UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON D.C., 20460 

MEMORANDUM 

PC Code: 125601 

DP Barcode: D4170 19 

Date: October 9, 2014 

SUBJECT: Response to Public Ct mments on the Paclobutrazol Registration Review 

Ecological Risk Asse sment 

TO: Khue Nguyen, Chemical Review Manager 

Melissa Grable, Actin~ Team Leader 

Neil Anderson, Branch Chief j 

Risk Management and Implementation Branch I 

Pesticide Re-Evaluatien Division 

Office of Pesticide Programs (7508P) 

FROM: Meghan Radtke, Ph.D., Acting RAPL 

Faruque Khan, Ph.D., Senior Scientist ..J-.1-. ~-"" 

Sujatha Sankula, Ph.D., Branch Chief 

Environmental Risk Branch I 

Environmental Fate and Effects Division (7507P) 

Office ofPesticide Programs 

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

Determination of Paclobutrazol" (DJJ93895), Syngenta Crop Protection submitted comments 

during the public comment period. 1 he Agency appreciates the feedback, in particular, the nine 

additional ecotoxicity studies that were submitted. 

This memo serves two purposes. Firlt, it documents the Agency's response to the public 

comments that were received from Syngenta. Comments that addressed similar topics were 

combined and summarized. Second, when applicable, it updates the analyses and results of the 

paclobutrazol Registration Review risk assessment. 

Syngenta Comme11t 

Syngenta believes that the method of using the 5th percentile in conjunction with Monte Carlo 

simulations (estimating a 5th percentile toxicity value using the most sensitive endpoint from 

structurally similar chemicals) was too conservative to generate toxicological endpoints when 

empirical data were not available. In response, they submitted nine studies that provided 

ecotoxicity data for acute exposure t1 estuarine/marine fish, chronic exposure to freshwater 

invertebrates, acute exposure to estuarine/marine invertebrates, exposure to terrestrial plants 
UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON D.C., 20460 

PC Code: 125601

DP Barcode: D417019 

Date: October 9, 2014 

MEMORANDUM 

SUBJECT: Response to Public Comments on the Paclobutrazol Registration Review 

Ecological Risk Assessment 

TO: Khue Nguyen, Chemical Review Manager 

Melissa Grable, Acting Team Leader 

Neil Anderson, Branch Chief 

Risk Management and Implementation Branch I 

Pesticide Re-Evaluation Division 

Office of Pesticide Programs (7508P) 

FROM: Meghan Radtke, Ph.D., Acting RAPL 

 Faruque Khan, Ph.D., Senior Scientist

 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 of Paclobutrazol” (D393895), Syngenta Crop Protection submitted comments 

during the public comment period. The Agency appreciates the feedback, in particular, the nine 

additional ecotoxicity studies that were submitted. 

This memo serves two purposes. First, it documents the Agency’s response to the public 

comments that were received from Syngenta. Comments that addressed similar topics were 

combined and summarized. Second, when applicable, it updates the analyses and results of the 

paclobutrazol Registration Review risk assessment. 

Syngenta Comment 

Syngenta believes that the method of using the 5th percentile in conjunction with Monte Carlo 

simulations (estimating a 5th percentile toxicity value using the most sensitive endpoint from 

structurally similar chemicals) was too conservative to generate toxicological endpoints when 

empirical data were not available. In response, they submitted nine studies that provided 

ecotoxicity data for acute exposure to estuarine/marine fish, chronic exposure to freshwater 

invertebrates, acute exposure to estuarine/marine invertebrates, exposure to terrestrial plants 
2 

(seedling emergence), acute contact/oral exposure to honeybees, and exposure to aquatic 

vascular plants, cyanobacteria, and aquatic non-vascular plants. A vegetative vigor study was 

also cited (as below), but it was not received by the Agency in time to incorporate into the 

revised Registration Review risk assessment: 

Martin, JA. 2013. Paclobutrazol formulation – vegetative vigor test. Smithers Viscient, 790 

Main Street, Wareham, MA 02571-1037 USA. Report number: 1781.6907, Study number: 

1781.6907, Task number: TK0176295. 294 pgs. 

In addition, Syngenta had several specific suggestions: 

 The acute-to-chronic ratio (ACR) would be an appropriate method to estimate the 

missing endpoints for chronic estuarine/marine fish and invertebrates. 

 For aquatic vascular plants, the watermilfoil and hydrilla study endpoints were too 

conservative, not based on robust endpoints, and not analytically verified. 

 The passerine acute oral toxicity test was not identified as a data gap in the Problem 

Formulation or the Data Call-In and that the available acute oral toxicity data for mallard 

duck and bobwhite quail should be sufficient to assess risk to passerine species. 

