010a). Modeled daily EDWCs for foliar and seed-treatment uses on tomatoes 

were combined in an Excel spreadsheet prior to adjusting for the PCA and calculating the 

EDWCs reported in Table 3.5.

Table 3.5. Abamectin EDWCs in Surface Water from Proposed and Current Uses.

A

Use PRZM Scenario 1-in-10-year Peak 

Exposure (ppb)

1-in-10-year Annual 

Mean Exposure (ppb)

30-year Mean 

Exposure (ppb)

Proposed Uses

Dry beans MI beans 2.3 [1.1] 1.3 [0.41] 0.94 [0.27]

Bulb onions GA onion 1.4 [0.81] 0.63 [0.26] 0.49 [0.19]

Chives OR mint 0.91 [0.27] 0.67 [0.15] 0.54 [0.12]

Current Uses

Cotton MS cotton 1.2 [0.74] 0.58 [0.22] 0.42 [0.15]

Tomatoes FL tomato 1.5 [0.67] 0.63 [0.22] 0.53 [0.18]

A

 Maximum values are in bold. Values based on degradation inputs used in previous assessments are in brackets.

Exposure estimates from the proposed maximum use pattern for dry beans are greater in 

this national screen than those from other proposed and currently labeled uses. This use pattern 

of maximum exposure results in a 1-in-10-year peak EDWC (2.3 μg/L) that is 5x higher than that 

reported in the previous assessments for use on cucurbits (0.46 μg/L; USEPA, 2007). The 1-in10-year annual mean EDWC (1.3 μg/L) for use on dry beans is 6x higher than that reported in 

the previous assessments for use on tomatoes (0.21 μg/L). However, these 1-in-10-year peak 

and annual mean exposure estimates for use on dry beans are only 2x and 3x higher, 

respectively, than the exposure estimates based on the degradation inputs used in the previous 

assessments for characterization (listed in brackets in Table 3.5). Addition of seed treatment to 

the foliar use on tomatoes did not change 1-in-10-year peak exposure estimates for foliar use and 

increased 30-year mean exposure estimates by only 15%.

Values provided for characterization should not be used quantitatively in exposure 

assessment because they are based on unacceptable data. They indicate that the current version 

of PE, with most of the same inputs (dates of initial application used in previous modeling are 

unknown), results in 1-in-10-year peak and annual mean values for use of abamectin on tomatoes 

that are similar (1.0-1.5x) to those resulting from the previous version of PE.

There are a number of factors inherent in the modeling that can affect the accuracy of this 

analysis; these include the availability and quality of the model input data and the use of 

maximum labeled practices and environmental conditions that may differ from actual practices 

and conditions. The exposure values listed above are conservative for a number of reasons. The 

actual application area of these uses in drinking water basins is likely to be less than the total 
Page 16 of 21

cropped area of the watershed. Also, degradation rates for abamectin residues of concern were 

assumed stable in the absence of acceptable photolysis and biodegradation data.

3.2.4. Monitoring Data

This assessment contains a cursory review of available surface water and ground water 

monitoring data sources for data on abamectin. Abamectin is not a regulated chemical under the 

Safe Water Drinking Act (SWDA) or related statutes. The compound is not an analyte listed in 

the USEPA STORET database (USEPA, 2010b), the California Department of Pesticide 

Regulation (DPR) Surface Water Database (CDPR, 2010), or the USGS NAWQA program 

(USGS, 2010a). Therefore, water monitoring data on abamectin have not been located.

3.2.5. Drinking Water Treatment

Because direct data regarding the effects of drinking water treatment on abamectin

residues of concern have not been provided to the Agency and could not be located in the public 

literature, drinking water treatment effects on abamectin have been considered only qualitatively 

based on the physicochemical properties of the compound. Carbon filtering is expected to 

reduce abamectin residue concentrations due to the compound’s affinity to organic carbon.

