UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON D.C., 20460 

PC Code: 112602 

DP Barcode: D416981 

Date: June 5, 2014 

MEMORANDUM 

SUBJECT: 

TO: 

FROM: 

Response to Public Comments on the Trinexapac-ethyl Registration Review 

Ecological Risk Assessment 

Kaitlin Keller, Chemical Review Manager 

Avivah Jakob, Team Leader 

Risk Management and Implementation Branch III 

Pesticide Re-Evaluation Division 

Office ofPesticide Programs (7508P) 

~~u~ os-.JV~ l.0/1 

Christopher Koper, M.S., Chemist~ • b /r/1_~/.l 

Stephen Wente, Ph.D., Biologist~ '1: /J.u,J);( ~ ~ /'6G(? Y Meghan Radtke, Ph.D., Acting RAPL , .j(__ f'c__~ l 

Sujatha Sankula, Ph.D., Branch Chief ==-

Environmental Risk Branch I 

Environmental Fate and Effects Division (7507P) 

Office of Pesticide Programs 

In response to the "Registration Review - Ecological Risk Assessment and Effects 

Determination ofTrinexapac-Ethyl" (DP09748), Syngenta Crop Protection and Crop Life 

America submitted comments during the public comment period. Syngenta submitted 36 

individual comments addressing the "Registration Review -Ecological Risk Assessment and 

Effects Determination ofTrinexapac-Ethyl", while Crop Life America submitted one comment. 

The Syngenta comments were numbered 1 to 36 in the order they appear in the Syngenta 

comment document (i.e., these numbers did not appear in the original document). Comments that 

addressed similar topics were combined and summarized to produce a more readable document. 

However, the comment numbers were retained to facilitate comparisons with Syngenta's original 

comments. The single Crop Life America comment appears after the Syngenta's comments near 

the end of this document (prior to the Literature Cited and Appendix). 

Comments by Syngenta Crop Protection 

Syngenta's comments largely addressed conservative assumptions made in the risk assessment, 

conservative assumptions in the models used in the risk assessment, and how less conservative 

assumptions would have changed the estimation of risk in the assessment. Typically, Syngenta's 
2

comments made an argument like: based on less-conservative assumptions A, B, and C, the 

estimated risk for taxa group X would be lower.

The EFED responses address each of the assumptions or other specific issues identified by 

Syngenta. In Appendix 1, EFED presents amended tables that reflect several changes that were 

proposed by Syngenta in their comments (Only tables where numbers or text changed are 

presented. Tables that did not change, and those for which the only change was omitting the 

unsupported 1.36 lb ai/A application rate, are not included.)

Bare-ground Terrestrial Field Dissipation Studies

Comment #1: 

Syngenta believes non-linear curve fitting is more appropriate for two bare-ground field 

dissipation studies, MRID 48043710 and 48043711 and suggests half-lives of 1.3 days (parent) 

and 10.8 days (trinexapac acid degradate). (Syngenta Crop Protection, p. 3)

EPA Response #1

The Agency agrees that a non-linear fit is more appropriate than linear in this case and accepts 

Syngenta’s half-life values. EFED is currently developing refined methods for estimating the 

formation and decline (half-lives and formation fractions) of parent and degradates. Therefore 

because the rate of decline of the parent has to agree with the rate of formation of the degradate, 

both of these half-life values (parent and degradate) may be revisited in the future. However, 

these half-life estimates would not be expected to radically change. Being a dissipation study 

rather than a degradation study, terrestrial field dissipation half-life values are not typically used 

directly as inputs to models, but rather compared to input values estimated from degradation 

studies. The next section concerns the exception to this rule where turf terrestrial field dissipation 

half-lives are used directly as an input to the T-REX model (for turf uses only).

Turf Foliar Residue Dissipation

Comment #9: 

Syngenta believes that use of the default T-REX half-life value of 35 days is overly conservative 

and does not accurately reflect the environmental fate and potential exposure of trinexapac-ethyl.

Field study data demonstrate the half-life value for trinexapac-ethyl on grass is <1 day (DER for

MRID 48043710, DER for MRID 48043711, MRID 41869539, and MRID 41869540). 

(Syngenta Crop Protection, p. 5)

EPA Response #9:

For turf uses (does not include grass grown for seed), the “grass total residues” half-lives 

calculated using PestDF are 1.96 days for CA (MRID 48043710), 0.386 days for NY (MRID

48043711), 0.245 days for NC (MRID 41869539), and 0.109 days for IL (MRID 41869540).

