Tier I method for estimating exposure 

of honey bees to pesticides

U.S. Environmental Protection Agency 1

Kris Garber, M.S.

Environmental Fate and Effects Division

Office of Pesticide Programs
Outline

• Overview of Tier I exposure assessment

• Food Consumption

• What do honey bees eat?

• Proposed food consumption rates for larval 

U.S. Environmental Protection Agency 2

• Proposed food consumption rates for larval 

and adult workers

• Discussion of conservativeness of proposed 

consumption rates

• Estimating Exposures for Tier I assessment

• Foliar Spray Applications

• Seed Treatments

• Soil Applications
Overview of Tier I exposure assessment

U.S. Environmental Protection Agency 3

Overview of Tier I exposure assessment
Purpose of Tier I Exposure Assessment

• The goal is to generate “reasonably conservative” estimates of 

pesticide exposures to bees

• Intended to distinguish between:

• Pesticides that do not pose a risk to bees and 

U.S. Environmental Protection Agency 4

• Pesticides that do not pose a risk to bees and 

• those that may need additional information

• Type I and II Errors

• Tier I assessment should not conclude that there is no effect when there 

actually is (Type II)

• It is more acceptable at the Tier I level to conclude that there is a 

potential effect when there is none (Type I)
1. Details of the product and its use pattern

2b. Is exposure of bee 

brood a concern?

Yes

No

Contact Exposure Oral Exposure Oral Exposure

No Tier 1 

brood 

assessment

Presumption 

of minimal 

risk

Tier I exposure assessment component of decision tree for 

foliar spray applications.

2a. Is exposure of adult 

bees a concern?

U.S. Environmental Protection Agency 5

3a. Calculate Tier 1 

screening-level EEC for 

adult contact exposure

3b. Calculate Tier 1 screeninglevel EEC for adult oral 

exposure via pollen and nectar

3c. Calculate Tier 1 

screening-level EEC for larval 

oral exposure via brood food

4a. Calculate Tier 1 

screening-level RQs for 

adult contact exposure

(RQ = EEC/adult acute 

contact LD50) 

4b. Calculate Tier 1 screeninglevel RQs for adult oral exposure

(RQ = EEC/adult acute oral LD50

& 

RQ = EEC/adult chronic 

NOAEC)* 

4c. Calculate Tier 1 

screening-level RQs for larval 

oral exposure

(RQ = EEC/larval acute LD50

& 

RQ= EEC/larval chronic 

NOAEC)* 
1. Details of the product and its use pattern

2b. Is exposure of bee 

brood a concern?

Yes

No

Oral Exposure Oral Exposure

No Tier 1 

brood 

assessment

Presumption 

of minimal 

risk

Tier I exposure assessment component of decision tree for 

seed and soil treatments.

2a. Is exposure of adult 

bees a concern?

U.S. Environmental Protection Agency 6

3a. Calculate Tier 1 screeninglevel EEC for adult oral 

exposure via pollen and nectar

3b. Calculate Tier 1 

screening-level EEC for larval 

oral exposure via brood food

4a. Calculate Tier 1 screeninglevel RQs for adult oral exposure

(RQ = EEC/adult acute oral LD50

& 

RQ = EEC/adult chronic 

NOAEC)* 

4b. Calculate Tier 1 

screening-level RQs for larval 

oral exposure

(RQ = EEC/larval acute LD50

& 

RQ= EEC/larval chronic 

NOAEC)* 
Food consumption of honey bees

U.S. Environmental Protection Agency 7

Food consumption of honey bees
• What do honey bees eat? • Pollen • Bee bread • Nectar • Honey