Flocculation and sedimentation removal may be effective at reducing abamectin residue 

concentrations, especially if residues are bound to suspended sediment. Lime softening is not 

expected to reduce concentrations of abamectin residues because the compound mixture does not 

indicate susceptibility to abiotic hydrolysis.

4. CONCLUSIONS

Drinking water exposure estimates for the proposed uses of abamectin on dry beans, 

chives, and bulb onions are represented by the use pattern of maximum abamectin exposure, dry

beans (Tables 1.1 and 3.5). Maximum acute, chronic, and cancer EDWCs in surface water are 

2.3, 1.3, and 0.94 μg/L, respectively. The maximum exposure estimate for ground water (from 

use on tomatoes and/or cucurbits) is only 1.6 × 10-3μg/L. Monitoring and drinking water 

treatment data on abamectin are not available.

Abamectin was modeled in this assessment as stable to degradation because it is stable to 

hydrolysis and submitted photolysis and biodegradation data are not acceptable. It could not be 

concluded, based on the unacceptable data, that any observed degradation of abamectin resulted 

in degradates of much less toxicity.

Assessment modeling relied on maximum labeled use patterns and a default national 

PCA value. Where actual use patterns are less than the labeled maximums and location-specific 

PCAs are less than assumed in this assessment, actual environmental exposures may be lower.
Page 17 of 21

5. REFERENCES

California Department of Pesticide Regulation (CDPR). 2010. Surface Water Database. 

California Environmental Protection Agency, Department of Pesticide Regulation, 

Surface Water Protection Program. Data available through June, 2008. Online at: 

http://www.cdpr.ca.gov/docs/sw/surfdata.htm

Jones, R. D., K. Costello, J. Hetrick, J. Lin, R. Parker, N. Thurman, C. Peck. 2010. 

Development and Use of the Index Reservoir in Drinking Water Exposure Assessments. 

U.S. Environmental Protection Agency, Office of Pesticide Programs. April 15, 2010. 

Online at: http://www.epa.gov/oppefed1/models/water/index_reservoir_dwa.html

Jones, R. D., K. Costello, J. Hetrick, J. Lin, R. Parker, N. Thurman, C. Peck, G. Orrick. 2010a. 

Development and Use of Percent Cropped Area Adjustment Factors in Drinking Water 

Exposure Assessments. U.S. Environmental Protection Agency, Office of Pesticide 

Programs. September 9, 2010. Online at: 

http://www.epa.gov/oppefed1/models/water/pca_adjustment_dwa.html

Loffler D., J. Rombke, M. Meller, and T. Ternes. 2005. Environmental Fate of Pharmaceuticals 

in Water/Sediment Systems. Environ. Sci. Technol., 39, 5209-5218.

Sherma, J. and Cairns, T. Comprehensive analytical profiles of important pesticides. 1993. 

CRC Press, Inc. Boca Raton, Fl. p 75.

Tomlin, C.D.S. (ed.). 1994. The Pesticide Manual - World Compendium. 10th ed. Surrey, UK. 

The British Crop Protection Council, p. 4.

U.S. Department of Agriculture (USDA). 2010. 2007 Census of Agriculture: United States: 

Summary and State Data. U.S. Department of Agriculture, National Agricultural 

Statistics Service. Issued February, 2009; updated December, 2009. Online at: 

http://www.agcensus.usda.gov/Publications/2007/index.asp

USEPA. 2007. Abdel-Saheb, I. Revised Drinking Water Assessment for Abamectin as a Seed 

Treatment and as a Foliar Application on Cucurbits and Tomato Based on New Seed 

Leaching Study Submitted to EFED. DP Barcode 335118. U.S. Environmental 

Protection Agency, Office of Prevention, Pesticides, and Toxic Substances, 

Environmental Fate and Effects Division. Memorandum to the Registration Division. 

Mar. 20, 2007.