Therefore the 90th percentile half-life estimate is 1.383 days. EFED will use this value for the TREX foliar dissipation half-life of turf scenarios (the default 35-day foliar dissipation half-life 

will be retained for grass grown for seed and other uses) since terrestrial wildlife would be 

expected to consume the live plant tissue (grass) rather than thatch (clippings).

Environmental Fate

Comment #3 and 5:
3

Syngenta recommended the OR Grass Seed STD scenario as a more representative PRZM crop 

scenario for modelling grasses grown for seed than the Pennsylvania and Florida turf scenarios

used in the risk assessment. (Syngenta Crop Protection, p. 4)

EPA Response #3 and 5:

The Agency considered using the Oregon Grass Seed scenario along with the Pennsylvania and 

Florida turf scenarios to model grasses grown for seed. However, the higher amounts of rainfall 

in the Pennsylvania and Florida turf scenarios typically result in higher exposures than the 

Oregon Grass Seed scenario. Unless the label restricts use to Oregon or to regions more similar 

to Oregon, the Pennsylvania and Florida turf scenarios will be utilized for a national risk 

assessment to assess grasses grown for seed.

Residues of Concern

Comment #2:

Based on the environmental fate data for trinexapac-ethyl, Syngenta believes it would not be 

possible for all of the included degradates to form under typical US soil pHs (pH 6-8).

Trinexapac-ethyl hydrolyzes to the free acid (CGA-179500) under alkaline conditions (pH 9, t1/2

= 8.1 days, MRID 41563930). Conversely, the free acid (CGA-179500) is stable at both neutral 

and basic pH (MRID 46809301; the major degradation products CGA-313458 and an unknown 

hydrolysis product (M3) were observed only at acidic pH (MRID 46809301). Syngenta believes 

the stressor of concern for aquatic exposure should consist of only parent trinexapac-ethyl plus 

the free acid (CGA-179500). (Syngenta Crop Protection, p. 4)

EPA Response #2:

The degradates included in the total toxic residue (parent plus degradates of concern) approach 

are based on recommendations from the Residues of Concern Knowledgebase Subcommittee 

(ROCKS) of EPA’s Health Effects Division (HED) (D390121). Only those degradates identified 

in the abiotic hydrolysis study at pH 7 are included in the total toxic residue abiotic hydrolysis 

half-life (degradates identified at other pHs do not affect this half-life). Assuming the other fate 

studies were conducted under circum-neutral conditions, all of the total toxic residue half-lives 

are based on circum-neutral conditions alone.

Turf Application Rates

Comment #4, 6, and 7:

Syngenta is amending their turf end-use product labels (Primo Maxx, EPA Reg. No. 100-937; 

Trinexapac-ethyl 1ME, EPA Reg. No. 100-1401; Lawn Primo G, EPA Reg. No.100-930; Lawn 

Primo F, EPA Reg. No. 100-931) to indicate that the single maximum application rate should not

exceed 88 fl. oz./A (0.68 lb ai. A). Further, Syngenta will voluntarily cancel the products Primo 

Liquid, EPA Reg. No. 100-729 and Primo WSB, EPA Reg. No. 100-752. Syngenta requests that 

the exposure modelling be updated to reflect the amended labelling. (Syngenta Crop Protection, 

p. 4)

EPA Response #4, 6, and 7:

The relevant amended tables appear in Appendix 1 and include the revised labeling and foliar 

dissipation half-life. There are still acute listed risk concerns for birds for turf use based on 8 
4

applications at 0.34 lb ai/A with a 4 week reapplication interval and 4 applications at 0.68 lb ai/A 

with a 8 week reapplication interval.

T-REX Modeling

Comment #10:

Syngenta notes that the RUD value for arthropods is calculated based on residue values from 

both field and laboratory studies. RUD values from laboratory studies are typically much higher 

than those from field experiments (Appendix B of the User’s Guide T-REX Version 1.5). This 

difference is attributable to insects being directly sprayed with pesticides in laboratory studies, 

whereas during field applications interception of spray by vegetation occurs. The extrapolated 

arthropod RUD value of 94 used in TREX v1.5.2 is therefore overly conservative and not 

indicative of environmentally relevant arthropod pesticide residue burdens. Syngenta, therefore; 

believes that arthropod-based EECs based for terrestrial vertebrates and invertebrates are likely 

overestimated in the current assessment. (Syngenta Crop Protection, p. 5-6)

EPA Response #10

EFED agrees that the data from two laboratory studies, one from the literature and one registrant 

submitted, present values that appear higher than those of other studies used in the distribution of 

residues. However, given that the data from some of the higher end residue measurements from 

the field do not report variance, it is not possible to make a definitive call that the laboratory 

values are completely outside field measurements. Moreover, the rebuttal comment seems to 

assume that all invertebrates during a spray event will be under cover and avoiding spray thus 

precluding the possibility that individual invertebrates would receive the full horizontal or 

vertical interception of the spray application. EFED is not prepared, in the absence of such data, 

to adopt that assumption. Therefore, the residue estimates will remain unchanged.