U.S. Environmental Protection Agency

8

Honey bee diet • Honey • Jelly • Royal Jelly • Brood Food • Food consumption varies • By caste • By age
Proposed food consumption rates 

for Tier I exposure assessment

• Adult honey bees = 292 mg food/day

• Based on nectar foraging worker 

• Represents consumption of nectar

• Pollen consumption is insignificant relative to nectar

U.S. Environmental Protection Agency 9

• Pollen consumption is insignificant relative to nectar

• Larvae = 120 mg food/day

• Based on 5 day old worker larvae

• Represents consumption of honey (converted to nectar equivalent) 

and pollen
• Pesticide does not dissipate while stored in the hive

• Pesticide concentrations in pollen and bee bread are equivalent

• Pesticide concentrations in nectar can be used to represent concentrations in 

honey

• Honey consumption rate can be converted to a nectar equivalent 

U.S. Environmental Protection Agency 10

Assumptions for Tier I Exposure Assessment 

(related to Food Consumption)

• Honey consumption rate can be converted to a nectar equivalent 

basis

• Using sugar consumption rates and sugar contents of honey and nectar

• Pesticide doses will be eqivalent

• Pesticide doses received from pollen and nectar are protective of 

doses from jelly

• Available data indicate that pesticides are ≥100x greater in food of nurse bees 

compared to royal jelly

• Pesticide concentration in foliage = conc. in nectar = conc. in pollen
Proposed food consumption rate: 

Adult Worker Bees

• Food Consumption rates for adult worker bees

• Nectar: Rortais et al. (2005)

• Pollen: Crailsheim et al (1992) 

• Nectar forager bees have highest food consumption rates

U.S. Environmental Protection Agency 11

Consumption rate (mg/day) Adult worker task Age (d) Consumption rate (mg/day)

Pollen Nectar Total food

Cell cleaning and capping 0-10 2.2-8.2 60 62-68

Brood and queen attending 6-17 1.7-9.5 113-167 115-177

Comb building, cleaning and 

food handling

11-18

1.7 60 62

Forager (pollen) >18 0.041 35-52 35-52

Forager (nectar) >18 0.041 107-428 107-428
Proposed food consumption rate: 

Adult Worker Bees = 292 mg/day

• Proposed value is estimated using modification to 

Rortais et al.’s method

• Monte Carlo Simulation of 5 variables

• Sugar required for flying

U.S. Environmental Protection Agency 12

50%

60%

70%

80%

90%

100%

P

erc

e

n

tile

• Sugar required for flying

• Number of foraging trips made in a day

• Duration of foraging trip

• Fraction of time spent flying during trip

• Amount of sugar present in nectar

• Analysis also included sugar requirements while resting

• Proposed value is median of 10,000 simulated bees

0%

10%

20%

30%

40%

50%

0 200 400 600 800

Nectar consumption (mg/day)

P

erc

e

n

tile
U.S. Environmental Protection Agency 13

Nectar and pollen consumption rates of adult worker bees by task and drones.

150

200

250

300

Pollen

Nectar

F

o

o

d

c

o

n

s

u

m

p

tio

n (m

g

/

d

a

y)

0

50

100

150

Worker (cell 

cleaning and 

capping)

Worker (brood 

and queen 

tending, nurse 

bees)

Worker (comb 

building, 

cleaning and 

food handling)

Worker 

(foraging for 

pollen)

Worker 

(foraging for 

nectar)

Worker 

(maintenance 

of hive in 

winter)

Drone

Caste (task)

F

o

o

d

c

o

n

s

u

m

p

tio

n (m

g

/

d

a

y)
150

200

250

300

Pollen

Honey

F

o

o

d

c

o

n

s

u

m

p

tio

n (m

g

/

d

a

y)

U.S. Environmental Protection Agency 14

Honey and pollen consumption rates of adult worker bees by task and drones.