EFED Response 

The 5th percentile method was chosen to estimate missing ecotoxicological endpoints because it 

was thought to be a conservative approach. Currently, the Agency is moving toward a 

probabilistic approach for ecotoxicology data. 

EFED evaluated the nine new studies that were submitted by Syngenta. Details of the evaluation 

can be found in USEPA 2014 (D418054). Information from “supplemental” and “acceptable” 

studies is incorporated into the analysis and risk conclusions of the revised Registration Review 

risk assessment, as described below. Revised risk conclusion summary tables are located in 

Appendix A (Tables 1-4). 

Acute risk to estuarine/marine fish 

The newly submitted study (MRID 49300405) with sheepshead minnow was classified as 

“invalid.” Consequently, the results of the original Registration Review risk assessment 

remain intact. There is the potential for direct acute effects to listed estuarine/marine fish 

from the potted ornamental drench use.

Chronic risk to estuarine/marine fish 

The acute-to-chronic ratio (ACR) approach that was proposed by Syngenta is inappropriate for 

two reasons. First, the ratio must be established with acute and chronic data from the same 

species. The acute data from grass carp and chronic data from fathead minnow cannot be used to 

generate the ratio. Secondly since the recently submitted acute sheepshead minnow study was 

found to be “invalid”, an acute estuarine/marine fish study with paclobutrazol is not available. 

Consequently, the results of the original Registration Review risk assessment remain in 

place. There is the potential for direct chronic effects to listed and non-listed 

estuarine/marine fish from the turf and potted ornamental drench uses. 
3 

Chronic risk to freshwater invertebrates 

The original Registration Review risk assessment indicated potential direct chronic risk concerns 

for listed and non-listed freshwater invertebrates for turf and potted ornamental drench uses. The 

newly submitted waterflea study (MRID 49300406) yielded a NOAEC of 0.32 mg ai/L, which is 

much less toxic than the endpoint generated by the 5th percentile approach (NOAEC = 0.009 mg 

ai/L). Using the new endpoint, only one scenario (Total Toxic Residues - TTR - Florida 

ornamentals, 2.5 lb) produced an RQ above the LOC of 1 (RQ = 1.04) (see Appendix A for RQs, 

Table 5). The updated risk conclusion is that there is a potential for direct chronic effects 

to listed and non-listed freshwater invertebrates only for the potted ornamental drench use. 

Acute risk to estuarine/marine invertebrates 

The original Registration Review risk assessment concluded that there were potential direct acute 

risk concerns for listed species to turf and ornamental uses, and non-listed species to ornamental 

uses. The newly submitted studies with Pacific oyster larvae (MRID 49300403) and mysid 

shrimp (MRID 49300404) yielded LC50s of > 9.9 and > 9.0 mg ai/L, respectively. These values 

are less toxic than the endpoint generated by the 5th percentile approach (LC50 = 0.24 mg ai/L). 

RQs cannot be calculated with the new endpoints because they are non-definitive (greater than 

values). Instead, when the highest estimated environmental concentration (EEC) (333 μg ai/L – 

TTR Florida ornamentals, 2.5 lb) is compared directly with > 9000 μg ai/L (mysid shrimp 

endpoint), the EEC is less than 1/20th of the LC50, indicating that direct acute risk is unlikely. 

The updated risk conclusion is that direct acute risk concerns to estuarine/marine 

invertebrates are unlikely for all uses. 

Chronic risk to estuarine/marine invertebrates 

Syngenta proposed that an acute-to-chronic ratio (ACR) approach be employed using existing 

acute and chronic waterflea data and the newly submitted mysid shrimp data (MRID 49300404; 

see above). The NOAEC generated by Syngenta using an ACR approach is not substantially 

different than the tebuconazole NOAEC that was used as a surrogate endpoint in the assessment 

(ACR NOAEC > 0.082 mg ai/L; Tebuconazole NOAEC = 0.035 mg ai/L) and would still result 

in RQs above the LOC. Consequently, the results of the original Registration Review risk 

assessment remain in place. There is the potential for direct chronic effects to listed and 

non-listed estuarine/marine invertebrates from the turf and potted ornamental drench 

uses. 