Comment #16:

Syngenta believes the modelling (T-REX) used to assess the risk to birds from consumption of 

treated granules is overly conservative. Fischer and Best (Environ. Toxicol. and Chem. 11:1495-

1508., 1995) applied untreated granules to multiple corn fields in Iowa at rates from 3.9 to 17.3 

kg/ha. Granules were applied as a banded application. Counts within the band immediately after 

application averaged 15 granules/ft2for every 1 kg/ha applied, or about 6% of the granules 

applied. Birds representing three taxonomic groups (256 birds total) were collected, and the 

number of granules consumed was determined by examination of gizzard contents. Nominal 

application rate had little, if any, effect on the number of granules present in bird gizzards. For 

the species with the highest consumption rate, the savannah sparrow (Passerculus 

sandwichensis), median consumption was 11 granules per day, with 95% of individuals 

estimated to consume ≤ 23 granules per day. In comparison, based on the highest registered 

trinexapac-ethyl (Primo G) granular application; T-REX calculated the number of granules that 

would have to be consumed by a small bird (20 g) in order to achieve the acute listed LOC of 0.1 

to be greater than 100. Taking the field data into consideration, Syngenta believes the T-REX 

model greatly overestimates the potential consumption of granules by foraging birds. (Syngenta 

Crop Protection, p. 9)
5

EPA Response #16:

As indicated by the rebuttal comment, the Fisher and Best paper involved the application of 

granules to corn fields. There is considerable bare ground in these environments and EFED can 

envision that the consumption of pesticide granules might be influenced by the availability of 

natural grit available in the bare ground areas. There have been past discussion of this data set 

and its portability across landscapes with different soil and different degrees of cover. Indeed, the 

Ecological Committee on FIFRA Risk Assessment Methods (ECOFRAM 1999) presented this 

very same concept when introducing the Granule Grit Preference factor which was a product of 

the availability of granules and the availability of natural grit. The pesticide being reviewed is 

applied to turf scenarios. There is likely quite a bit of difference between the background 

amounts of natural grit available for birds in these areas. Therefore EFED is not prepared to 

assume that the Fisher and Best data represent soil and cover conditions likely encountered in 

turf scenarios.

Comment #15:

Syngenta notes that deriving RQs for small birds, such as passerine species, based on some food 

groups is highly conservative and over-predicts potential exposure. The unlikely occurrence of 

small avian species feeding exclusively on short grass or other solely vegetation-based diets is 

described in the US EPA T-REX v1.5.2 Manual Section 4.1: Acute and Reproduction Dietary 

Discussions. Therefore, using vegetation-based EECs values for screening level assessment to 

derive RQs estimates may be overly conservative and greater focus should be placed on dietary 

items that have greater ecological realism for small birds given their feeding preferences (i.e. 

fruits/pods/seeds and arthropods; Dunn, 2011). (Syngenta Crop Protection, p. 8)

EPA Response #15:

EFED agrees. This kind of characterization should be conveyed within a refined risk assessment.

For non-listed avian species, there are no exceedances of LOCs other than granular 0.88 lbs. 

ai/A. (The previous chronic exceedances for turf were addressed through clarification of the 

application rates and use of the turf terrestrial field dissipation half-life as the turf foliar 

dissipation half-life.) Therefore, such characterization is unnecessary in the interim decision on 

non-listed species, but will be considered as part of the endangered species assessment.

Avian Data

Comment #12:

Syngenta disagrees with the Agency’s interpretation of a zebra finch study (MRID 48894501) 

which yielded the most sensitive acute oral endpoint (LD50 = 813 mg a.i/kg-bw) and the only 

acute definitive endpoint available for birds. The study produced an irregular dose response 

curve with mortalities at the top two treatment levels (2000 and 25000 mg ai/kg-bw) being lower 

than mortalities at the second and third treatments levels (500 and 1000 mg/kg-bw). Therefore, 

the Agency calculated a conservative LD50 using just the first three treatment levels (500, 1000, 

and 1500 mg/kg-bw). According to Syngenta:

Syngenta believes that all data should be considered in the calculation of an LD50. The 

endpoint described in the original study report for MRID 48894501 (LD50=1684 mg 

ai/kg-bw) is supported by the data. It is also important to note that in addition to the two 

avian dietary studies with LC50 values >5200 mg/kg-diet (and no sub-lethal effects at 
6

any concentration) described in the current EPA assessment (MRIDs 41563902, 

41563903), data are also available for an avian acute oral toxicity study with the mallard 

duck (MRID 41563901). The avian acute oral study yielded LD50 values >2000 mg/kgbw. Dose-dependent sublethal effects associated with body weight and food consumption 

were only observed at the highest treatment level (2000 mg/kg-bw). If requested by the 

Agency, Syngenta will submit a bobwhite acute oral toxicity study (Campbell et al. 1991) 

that also supports LD50 values >2000 mg/kg-bw. Syngenta seeks additional information 

about the TOXANAL statistical program (2009) that was used to calculate the revised 

toxicity endpoint (DER for MRID 48894501) and requests the opportunity to discuss 

statistical analysis of MRID 48894501 data with the Agency. (Syngenta Crop Protection, 

p. 6)

EPA Response #12:

The Agency specifically requests passerine data because passerines have characteristics which 

may make them more sensitive to pesticide effects. Therefore mallard and bobwhite quail data 

are of limited value in evaluating passerine data; however, the Agency will consider all data that 

are submitted and potentially could incorporate such data in the refined listed species assessment 

or other future assessments.

Given the non-monotonic dose response that was observed, the evaluation of MRID 48894501 

was conducted in collaboration with EFED’s Terrestrial Biology Technical Team and the 

Statistics Technical Team. Both teams concurred that the moving average method was an 

appropriately conservative approach in the estimation of the LD50. EFED welcomes the 

opportunity to discuss this further with Syngenta.

Terrestrial Invertebrates

Comment #8:

Syngenta is submitting with this response a study entitled “Acute toxicity of CGA 163935 tech. 

to the honeybee Apis mellifera L. Under Laboratory Conditions (Barth 2000)” for Agency 

review. The study was completed according to OECD 213 and 48-h oral and contact endpoints 

were >200 μg ai/bee; confirming that trinexapac-ethyl is practically non-toxic to honey bees.

Comment #13:

Syngenta believes the LD50 of 47 μg ai/bee reported in the acute contact toxicity study (MRID 

41563928) should be recalculated to account for mortality (20%) in the solvent control. The

DER for MRID 41563928 calculated LD50 values and accounted for control mortality by using 

Abbott’s formula. The resulting LD50 value was 58 μg ai/bee (48.5 - 68.5).

In addition, Syngenta is submitting a newer bee acute toxicity study (Barth 2000) that is of 

higher quality because it was completed according to the OECD 213 guideline, had no control 

mortality in the solvent control, and also concurrently tested the reference item, Dimethoate 400 

EC. The endpoint for the Barth (2000) study was > 200 μg ai/bee as mortality was 3.3 and 16.7% 

in the 100 and 200 μg ai/bee treatments, respectively.

While control performance was within acceptable limits, the relatively high control mortality in 

the solvent control treatment, lack of a positive reference test, and substantially lower endpoint 
7

compared to the Barth (2000) study causes Syngenta to question the health of the bee population 

and/or handling of the bees used in MRID 41563928. Based on trinexapac-ethyl’s mode of 

action, there is no reason to believe that terrestrial invertebrates would be susceptible to 

exposure; this statement is further supported by lack of sensitivity observed in studies completed 

with aquatic invertebrates. Taking these factors into consideration, Syngenta believes the Barth 

(2000) study represents best available data and should be used in ecological risk assessments 

rather than MRID 41563928. (Syngenta Crop Protection, p. 6-7)

Comment #20 and 28:

Syngenta supports the use of a science-based approach for establishing an appropriate LOC for 

listed terrestrial invertebrates and believes that the interim LOC of 0.05 for all terrestrial 

invertebrates is overly conservative. (Syngenta Crop Protection, p. 10 and 12)

EPA Response # 8, #13, #20, and #28:

EFED is in the process of reviewing the Barth (2000) study and will update the risk conclusions, 

accordingly. In regards to MRID 41563928, EFED’s policy is to compare the treatment group to 

the negative control and not to adjust for solvent control mortality. The Terrestrial Biology 

Technical Team is considering the development of a terrestrial invertebrate LOC; Syngenta is 

welcome to submit information that supports a science-based approach.

Aquatic Plant Study Interpretation

Comment #14:

Effect concentrations (ECx) calculated based on growth rate are considered more robust than 

those based on standing crop (cell density and biomass) for a given time-point since growth rate 

is independent of exposure time. Moreover, as indicated by Bergtold and Dohmen (2011), the 

Organization for Economic Cooperation and Development (OECD) indicates a clear preference 

for growth rate as the appropriate endpoint in the Lemna and algal toxicity test guideline (OECD 

2006a,b). (Syngenta Crop Protection, p. 7-8)

EPA Response #14:

The argument provided by Syngenta regarding the transformation of this value to a growth rate is 

noted and EFED will consider the transformation for this endpoint in future model updates. 

