Illustrating Use of Simple Filters to Reduce the Scope of a National 

Endangered Species Assessment for a Herbicide

Dwayne R.J. Moore – Intrinsik Environmental Sciences (US), Inc., New Gloucester, ME

April 22nd, 2014
Interim Approach

• Step 1 proximity analysis

– “May affect” if overlap between species range and critical habitat and action area 

– Otherwise, “No effect”

• Action area

– Includes potentially treated areas and off-site transport area

– Outer boundary of off-site transport area occurs where conservative estimate of exposure 

crosses below conservative effects metric (e.g., dose causing one-in-a-million mortality to 

most sensitive animal species) for the most at-risk receptor group

• Outcome will be an enormous number of species status records (SSRs) for listed 

species, many of which are not even remotely at risk

Cotton Location

Off-site transport zone

Species 2

Range

Species 1

Range

Better approach would be 

to define separate off-site 

transport areas for each 

use pattern and receptor 

group for both direct and 

indirect effects
Establishing Off-site Transport 

Areas

• Identify toxicity thresholds for each receptor group for both direct and 

indirect effects

• Determine distances from treated areas at which thresholds will no 

longer be exceeded

• Exposure analyses: Drift models or field study results, T-REX, 

TerrPlant, PRZM/EXAMS

• Effects metrics and obligate relationships

– Animals: Acute concentration or dose causing 1-in-a-million mortality either for 

most sensitive species or HC05; most sensitive chronic NOEC

– Plants: Lowest of NOAEC and EC05

• Indirect effects metrics

– Animals: Acute concentration or dose affecting 10% of individuals from HC05 

hypothetical species or most sensitive LC/EC50; chronic LOAEC or defensible 

ECx

– Plants: Lowest of LOAEC and EC25

• Incorporate labeled risk mitigations, e.g., buffers, DRTs
Case Study

• Widely used herbicide that provides post-emergence control of 

broadleaf weeds, woody brush and vines

• New formulation proposed, but numbers changed for this 

example

• Assume rates vary from 0.1 to 1 lb ai/A depending on use 

pattern

• Uses include row crops, grains, pastures, etc

• Mitigations on proposed label

– Extremely coarse to ultra coarse spray drift nozzles

– Boom height ≤ 24 inches from target

– 150 ft buffer

• EFED has conducted full risk assessment on ai
Use Patterns

Use Pattern

Maximum Single 

Application Rate (lb

ai/A)

Number of 

Applications

Minimum 

Application 

Interval (d)

Vegetable 0.5 1 -

Between crop application 1 1 -

Corn and popcorn 0.5 2 14 

Cotton 1 2 14

Turf 1 1 -

Grass seed 0.125 1 -

Pasture 1 1 -

Conservation Reserve Program 1 2 20

Small grain 0.25 2 14

Sorghum 0.25 2 14

Soybean 1 3 14

Sugarcane 1 2 14

Sod farm 0.5 2 14
EFED Assessment

• Terrestrial biota

– Using T-REX and TerrPlant, modeled application rates ranging from 1-3 lb

ai/A

– Proposed maximum for new formulation = 1 lb ai/A

– RQs > Listed Species LOC for acute exposure to birds (on-field), but not for 

chronic exposure

– RQs > Listed Species LOC for chronic exposure to mammals (on-field), but 

not for acute exposure

– RQs > 1 for terrestrial plants for nearly all use patterns

• Aquatic biota

– No RQs exceeded listed species LOCs for fish, aquatic invertebrates, nonvascular plants or vascular plants at application rate of 1 lb ai/A

• No need to conduct proximity analyses for direct effects to listed 

aquatic species or for listed species that depend on aquatic prey 

and habitat
Additional Filters

• Determine off-site transport area 

for remaining receptor groups – Used actual spray drift curve from field 

study conducted with DRT nozzle

– Incorporated no-spray buffer of 150 ft

• Used Proximity Distance 

Calculator – Determines distance to toxicity 

threshold for each use 

pattern/receptor group

– For terrestrial biota, replicates T-REX 

and TerrPlant

– For aquatic biota, uses outputs from 

PRZM/EXAMS

– Adjusts results for distance from field 

assuming spray drift curve and buffer 

size

0

0.001

0.002

0.003

0 200 400 600

Fraction of Applied

Distance (feet)

Raw Data

Model
Results

Maximum Single 

Application Rate (lb/A Crops

Exposure at 

Downwind Field 

Edge (lb ai/A)

Potentially at Risk? Action Area 

(Distance Beyond 

Monocots Dicots Field Edge, feet)

0.125 Grass seed 0.000049 No No NA

0.25 Small grain and sorghum 0.000098 No No NA

0.5 Vegetable, sod farm 0.000196 No No NA

1

Pasture, cotton, CRP, soybean, 

sugarcane

0.000392 No Yes 116

• For direct effects to listed bird and mammal species

– Concerns only exist for treated areas of field assuming 100% foraging in those areas

– No concern beyond field edge

• Relative rarity of wildlife foraging exclusively on treated portions of crop 

fields also precludes effects to listed species that depend on wildlife for prey

• Terrestrial plant results shown below
Results, continued

Taxon

EPA Step 1 

Determination

Our Direct 

Effects 

Determination

Our Indirect 

Effects 

Determination

Comments

Mammals, birds, reptiles

and terr. amphibians May affect May affect May affect

Only listed species that 

forage in treated areas or that 

depend on listed dicots are 

potentially at risk

Terrestrial invertebrates May affect No effect May affect Concern is only for listed 

species that depend on dicots

Terrestrial monocots May affect No effect No effect

Terrestrial dicots May affect May affect No effect Only for highest application 

rate

Freshwater fish and aq. 

amphibians May affect No effect No effect

Estuarine and marine fish May affect No effect No effect

Freshwater invertebrates May affect No effect No effect

Estuarine and marine 

invertebrates May affect No effect No effect

Aquatic vascular plants May affect No effect No effect

Aquatic non-vascular 

plants Not applicable because there are no listed non-vascular plants
Case Study Conclusions

• Using interim approach, all listed species within 116 feet of 

labeled uses would screen through to Step 2

• No concern for listed bird and mammal species that do not 

forage on treated areas

• No concern for listed species that depend on birds and mammals 

for prey

• No concern for listed monocot species or listed species that 

depend on monocots for food and habitat

• Proximity analysis only necessary for:

– Listed dicot species and the species that depend on them 

– Only for use patterns with maximum application rate of 1 lb ai/A 

• Overall, scope of assessment considerably reduced through use 

of simple, conservative filters