USEPA. 2008. Piper, S. Abamectin: (Acute and Chronic Aggregate Dietary Exposure and Risk 

Assessments for the Section 3 on Stone Fruit Crop Group 12, Tree Nut Crop Group 14, 

Pistachio, Tuberous and Corm Vegetables Subgroup 1C and Cattle Ear Tag Use. DP 

Barcode 359583. U.S. Environmental Protection Agency, Office of Prevention, 

Pesticides, and Toxic Substances, Health Effects Division. Memorandum to the Health 

Effects Division and Registration Division. Dec. 13, 2008.
Page 18 of 21

USEPA. 2009. Guidance for Selecting Input Parameters in Modeling the Environmental Fate 

and Transport of Pesticides. Version 2.1. U.S. Environmental Protection Agency, Office 

of Pesticide Programs, Environmental Fate and Effects Division, Oct. 22, 2009. Online 

at: http://www.epa.gov/oppefed1/models/water/input_parameter_guidance.htm

USEPA. 2010. Screening Level Usage Analysis (SLUA) of Abamectin (122804). U.S. 

Environmental Protection Agency, Office of Chemical Safety and Pollution Prevention, 

Biological and Economic Analysis Division. Oct. 6, 2010.

USEPA. 2010a. Water Models. U.S. Environmental Protection Agency, Pesticides: Science 

and Policy. Last updated: Dec. 8, 2010. Online at: 

http://www.epa.gov/oppefed1/models/water/

USEPA. 2010b. STORET. U.S. Environmental Protection Agency, Office of Water. Online at: 

http://www.epa.gov/STORET/

United States Geological Survey (USGS). 2010. 2002 Pesticide Use Maps. United States 

Geological Survey, National Water-Quality Assessment (NAWQA) Program, Pesticide 

National Synthesis Project. Online at: http://water.usgs.gov/nawqa/pnsp/usage/maps/

USGS. 2010a. National Water-Quality Assessment (NAWQA) Program. United States 

Geological Survey. Data available through Sep. 30, 2007. Last updated: Dec. 14, 2010. 

Online at: http://infotrek.er.usgs.gov/apex/f?p=NAWQA:HOME:3966441777962223

Submitted Environmental Fate and Product Chemistry Data

MRID 131083. Maynard, M.; Ku, C.; Gruber, V.; et al. (1982) Hydrolysis of Avermectin B1a 

(MK-0936). (Unpublished study received Jan 3, 1983 under 618-EX-10; submitted by

Merck & Co., Inc., Rahway, NJ; CDL:249152-F)

MRID 159163. MSD AGVET, Inc. (19??) Chemical and Physical Properties of Technical MK936 (Avermectin B1) and Avid 0.15 EC Miticide/Insecticide for Experimental Use on 

Citrus. (Compilation; unpublished study received Dec 29, 1983 under 50658-EX-1; 

CDL:252113-A)

MRID 40856301. Wislocki, P. (1988) Position Paper: Abamectin--Soil Dissipation and Crop 

Rotation: Supplemental: Project ID: 618-936-SD-CR. Unpublished study prepared by

Merck Sharp & Dohme Research Labora- tories. 47 p.

MRID 41191501. Wehner, T. (1989) Additional Soil Residue Data in Support of Applications 

for Registration of Abamectin Soil Leaching and Dissipation Study: Project ID: Merck 

No. 001-87-6045R; ADC No. 992. Unpublished study prepared by Merck Sharp & 

Dohme Research Laboratories and Analytical Development Corp. 18 p.
Page 19 of 21

MRID 42627601. Wertz, P. (1993) Soil Residue Data in Support of an Application for 

Registrations of Abamectin: Soil Leaching and Dissipation Study 001-87-6045R: A 

Supplement: Lab Project Number: 618-936-6045. Unpublished study prepared by

Analytical Development Corp. 603 p.

MRID 46877502. Friis, M. (2006) Product Chemistry - Abamectin Technical (CHA 204) - EPA 

Reg. No. 4787 - (none yet) Group B - Physical /Chemical Properties. Project Number: 

MVF/060500/ABA/204/B/USA, PYC/170, PYC/171. Unpublished study prepared by

Cheminova Agro A/S. 399 p.