TerrPlant and SIP Modeling

Comment #21 and 35:

Syngenta believes that the runoff assumptions in the TerrPlant model are highly conservative and 

EECs should instead be estimated by integrating TerrPlant spray drift assumptions with PRZM 

edge of field (EOF) runoff flux values.

SIP Modeling

Comment #26:

Syngenta agrees that use of the water solubility limit for trinexapac-ethyl in the SIP model is 

highly conservative and refinements should be applied. (Syngenta Crop Protection, p. 11)
8

EPA Response #21, #26, and #35:

EFED is currently developing a new terrestrial plant exposure model (Audrey III) that is based 

on PRZM edge of field estimations to replace the TerrPlant model. However, until these new 

models have been formally adopted, EFED will continue to rely on the existing models for 

conducting quantitative exposure assessments for risk to non-target terrestrial plants.

Risk Calls

Comment #36:

Refinements described in previous comments indicate that potential risks to birds (reptiles and 

land-phase amphibians), terrestrial plants, and terrestrial invertebrates are negligible as screening 

level RQs are less than respective LOCs. As such, Syngenta believes the use of trinexapac-ethyl

will not cause either direct or indirect effects to any listed species. Please refer to previous

comments for additional details.

An overview document entitled “Risk Assessment Considerations Concerning the Potential 

Impacts of Pesticides on Endangered Species” (MRID No. 49200601), by R. Brain et. al, was 

submitted August 26, 2013. Syngenta, as the applicant, is available to discuss, with EPA, higher 

tier refinements in the trinexapac-ethyl endangered species assessment.

EPA Response #36:

Based on changes to turf application rates and turf foliar dissipation half-life, the RQs declined 

for these turf uses (see tables in Appendix 1). There are no risks to non-listed animal taxa. The

amended screening-level assessment still indicates the potential for direct risk concerns to listed 

birds, reptiles, and land-phase amphibians, terrestrial invertebrates, and both listed and non-listed 

terrestrial plants from the use of trinexapac-ethyl on some of its registered use sites. Listed 

species of all taxa may also be affected through indirect effects because of the potential direct 

effects on listed and non-listed species. Potential direct effects on listed birds, reptiles, and landphase amphibians, terrestrial invertebrates, and terrestrial plants from the use of trinexapac-ethyl 

may be associated with modification of primary constituent elements of designated critical 

habitat, where such designations have been made.

Comments by Crop Life America

Crop Life America submitted a two-page document that gave general comments concerning the 

Registration Review risk assessment process for human health and ecological effects of 

pesticides. They argue that the risk assessment being done in the Preliminary Risk Assessment 

provide only a screening-level assessment of risk. They argue that these early-tier assessment 

generally would not be robust enough for concluding that a pesticide does pose a risk, but rather 

only indicate when a more refined risk assessment is needed, with additional data to reduce 

uncertainties. They state that they generally would regard it premature make definitive risk 

conclusions or to discuss options for risk mitigation based solely on the early-tier risk 

assessment. (Crop Life America, p. 1-2).

EPA Response:

The comments of Crop Life America concerns the general tiered risk assessments process for 

regulating pesticides within the Registration Review process. The comments were not specific to 
9

trinexapac-ethyl, and do not address any specific concerns with the trinexapac-ethyl ecological 

risk assessment. Therefore, the comments do not suggest any need to change the ecological risk 

assessment for trinexapac-ethyl. EFED defers to the Pesticide Re-registration Division to 

respond to the general comments on the tiered risk assessment process.

References

Bergtold, M. and G.P. Dohmen. 2011. Biomass or growth rate endpoint for algae and aquatic 

plants: Relevance for the aquatic risk assessment of herbicides

Dunn JL. (2011). National Geographic Field Guide to the Birds of North America, 6th Edition. 

Publisher: National Geographic Society. Pg. 574.

OECD (Organization for Economic Cooperation and Development). 2006b. OECD Guideline 

201: Freshwater alga and cyanobacteria, growth inhibition test. Adopted 23 March 2006. 

Paris.

OECD (Organization for Economic Cooperation and Development). 2006b. OECD Guideline 

221: Lemna sp. growth inhibition test. Adopted 23 March 2006. Paris.