0

50

100

150

Worker (cell 

cleaning and 

capping)

Worker (brood 

and queen 

tending, nurse 

bees)

Worker (comb 

building, 

cleaning and 

food handling)

Worker 

(foraging for 

pollen)

Worker 

(foraging for 

nectar)

Worker 

(maintenance 

of hive in 

winter)

Drone

Caste (task)

F

o

o

d

c

o

n

s

u

m

p

tio

n (m

g

/

d

a

y)
Comparison of proposed food consumption 

rates (mg/day) to non-Apis bees: Adults

Species Nectar Pollen Total food

Honey bee worker

(Apis mellifera)

292 0.04 292

Bumblebee

U.S. Environmental Protection Agency 15

(Bombus spp.)

183-372 27-30 210-402

European mason bee

(Osmia cornuta) 45-193 na 45-193

Alfalfa leaf-cutting bee

(Megachile rotundata)

110-165 na 110-165
Proposed food consumption rate:

Larval Worker Bees

• It is assumed that larvae grow exponentially and that their daily 

food consumption rate doubles every day 

• Consume 120 mg total food during days 4 and 5

• 5.4 mg pollen

• 115 mg honey (diluted to 45% sugar; Rortais et al. 2005)

U.S. Environmental Protection Agency 16

Day of life 

stage 

Daily food consumption rate (mg/day) 

Brood food / 

royal jelly Honey Pollen Total food

1 3.8 none none 3.8

2 7.5 none none 7.5

3 15 none none 15

4 none 37 2.7 40

5 none 77 2.7 80
Proposed food consumption rate:

Larval Worker Bees

• Proposed value of 120 mg/day is based on 5th day of life stage

• Highest food consumption value compared to other days of larval life 

stage

• Consumption of 2.7 mg pollen

• Honey consumption rate is converted to nectar equivalent rate 

U.S. Environmental Protection Agency 17

(117 mg/day)
150

200

250

300 Royal jelly/brood food

Pollen

Honey

F

o

o

d

c

o

n

s

u

m

p

tio

n (m

g

/

d

a

y) Honey, pollen and brood food or royal jelly consumption rates of larvae of 

different castes and ages. 

U.S. Environmental Protection Agency 18

0

50

100

150

Worker 

(1)

Worker 

(2)

Worker 

(3)

Worker 

(4)

Worker 

(5)

Drone 

(5)

Drone 

(6)

Queen 

(1)

Queen 

(2)

Queen 

(3)

Queen 

(4)

Queen 

(5)

Caste (day of life stage)

F

o

o

d

c

o

n

s

u

m

p

tio

n (m

g

/

d

a

y)
150

200

250

300 Royal jelly/brood food

Pollen

Honey

F

o

o

d

c

o

n

s

u

m

p

tio

n (m

g

/

d

a

y)

U.S. Environmental Protection Agency 19

Honey, pollen and brood food or royal jelly consumption rates of larvae of 

different castes and ages. 

0

50

100

150

Worker 

(1)

Worker 

(2)

Worker 

(3)

Worker 

(4)

Worker 

(5)

Drone 

(5)

Drone 

(6)

Queen 

(1)

Queen 

(2)

Queen 

(3)

Queen 

(4)

Queen 

(5)

Caste (day of life stage)

F

o

o

d

c

o

n

s

u

m

p

tio

n (m

g

/

d

a

y)
150

200

250

300 Royal jelly/brood food

Pollen

Honey

F

o

o

d

c

o

n

s

u

m

p

tio

n (m

g

/

d

a

y)

U.S. Environmental Protection Agency 20

Honey, pollen and brood food or royal jelly consumption rates of larvae of 

different castes and ages. 

0

50

100

150

Worker 

(1)

Worker 

(2)

Worker 

(3)

Worker 

(4)

Worker 

(5)

Drone 

(5)

Drone 

(6)

Queen 

(1)

Queen 

(2)

Queen 

(3)

Queen 

(4)

Queen 

(5)

Caste (day of life stage)

F

o

o

d

c

o

n

s

u

m

p

tio

n (m

g

/

d

a

y)
U.S. Environmental Protection Agency 21

150

200

250

300

Royal jelly/brood food

Pollen

Nectar

F

o

o

d

c

o

n

s

u

m

p

tio

n (m

g

/

d

a

y)

Nectar, pollen and brood food or royal jelly consumption rates of larvae of 

different castes and ages. 