Risk to terrestrial plants 

The original Registration Review risk assessment concluded there were potential direct risk 

concerns for listed and non-listed dicots (turf, potted ornamental spray, soil injection/basal 

drench, and potted ornamental drench) and listed monocots (turf, soil injection/basal drench, and 

potted ornamental drench). The newly submitted seedling emergence study (MRID 49300408) 

yielded paclobutrazol endpoints that were lower than those estimated using the 5th percentile 

approach (Table 1). Note, a new paclobutrazol vegetative vigor study was submitted shortly 

before the draft interim decision document was published. The data will be considered in future 

analyses. 
4 

Table 1. Comparison of seedling emergence endpoints 

 5th Percentile Empirical Data 

Monocot EC25 > 0.098 lb ai/A 

NOAEL = 0.071 lb ai/A 

EC25 = 0.0824 lb ai/A 

NOAEL = 0.0098 lb ai/A 

Dicot EC25 = 0.009 lb ai/A 

NOAEL = 0.002 lb ai/A 

EC25 = 0.00631 lb ai/A 

EC05 = 0.000567 lb ai/A* 

*The EC05 was used because the NOAEL was calculated to be greater than the EC25. 

Incorporating the new endpoints into the assessment, the resultant RQs are higher, now resulting 

in risk concerns for non-listed monocots (turf, soil injection/basal drench, and potted ornamental 

drench) in addition to the risk concerns identified in the original assessment. No risk concerns 

were identified for any of the seed treatments (see Appendix A for RQs, Table 6). 

Consequently, there is the potential for direct effects to listed and non-listed dicots and 

listed monocots for the turf, soil injection/basal drench, potted ornamental spray, and 

potted ornamental drenches. In addition, there are potential direct risk concerns for nonlisted monocots for the turf, soil injection/basal drench, and potted ornamental drench 

uses. 

Acute risk to terrestrial invertebrates 

The original Registration Review risk assessment concluded that there were no potential direct 

risk concerns for listed and non-listed terrestrial invertebrates. The conclusions were based upon 

triticonazole (LD50 >24 μg ai/bee), which was used as a surrogate for paclobutrazol. A newly 

evaluated honeybee acute oral and contact toxicity study (MRID 49300407) yielded less 

sensitive LD50s of >237 μg ai/bee (oral) and >235 μg ai/bee (contact). Consequently, the 

conclusions from the original Registration Review risk assessment are unchanged. No 

direct risk concerns are expected for listed and non-listed terrestrial invertebrates for any 

of the uses.

Risk to aquatic vascular plants 

The original Registration Review risk assessment concluded that there were potential risk 

concerns for non-listed aquatic vascular plants from the turf and ornamental drench uses. The 

newly evaluated study yielded an endpoint (EC50 = 4.61 μg ai/L) that is lower than the endpoint 

generated by the 5th percentile approach (EC50 = 8 μg ai/L). Using the new EC50 endpoint, the 

RQs are higher, but the risk picture does not change. In addition, the new study reported a 

NOAEC of 1.65 μg ai/L, which was used to calculate RQs for listed aquatic vascular plant 

species (see Appendix A for RQs, Table 5). All of the scenarios exceeded the LOC for listed 

aquatic vascular plants. 

In addition, Syngenta was concerned that the toxicity information from the open literature on 

watermilfoil and hydrilla was limited in its scope. EFED agrees; the open literature information 

was only used to qualitatively characterize risks to aquatic monocots and dicots, but was not used 

to calculate RQs. 

The conclusions from the original Registration Review risk assessment are unchanged for 

non-listed species, with the exception that the RQs are higher. There are potential direct 
5 

risk concerns for listed and non-listed aquatic vascular plants for the turf and ornamental 

drench uses. 

Risk to aquatic non-vascular plants 

The endpoints from the newly submitted studies for blue-green algae (EC50 > 25 mg ai/L) 

(MRID 49300401) and green algae (EC50 = 6.4 mg ai/L) (MRID 49300402) were lower than the 

paclobutrazol endpoint used in the risk assessment (green algae EC50 = 40.8 mg ai/L). New RQs 

were calculated using the most sensitive endpoint (6.4 mg ai/L); although the RQs are higher, 

they do not exceed the non-listed species LOC (see Appendix A for RQs, Table 5). 

Consequently, the conclusions from the original Registration Review risk assessment are 

unchanged for non-listed species (there are currently no listed species of aquatic nonvascular plants). Potential risk concerns for non-listed aquatic non-vascular plant species 

are unlikely. 

Acute risk to birds 

Syngenta indicated that mallard duck and bobwhite quail data could be used in place of acute 

oral passerine data to assess risk to these avian species. The original Registration Review risk 

assessment adopted this approach and concluded that there were no potential risk concerns 

for acute exposures to birds. This conclusion remains unchanged. An acute oral study with 

a passerine species will remain a data gap, in accordance with the data requirements in the Code 

of Federal Regulations. 

Syngenta Comment 

Syngenta indicated that further refinements are needed beyond the simple deterministic 

screening-level assessment methods that were used in the Registration Review risk assessment. 