MRID 47436204. Wagner, J. (2008) Abamectin Technical Herbicide: Stability to Normal and 

Elevated Temperatures, Metals, and Metal Ions: Oxidation/Reduction: Flammability: 

Explodability: Storage Stability: Miscibility: Corrosion Characteristics: Dielectric 

Breakdown Voltage: Viscosity: Boiling Point: Dissociation Constants in Water: Particle 

Size, Fiber Length, and Diameter Distribution: Partition Coefficient: Water Solubility: 

Vapor Pressure. Unpublished study prepared by Wagner Regulatory Associates, Inc. 6 p.

MRID 47000503. Clark, A. (2005) Abamectin FS 400 Pre-Treated Tomato Seeds and Abamectin 

FS 500 Pre-Treated Corn Seeds: Desorption of (Carbon 14) Abamectin Following 

Applications of Pre-Treated Corn and Tomato Seeds to Various Soils: Final Report. 

Project Number: T005067/05. Unpublished study prepared by Syngenta Crop Protection. 

63 p.
Page 20 of 21

Appendix A. SCI-GROW Input/Output Data.

SCI-GROW Input/Output File.

SciGrow version 2.3

chemical:Abamectin

time is 1/10/2011 13:47:22

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

 Application Number of Total Use Koc Soil Aerobic

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

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

 0.064 1.0 0.064 4.70E+03 10000.0

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

groundwater screening cond (ppb) = 1.58E-03 

************************************************************************

SciGrow version 2.3

chemical:Abamectin

time is 1/10/2011 15:22:12

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

 Application Number of Total Use Koc Soil Aerobic

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

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

 0.064 1.0 0.064 4.70E+03 101.0

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

groundwater screening cond (ppb) = 9.14E-04 

************************************************************************
Page 21 of 21

Appendix B. PE Input Files.

Table B.1. Summary of PE Input File Names.

Use File names Date

Dry beans Aba_bean.out Aba_bean_char.out

Jan. 11, 2010

Bulb onions Aba_onio.out Aba_onio_char.out

Chives Aba_mint.out Aba_mint_char.out

Cotton Aba_cott.out Aba_cott_char.out

Tomatoes Aba_toma.out Aba_toma_char.out

Aba_toma_seed.out Aba_toma_seed_char.out

Example Input File (Use on Dry Beans)

Output File: Aba_bean

Metfile: w14826.dvf

PRZM scenario: MIbeansSTD.txt

EXAMS environment file: ir298.exv

Chemical Name: Abamectin

Description Variable Name Value Units Comments

Molecular weight mwt 873.1 g/mol

Henry's Law Const. henry 1.7e-7 atm-m^3/mol

Vapor Pressure vapr torr

Solubility sol 0.010 mg/L

Kd Kd 77 mg/L

Koc Koc mg/L

Photolysis half-life kdp 0 days Half-life

Aerobic Aquatic Metabolism kbacw 0 days Halfife

Anaerobic Aquatic Metabolism kbacs 0 days Halfife

Aerobic Soil Metabolism asm 0 days Halfife

Hydrolysis: pH 7 0 days Half-life

Method: CAM 2 integer See PRZM manual

Incorporation Depth: DEPI cm

Application Rate: TAPP 0.021 kg/ha

Application Efficiency: APPEFF 0.95 fraction

Spray Drift DRFT 0.16 fraction of application rate applied to pond

Application Date Date 01-08 dd/mm or dd/mmm or dd-mm or dd-mmm

Interval 1 interval 6 days Set to 0 or delete line for single app.

app. rate 1 apprate kg/ha

Interval 2 interval 12 days Set to 0 or delete line for single app.

app. rate 2 apprate kg/ha

Record 17: FILTRA

IPSCND 1

UPTKF

Record 18: PLVKRT

PLDKRT

FEXTRC 0.5

Flag for Index Res. Run IR Reservoir

Flag for runoff calc. RUNOFF total none, monthly or total(average of entire run)