0

50

100

150

Worker 

(1)

Worker 

(2)

Worker 

(3)

Worker 

(4)

Worker 

(5)

Drone 

(5)

Drone 

(6)

Queen 

(1)

Queen 

(2)

Queen 

(3)

Queen 

(4)

Queen 

(5)

Caste (day of life stage)

F

o

o

d

c

o

n

s

u

m

p

tio

n (m

g

/

d

a

y)
8

10

12

14

Royal jelly/brood food

Pollen

Nectar

g

a.i./

b

e

e) U.S. Environmental Protection Agency 22

Pesticide doses received by larvae of different castes and ages through 

consumption of pollen and nectar containing 100 μg a.i./kg 

and royal jelly containing 1 μg a.i./kg. 

0

2

4

6

8

Worker 

(1)

Worker 

(2)

Worker 

(3)

Worker 

(4)

Worker 

(5)

Drone 

(5)

Drone 

(6)

Queen 

(1)

Queen 

(2)

Queen 

(3)

Queen 

(4)

Queen 

(5)

Caste (day of life stage)

P

e

s

ticid

e

d

o

s

e (μ

g

a.i./

b

e

e)
Comparison of proposed food consumption 

rates (mg/day) to non-Apis bees: Larvae

U.S. Environmental Protection Agency 23

Species Male/ female Nectar Pollen Total food

Honey bee

(Apis mellifera) Female 117 2.7 120

Bumblebee

(Bombus spp.)

unknown 60 22-23 82-83

European mason bee

(Osmia cornuta)

Female 1.8 16.3 18

Male 1.1 9.5 11

Alfalfa leaf-cutting bee

(Megachile rotundata)

Female 6.2 3.1 9.3

Male 5.2 2.6 7.8
Summary of Food Consumption Analysis

• Assume that pesticide doses received through consumption of pollen 

and nectar can be used to conservatively represent other types of 

food

• Proposed food consumption rates

• Adults = 292 mg/day (nectar forager)

Larvae = 120 mg/day (5 day old)

U.S. Environmental Protection Agency 24

• Larvae = 120 mg/day (5 day old)

• Appear to be protective for other honey bees and some non-Apis bees

• Interested in SAP comments on proposed food consumption rates, 

related assumptions, strengths and limitations

• Charge question 5

• Interested in SAP comments on relative protectiveness of proposed 

food consumption rates in representing exposures to non-Apis bees

• Charge question 3
Tier I methods for estimating pesticide 

concentrations on bees and in pollen and nectar

U.S. Environmental Protection Agency 25

concentrations on bees and in pollen and nectar
Estimating pesticide exposures for Tier I assessment

• Foliar applications

• Contact EEC- Koch and Weisser 1997

• Dietary EEC- T-REX tall grass upper bound

• Seed treatments

Dietary EEC - EPPO default value of 1 mg a.i./kg

U.S. Environmental Protection Agency 26

• Dietary EEC - EPPO default value of 1 mg a.i./kg

• Soil treatments

• Dietary EEC - Modified Briggs’ Model

• All dietary EECs converted to a dose using proposed food 

consumption rates for adult (292 mg/day) and larval (120 

mg/day) workers
Identification and Evaluation of Methods

• Methods considered

• Many are currently used for regulatory purposes

• Other methods available in the open literature

• Evaluation of Methods

U.S. Environmental Protection Agency 27

• Evaluation of Methods

• Compared to empirical data from scientific literature and unpublished 

registrant studies

• Amount of data available to evaluate each method varied

Application type (route) Method Number of studies

Foliar spray (contact) Koch and Weisser 2

Foliar spray (diet) T-REX (tall grass) 11

Seed treatment (diet) EPPO (1 mg/kg) 12

Soil treatment (diet) Briggs’ Model 6
• Description of Proposed Method

• Contact Dose = 2.7 μg a.i./bee * Application rate (in lb a.i./A)