US EPA, 2012. Proposed Methodology for Specifying Atrazine Levels of Concern for

Protection of Plant Communities in Freshwater Ecosystems. Report To: Environmental

Fate and Effects Division, Office of Pesticide Programs, U.S. Environmental Protection

Agency, Washington, DC. EPA/600/R-12/019. Mid-Continent Ecology Division, 

National Health and Ecological Effects Laboratory, Office of Research and Development, 

U.S. Environmental Protection Agency, Duluth, MN. March 5, 2012.
10

Appendix 1. Amended Tables from the Trinexapac-ethyl Registration Review Risk 

Assessment

The following tables amend the tables presented in “Registration Review – Ecological Risk 

Assessment and Effects Determination of Trinexapac-Ethyl” (DP09748). Rather than repeat the 

entire tables, only the changes to turf uses are presented. These changes were to turf application 

rates (eliminated the 4 applications at 1.36 lb ai/A with an 8 week reapplication interval use) and 

and the turf foliar dissipation half-life (changed from 35 days to 1.383 days). (Note that the Table 

number of the table in the PRA that is amended appears at the end of each table caption.)

Estimated Environmental Concentrations

In Tables 1, 2, and 3, the reduced turf EECs are presented for birds, mammals, and terrestrial 

insects, respectively.

Table 1. Avian trinexapac-ethyl upper bound EEC values (amends PRA Table 9)

Feeding Category

Dietarybased EECs 

(mg/kg-food 

item)

Dose-based EECs (mg/kg-bw)

Small

(20 g)

Medium

(100 g) Large

(1000 g)

Turf

8 apps at 0.34 lb ai/A (4 weeks)

Short grass 81.60 92.93 53.00 23.73

Tall grass 37.40 42.59 24.29 10.87

Broadleaf plants 45.90 52.28 29.81 13.35

Fruits/pods 5.10 5.81 3.31 1.48

Arthropods 31.96 36.40 20.76 9.29

Seeds 5.10 1.29 0.74 0.33

Turf

4 apps at 0.68 lb ai/A (8 weeks)

Short grass 163.20 185.87 105.99 47.45

Tall grass 74.80 85.19 48.58 21.75

Broadleaf plants 91.80 104.55 59.62 26.69

Fruits/pods 10.20 11.62 6.62 2.97

Arthropods 63.92 72.80 41.51 18.59

Seeds 10.20 2.58 1.47 0.66

Table 2. Mammalian trinexapac-ethyl upper bound EEC values (amends PRA Table 10)

Feeding Category

Dietarybased EECs 

(mg/kg-food 

item)

Dose-based EECs (mg/kg-bw)

Small

(15 g)

Medium

(135 g) Large

(1000 g)

Turf

8 apps at 0.34 lb ai/A (4 weeks)

Short grass 81.60 77.80 53.77 12.47

Tall grass 37.40 35.66 24.64 5.71
11

Feeding Category

Dietarybased EECs 

(mg/kg-food 

item)

Dose-based EECs (mg/kg-bw)

Small

(15 g)

Medium

(135 g) Large

(1000 g)

Broadleaf plants 45.90 43.76 30.25 7.01

Fruits/pods 5.10 4.86 3.36 0.78

Arthropods 31.96 30.47 21.06 4.88

Seeds 5.10 1.08 0.75 0.17

Turf

4 apps at 0.68 lb ai/A (8 weeks)

Short grass 163.20 155.60 107.54 24.93

Tall grass 74.80 71.32 49.29 11.43

Broadleaf plants 91.80 87.52 60.49 14.03

Fruits/pods 10.20 9.72 6.72 1.56

Arthropods 63.92 60.94 42.12 9.77

Seeds 10.20 2.16 1.49 0.35

Table 3. Adult insect foliar spray upper-bound EEC values (amends PRA Table 11)

Use

EEC (based on tall 

grass)

(mg/kg-diet) (dose 

in μg ai/bee*)

EEC (based on 

arthropod)

(mg/kg-diet) 

(dose in μg 

ai/bee**)1

Turf

8 apps at 0.34 lb ai/A (4 weeks) 37.40 (10.92) 31.96 (4.09)

Turf

4 apps at 0.68 lb ai/A (8 weeks) 74.80 (21.84) 63.92 (8.18)

*Conversion made by multiplying EEC by food consumption rate of adult bee (0.292 g/day)

**Conversion made by multiplying EEC by body weight of adult bee (0.128 g)

1Based on single app at the highest single application rate if application interval is >5 days

Risk Quotients

Tables 4, 5, and 6 present the reduced risk quotients for turf uses for birds, mammals, and 

terrestrial insects, respectively. For birds, risks still remain for sugarcane, grasses grown for seed, 

and both modeled turf application rates. It would be prudent to consider Syngenta’s comment:

Syngenta notes that deriving RQs for small birds, such as passerine species, based on 

some food groups is highly conservative and over-predicts potential exposure. The 

unlikely occurrence of small avian species feeding exclusively on short grass or other 

solely vegetation-based diets is described in the US EPA T-REX v1.5.2 Manual Section 

4.1: Acute and Reproduction Dietary Discussions. Therefore, using vegetation-based 

EECs values for screening level assessment to derive RQs estimates may be overly 

conservative and greater focus should be placed on dietary items that have greater 
12

ecological realism for small birds given their feeding preferences (i.e. fruits/pods/seeds 

and arthropods; Dunn, 2011). (Syngenta Crop Protection, p. 8)

All of the RQ exceedances for listed birds (no non-listed LOC exceedances) are for “short grass 

or other solely vegetation-based diets”.

Table 4. Upper bound risk quotients for birds derived from T-REX (amends PRA Table 

20)

Risk Quotients 

Based

on Kenaga

Upper Bound EEC

Dose-Based RQs Chronic 

DietaryBased RQs

20 g 100 g 1000 g

Acute Acute Acute

Turf

8 apps at 0.34 lb ai/A (4 weeks)

Short grass 0.11* 0.05 0.01 0.41

Tall grass 0.05 0.02 0.01 0.19

Broadleaf plants 0.06 0.03 0.01 0.23

Fruits/pods 0.01 0.00 0.00 0.03

Arthropods 0.04 0.02 0.01 0.16

Seeds 0.00 0.00 0.00 0.03

Turf

4 apps at 0.68 lb ai/A (8 weeks)

Short grass 0.21* 0.09 0.03 0.82

Tall grass 0.10* 0.04 0.01 0.37

Broadleaf plants 0.12* 0.05 0.02 0.46

Fruits/pods 0.01 0.01 <0.01 0.05

Arthropods 0.08 0.04 0.01 0.32

Seeds <0.01 <0.01 <0.01 0.05

Bold text indicates an LOC was exceeded.

* Exceeds the acute listed species LOC of 0.1

**Exceeds or meets the acute non-listed species LOC of 0.5

***Exceeds the chronic LOC of 1

There were no exceedances for mammals before these changes and are none after the changes 

either.

Table 5. Dietary upper bound risk quotients for mammals (amends PRA Table 21)

Risk Quotients 

Based

on Kenaga

Upper Bound EEC

Dose-Based RQs Chronic 

DietaryBased RQs

15 g 35 g 1000 g

Acute Chronic Acute Chronic Acute Chronic

Turf

8 apps at 0.34 lb ai/A (4 weeks)

Short grass 0.01 0.06 0.01 0.05 <0.01 0.03 0.01
13

Risk Quotients 

Based

on Kenaga

Upper Bound EEC

Dose-Based RQs Chronic 

DietaryBased RQs

15 g 35 g 1000 g

Acute Chronic Acute Chronic Acute Chronic

Tall grass <0.01 0.03 <0.01 0.02 <0.01 0.01 <0.01

Broadleaf plants <0.01 0.03 <0.01 0.03 <0.01 0.02 <0.01

Fruits/pods <0.01 <0.01 <0.01 0.00 <0.01 <0.01 <0.01

Arthropods <0.01 0.02 <0.01 0.02 <0.01 0.01 <0.01

Seeds <0.01 <0.01 <0.01 0.00 <0.01 <0.01 <0.01

Turf

4 apps at 0.68 lb ai/A (8 weeks)

Short grass 0.02 0.12 0.01 0.10 0.01 0.05 0.01

Tall grass 0.01 0.05 0.01 0.05 <0.01 0.03 0.01

Broadleaf plants 0.01 0.07 0.01 0.06 <0.01 0.03 0.01

Fruits/pods <0.01 0.01 <0.01 0.01 <0.01 <0.01 <0.01

Arthropods 0.01 0.05 0.01 0.04 <0.01 0.02 0.01

Seeds <0.01 <0.01 <0.01 <0.01 <0.01 0.00 <0.01

The adult insect foliar spray risks (listed species only) still exist for sugarcane, grasses grown for 

seed, and both modeled turf application rates.

Table 6. Adult insect foliar spray risk quotient values (amends PRA Table 23)

Use RQ

Turf

8 apps at 0.34 lb ai/A (4 weeks) 0.09*

Turf

4 apps at 0.68 lb ai/A (8 weeks) 0.17*

*Exceeds the listed species interim LOC of 0.05

Table 7 updates the odds of mortality for a bird or terrestrial invertebrate from trinexapac-ethyl

applications.