• From Koch and Weisser (1997)

U.S. Environmental Protection Agency 28

Estimating Contact Exposures for 

Foliar Spray Applications

• From Koch and Weisser (1997)

• Based on maximum concentration of chemical tracer measured on bees 

foraging on treated areas

• 5 trials on Phacelia fields (total number of bees analyzed = 1724)

• 9 trials on apple orchards (total number of bees analyzed = 4316)
U.S. Environmental Protection Agency 29

Estimating Contact Exposures for 

Foliar Spray Applications

70

80

90

100

Trial 1

Trial 2

o

f

b

e

e

s in

trial

Frequency distribution of measured tracer on individual bees during 5 trials with 

Phacelia fields. 

43% of 

bees

0

10

20

30

40

50

60

70

<=0.26 >0.26 to 

<=0.53

>0.53 to 

<=0.79

>0.79 to 

<=1.1

>1.1 to 

<=1.3

>1.3 to 

<=1.6

>1.6 to 

<=1.8

>1.8 to 

<=2.1

>2.1 to 

<=2.4

>2.4

Trial 3

Trial 4

P

Trial 5

erc

e

n

t

o

f

b

e

e

s in

trial

Mass of tracer measured on individual bees (μg a.i./bee per 1 lb/A)
70

80

90

100

Trial 1

Trial 2

Trial 3

o

f

b

e

e

s in

trial

U.S. Environmental Protection Agency 30

Estimating Contact Exposures for 

Foliar Spray Applications

Frequency distribution of measured tracer on individual bees during 9 trials with 

apple orchards. 

71% of 

bees

0

10

20

30

40

50

60

70

<=0.26 >0.26 to 

<=0.53

>0.53 to 

<=0.79

>0.79 to 

<=1.1

>1.1 to 

<=1.3

>1.3 to 

<=1.6

>1.6 to 

<=1.8

>1.8 to 

<=2.1

>2.1 to 

<=2.4

>2.4

Trial 4

Trial 5

Trial 6

Trial 7

Trial 8

Trial 9

P

erc

e

n

t

o

f

b

e

e

s in

trial

Mass of tracer measured on individual bees (μg a.i./bee per 1 lb/A)
• Discussion of Relevance of T-REX arthropod residue value

• Contact Dose = 12 μg a.i./bee * Application rate (in lb a.i./A)

• Value represents 95th percentile residue value (94 mg a.i./kg) converted to a 

dose using weight of a bee (0.128 g)

U.S. Environmental Protection Agency 31

Estimating Contact Exposures for 

Foliar Spray Applications

• Based on analysis of pesticide residues on crickets, grasshoppers, beetles, 

etc. located on treated field at the time of the application

• Limitation: data set does not include residue data for honey bees
• Method Evaluation

• No upper bound residues available to evaluate proposed value

• Mean Koch and Weisser (1997) data and T-REX arthropod residue values 

are consistent with means of empirical data from 2 other studies

U.S. Environmental Protection Agency 32

Estimating Contact Exposures for 

Foliar Spray Applications

8

9

a.i./

b

e

e

p

er 1 lb

a.i./

A)

0

1

2

3

4

5

6

7

8

T-REX 

(arthropod) 

Koch and 

Weisser 1997 

(Phacelia)

Koch and 

Weisser 1997 

(apple)

Delabie et al. 

1985 

Hanny and 

Harvey 1982 

Hanny and 

Harvey 1982 

Source

M

e

a

n

c

o

n

t

a

c

t

d

o

s

e

(μ

g

a.i./

b

e

e

p

er 1 lb

a.i./

A)
• Assumptions and Uncertainties

• Limited number of studies available for evaluation of method

• Based on only two crops

• Based on one study site 

U.S. Environmental Protection Agency 33

Estimating Contact Exposures for 

Foliar Spray Applications

• Strengths of proposed method

• Koch and Weisser (1997) maximum value appears to be conservative

• Robust study design

• Maximum value is based on measurements of >6000 bees

• Tracer did not impact study results

• Consistent with other methods that are empirically based

• T-REX arthropod residue value (factor of 5 different)

• Atkins et al. 1981
• Description of Proposed Method

• Use T-REX upper bound residue value on foliage as a surrogate for pollen 

and nectar

• Sufficient data are not available to derive nectar or pollen specific residue 

values

U.S. Environmental Protection Agency 34

Estimating Dietary Exposures for 

Foliar Spray Applications

Plant Description

Concentration (mg a.i./kg per 1 lb a.i./A)

Upper-bound Mean

Short grass 240 85

Broadleaf plants 135 45

Tall grass 110 36

Fruit, pods and seeds 15 7
• Method Evaluation: mean empirical data for nectar (n = 10)

• Mean residues for short grass, broad leaf plants and tall grass are all higher than mean 

empirical data

• 4 empirical values exceed the mean residue for fruit, pods and seeds

• Maximum residues only available for some studies (0.17-2.2 mg a.i./kg per 1 lb a.i./A)

U.S. Environmental Protection Agency 35

Estimating Dietary Exposures for 

Foliar Spray Applications

90

100

A)

Short grass

0

10

20

30

40

50

60

70

80

90

chemical

m

e

a

n

c

o

n

c

e

n

tra

tio

n

(m

g

a.i./

k

g

p

er 1 lb

a.i./

A)

Short grass

Broadleaf

Tall grass

Fruit, etc.
• Method Evaluation: mean empirical data for pollen (n = 9)

• Mean residue for short grass is higher than mean empirical data

• Mean residues for broad leaf plants and tall grass higher than all but one residue value

• 5 empirical values exceed the mean residue for fruit, pods and seeds

U.S. Environmental Protection Agency 36

Estimating Dietary Exposures for 

Foliar Spray Applications

80

90

100

A)

Short grass

0

10

20

30

40

50

60

70

80

chemical

m

e

a

n

c

o

n

c

e

n

tra

tio

n

(m

g

a.i./

k

g

p

er 1 lb

a.i./

A)

Short grass

Broadleaf

Tall grass

Fruit, etc.
• Method Evaluation: maximum empirical data for pollen (n = 14)

• Upper bound residues for short grass, broadleaf plants and tall grass are 

higher than empirical data

• 7 empirical values exceed the mean residue for fruit, pods and seeds

U.S. Environmental Protection Agency 37

Estimating Dietary Exposures for 

Foliar Spray Applications

200

250

M

a

x

c

o

n

c

e

n

tra

tio

n

a.i./

k

g

p

er 1 lb

a.i./

A)

Short grass

0

50

100

150

200

chemical

M

a

x

c

o

n

c

e

n

tra

tio

n

(m

g

a.i./

k

g

p

er 1 lb

a.i./

Broadleaf

Tall grass

Fruit, etc.
• Summary of Evaluation and Proposed Method

• Short grass, broad leaf plant and tall grass residues are consistently 

conservative relative to mean and maximum residue data for pollen and 

nectar

Only one value exceeds tall grass and broad leaf plant residues

U.S. Environmental Protection Agency 38

Estimating Dietary Exposures for 

Foliar Spray Applications

• Only one value exceeds tall grass and broad leaf plant residues

• Tall grass value is closest to empirical data

• Proposed residue concentration is 110 mg a.i./kg per 1 lb a.i./A

• Adult dose: 32 μg a.i./bee per 1 lb a.i./A

• Larval dose: 13μg a.i./bee per 1 lb a.i./A
• Assumptions and Uncertainties

• Assume that tall grass upper bound is representative of pollen and nectar

• Assume that concentration from direct foliar spray at time of application 

exceeds later concentration resulting from systemic transport

U.S. Environmental Protection Agency 39

Estimating Dietary Exposures for 

Foliar Spray Applications

• Strengths of proposed method

• Tall grass upper bound concentration appears to be reasonably 

conservative compared to empirical concentrations on pollen and nectar
Estimating Dietary Exposures for 