Table 7. Odds of mortality for a bird or terrestrial invertebrate for trinexapac-ethyl

(amends PRA Table 28)

RQ Slope 95% Confidence 

Interval

Odds 

(1 in...)

Birds

(turf at 0.34 lb ai/A)

0.11 4.5 2, 9 1.25E+05

Birds

(turf at 0.68 lb ai/A)

0.21 4.5 2, 9 874

Terrestrial invertebrates

(turf at 0.34 lb ai/A)

0.09 2.43 1.77, 3.09 181

Terrestrial invertebrates

(turf 0.68 lb ai/A)

0.17 2.43 1.77, 3.09 33
14

Table 8 presents the distance from a treated field (buffer distances) in which LOC exceedances 

would be expected. Oddly the aerial applications (sugarcane and grasses grown for seed) produce 

0 ft buffers, while ground applications (sugarcane, grasses grown for seed, and both modeled turf 

application rates) produce 3.82 ft buffer distances. It appears that AgDRIFT rounds to the nearest 

meter and then converts to English units (1 meter = 3.28 ft). It is likely that both the aerial and 

ground buffer predictions are small distances with the aerial buffer being less than 0.5 meter 

(rounding to 0) and ground buffers being between 0.5 and 1.5 meters (rounding to 1 meter or 

3.28 ft).

Table 8. Avian, reptile, and terrestrial-phase amphibian Buffers for trinexapac-ethyl 

calculated with AgDRIFT (amends PRA Table 29)

Use Single Application Rate (lb ai/A) Fraction of Applied Buffer Distance (ft)

Turf (ground) 0.34 0.67 3.28*

Turf (ground) 0.68 0.48 3.28*

* AgDRIFT appears to default to 3.28 ft (1 meter) for ground applications producing small buffer distances. 

Possibly AgDRIFT rounds to the nearest meter before converting to English units.

Table 9 summarizes risks to listed species for the amended turf scenarios. Again there are no 

non-listed exceedances for birds.

Table 9. Trinexapac-ethyl uses that exceeded the acute dose-based listed LOC (0.1) for 

birds (amends PRA Table 30)

Use Dose-based RQs

Short grass Tall grass Broadleaf plants Arthropods

20 g 100 g 1000 g 20 g 100 g 1000 g 20 g 100 g 1000 g 20 g 100 g 1000 g

Turf (0.34 lb ai/A) Yes No No No No No No No No No No No

Turf (0.68 lb ai/A) Yes No No Yes No No Yes No No No No No

In Table 10 the buffer distances are for listed terrestrial invertebrates only (no non-listed LOC 

exceedances for terrestrial invertebrates).

Table 10. Terrestrial invertebrate buffers for trinexapac-ethyl calculated with AgDRIFT

(amends PRA Table 31)

Use Single Application Rate (lb ai/A) Fraction of Applied Buffer Distance (ft)

Turf (ground) 0.34 0.56 6.56

Turf (ground) 0.68 0.29 9.84

Tables 11 and 12 summarize the exceedances of listed and non-listed LOCs. Non-listed 

monocots exceed for only turf applications at 0.68 lb ai/A, while non-listed dicots only exceed 

for grasses grown for seed and turf applications at 0.68 lb ai/A. Both listed monocots and listed 

dicots exceed for all uses.
Table 11. Trinexapac-ethyl application scenarios that exceeded the LOC for listed and non-listed monocots (amends PRA 

Table 32)

Use Non-listed monocot Listed monocot

Dry Semi-aquatic Spray drift Dry Semi-aquatic Spray drift

Ground Aerial/ 

Chem

Ground Aerial/ 

Chem

Ground Aerial/

Chem

Ground Aerial/ 

Chem

Ground Aerial/ 

Chem

Ground Aerial/ 

Chem

Turf (0.34 lb ai/A0 No No No No No No No No Yes Yes No No

Turf (0.68 lb ai/A) No No Yes Yes No No No No Yes Yes No No

Table 12. Trinexapac-ethyl application scenarios that exceeded the LOC for listed and non-listed dicots (amends PRA Table 

33)

Use Non-listed dicot Listed dicot

Dry Semi-aquatic Spray drift Dry Semi-aquatic Spray drift

Ground Aerial/ 

Chem

Ground Aerial/ 

Chem

Ground Aerial/

Chem

Ground Aerial/ 

Chem

Ground Aerial/ 

Chem

Ground Aerial/ 

Chem

Turf (0.34 lb ai/A0 No No No No No No No No Yes Yes No No

Turf (0.68 lb ai/A) No No Yes Yes No No No No Yes Yes No No