Seed Treatments

• Description of Proposed Method

• Assume that pesticide concentration in pollen and nectar of seed treated 

crops is 1 mg a.i./kg (1 μg a.i./g)

• No adjustment is made for application rate

U.S. Environmental Protection Agency 40

• No adjustment is made for application rate

• Based on EPPO’s recommended screening value

• Final doses calculated by multiplying 1 μg a.i./g by food intake rates

• Adult Dose = 0.29 μg a.i./bee

• Larval Dose = 0.12 μg a.i./bee
Estimating Dietary Exposures for 

Seed Treatments

• Method Evaluation: empirical data for pollen (n = 18)

• 1 mg a.i./kg screen is factor of 28 above highest concentration

U.S. Environmental Protection Agency 41

0.1

1

C

o

n

c

e

n

tra

tio

n (m

g

a.i./

k

g)

0.0001

0.001

0.01

0.1

Chemical

C

o

n

c

e

n

tra

tio

n (m

g

a.i./

k

g)

< <

<

<
Estimating Dietary Exposures for 

Seed Treatments

• Method Evaluation: empirical data for nectar (n = 6)

• 1 mg a.i./kg screen is 3 orders of magnitude above empirical data

U.S. Environmental Protection Agency 42

0.1

1

C

o

n

c

e

n

tra

tio

n (m

g

a.i./

k

g)

0.0001

0.001

0.01

0.1

Clothianidin Clothianidin Imidacloprid Imidacloprid Thiamethoxam Unnamed 

Chem # 2

Chemical

<

<

C

o

n

c

e

n

tra

tio

n (m

g

a.i./

k

g)

<
Estimating Dietary Exposures for 

Seed Treatments

• Assumptions and Uncertainties

• Assumed that pesticides applied to seeds are systemically transported

• Does not account for application rate

• Does not account for fate of pesticide

U.S. Environmental Protection Agency 43

• Strengths of proposed method

• 1 mg a.i./kg value is conservative relative to empirical data 

• By a factor of 28 for pollen

• By a factor of 333 for nectar
Estimating Dietary Exposures for 

Soil Applications

• Description of Proposed Method

• Based on Briggs’ model (Briggs et al. 1982, 1983) 

• Predicts concentration in stems using:

• Octanol-water partition coefficient (Kow)

• Concentration in water (Cwater) 

• Transpiration Stream Concentration Factor (TSCF)

U.S. Environmental Protection Agency 44

• Transpiration Stream Concentration Factor (TSCF)

• Calculated using Kow

  water

LogKow Cstem TSCF * 10 0.82 *C(0.95* 2.05)

 


Estimating Dietary Exposures for 

Soil Applications

• Description of Proposed Method (continued)

• Includes soil water partitioning as proposed by Ryan et al. (1988)

• Requires organic-carbon partition coefficient (Koc) 

or soil partition coefficient (Kd)

• Requires concentration in soil (Csoil) instead of Cwater

• Requires basic soil properties

U.S. Environmental Protection Agency 45

• Requires basic soil properties

• Soil bulk density (ρ)

• Soil water content (θ)

• Fraction of organic carbon (foc)

 











 



Koc foc C TSCF Csoil

LogKow

stem * *

* 10 0.82 * *

(0.95* 2.05)

 


Estimating Dietary Exposures for 

Soil Applications

• Description of Proposed Method (continued)

• Modifications to the TSCF calculation were made by EPA to generate 

more conservative estimates of the concentration in stems (Appendix 5)

U.S. Environmental Protection Agency 46

Estimates of median and 95th percentile TSCF values based on empirical 

dataset reported by Briggs et al. 1982. 

0

0.2

0.4

0.6

0.8

1

-1 0 1 2 3 4 5

Median TSCF

95th percentile TSCF

Log Kow

T

S

C

F

dataset reported by Briggs et al. 1982. 
Estimating Dietary Exposures for 

Soil Applications

• Method Evaluation: empirical data for pollen (n = 14)

• Model predictions are generally conservative compared to empirical data

U.S. Environmental Protection Agency 47

1.4

1.6

1.8

2

k

g

p

er 1 lb

a.i./

A)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Chemical

C

o

n

c

e

n

tra

tio

n (m

g

a.i./

k

g

p

er 1 lb

a.i./

A)
Estimating Dietary Exposures for 

Soil Applications

• Method Evaluation: empirical data for nectar (n = 16)

• Model predictions are generally conservative compared to empirical data

U.S. Environmental Protection Agency 48

1.4

1.6

1.8

2

p

er 1 lb

a.i./

A)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Chemical

C

o

n

c

e

n

tra

tio

n (m

g

a.i./

k

g

p

er 1 lb

a.i./

A)
Estimating Dietary Exposures for 

Soil Applications

• Consideration of EPPO’s 1 mg a.i./kg screen

• Conservative for all but one value from empirical data set

• Dimethoate concentration in nectar (4.82 mg a.i./kg; Lord et al. 1968) 

• High application rate (17 lb a.i./A)

U.S. Environmental Protection Agency 49

• High application rate (17 lb a.i./A)

• Does not account for application rate 

• Does not account for fate of chemical
Estimating Dietary Exposures for 

Soil Applications

• Assumptions and Uncertainties of modified Briggs’ Model

• Uses stem concentrations as surrogates for pollen and nectar

• Assumed that pesticides applied to soil are systemically transported

• Data from barley only

• Limited number and type of chemicals (2 classes of non-ionic pesticides)

• May have limited application to ionic chemicals

U.S. Environmental Protection Agency 50

• May have limited application to ionic chemicals

• Xylem based

• Strengths of proposed method

• Estimates appear to be reasonably conservative

• Accounts for some basic chemical specific parameters 

• Application rate

• Kow

• Koc (or Kd)
Summary of Proposed 

Tier I Exposure Assessment Methods

U.S. Environmental Protection Agency 51

Application 

Type

Exposure 

Route

Method Description Exposure Estimate

Example Dose

(μg a.i./bee)

From 1 lb a.i./A 

application

Adult Bees

Foliar spray Contact Koch and Weisser

(1997) Max (2.7 μg a.i./bee)*App. rate 2.7

Foliar spray Diet T-REX tall grass upper 

bound Foliar spray Diet (32 μg a.i./bee) *App. rate 32 T-REX tall grass upper bound (32 μg a.i./bee) *App. rate 32

Seed 

Treatment

Diet EPPO screen 0.29 μg a.i./bee 0.29

Soil 

Treatment

Diet Modified Briggs 

model (Briggs EEC)(0.29 g/day) 0.42

Larvae

Foliar spray Diet T-REX tall grass upper 

bound (13 μg a.i./bee) *App. rate 13

Seed 

Treatment

Diet EPPO screen 0.12 μg a.i./bee 0.12

Soil 

Treatment

Diet Modified Briggs 

model (Briggs EEC)*(0.12 g/day) 0.18
Summary of Tier I Methods for estimating 

pesticide concentrations on bees and in pollen 

and nectar

• Estimated concentrations on bees, pollen and nectar are 

reasonably conservative relative to empirical data

U.S. Environmental Protection Agency 52

• As more data become available, EPA may re-evaluate methods

• Interested in SAP comments on the proposed methods for 

estimating tier 1 exposure values 

• Contact exposure: charge question 4

• Dietary exposure: charge question 6
Questions

U.S. Environmental Protection Agency 53

Questions