If risk concerns are identified, further information such as community-level effects data, 

environmental fate and exposure data, monitoring data, geospatial location and proximity data, 

and species biology data can be incorporated into the assessment. Refined modeling techniques 

and probabilistic methods can be employed to more accurately characterize the risk. 

Specifically, Syngenta recommended that the aquatic exposure numbers could be further refined 

by predicting exposure concentrations over time and space that are specific to listed species and 

their habitats, and by distinguishing flowing water from static water bodies, as outlined in the 

National Academy of Sciences report. For terrestrial plants, Syngenta suggests that Pesticide 

Root Zone Model (PRZM) output be used to generate RQs for runoff exposures. 

 

EFED Response 

EFED agrees that further refinements are needed, particularly for the endangered species portion 

of the assessment. In conjunction with the National Marine Fisheries Service and the U.S. Fish 

and Wildlife Service, EPA is developing a suite of tools and a process to more realistically assess 

risk to listed species. Many of the techniques, tools, and data referenced in Syngenta’s comment 

are under consideration. 

Regarding the suggested refinements for assessing risks to terrestrial plants from runoff, EFED is 

currently developing a model based on PRZM output for terrestrial plants; however, until the 
6 

model has been fully vetted and approved, TerrPlant will be used as a Tier 1 model for screening 

risks to plants. 

Syngenta Comment 

Syngenta indicated than an acceptable bioaccumulation study is available, Study RJ 0321B 

PP333: Accumulation in Bluegill Sunfish in a Flow-Through System, D. Riley (10/21/83). 

Of particular note, bluegill sunfish were exposed to 14C triazole labelled PP333 in a flow-through 

system (~0.5 mg/L). After exposure, fish were transferred to uncontaminated flowing water for 

14 days (depuration). During the exposure period, the maximum concentration in whole fish, 

muscle, and viscera were 23.2, 10.7, and 131.4 mg/kg wet weight respectively which is 

equivalent to bioconcentration factors of 44, 20, and 248 respectively. There was no significant 

increase in 14C residues after 3 days of exposure. During the depuration period, the 14C levels 

fell rapidly and returned to background levels in all tissues in 7 days. 

EFED Response 

The study (RJ 0321B) author’s conclusion that 14C residues in fish tissue had plateaued was 

contradicted by the data provided in Table 4 and Figure 3 of the submitted study and captured in 

the Data Evaluation Record. The maximum bioconcentration factor in various tissues is not 

reliable because the study was terminated before it was certain that the concentration of 14C 

residues reached a plateau in the fish tissues. 

Syngenta Comment 

With respect to drift, EPA assesses potential drift onto non-target areas using the AgDRIFT 

model. Tier 1 modelling with AgDRIFT produces a highly conservative estimation of spray drift 

deposition fractions using default and generic assumptions that can be refined using realistic 

model adjustments and alternatives (e.g., evaporation rate, boom height, droplet size, use of the 

50th centile deposition data). The only inputs available to the user in Tier 1 AgDRIFT are boom 

height, data percentile and droplet size distribution. No further information about the spray 

material, equipment or climate conditions is considered. To improve understanding of factors 

influencing drift, two higher tiered models are proposed; REGDISP for ground application and 

AGDISP for aerial application. The existing aerial application models take a ‘mechanistic’ 

approach with consideration of physical and chemical properties while the current ground 

application models use empirical data (deposition data) and representative curve fits. Accuracy 

of those drift model performances relies on how well a model predicts upper centile of a biased 

data set. 

EFED Response 

While EFED has evaluated the spray drift deposition data submitted by Crop Life America (i.e., 

REGDISP), issues with the completeness of data prevent EFED from using the information for 

quantitative purposes. EFED is in the process of evaluating all spray drift data that have been 

submitted to the Agency and developing a policy for the use of existing and future deposition 

data in the conduct of its assessments. Until this policy has been formalized and approved by 

management, EFED will continue to use methods developed in its Guidance on Modeling Offsite 

Deposition of Pesticides Via Spray Drift for Ecological and Drinking Water Assessments 

document in the quantitative assessment of spray drift, while using supplemental data submitted 

to the Agency for characterization purposes. The only exception would be for spray drift 
7 

deposition data, generated specifically for the active ingredient/formulated product being 

assessed. Such data could be used for quantitative purposes, if found to be acceptable to the 

Agency. 

Syngenta Comment 

There were a number of minor errors identified in the Registration Review risk assessment. 

They usually consisted of typos, differences in species names, slight variations in environmental 

fate parameters, and other similar issues. 

EFED Response 

EFED appreciates Syngenta’s careful review of the risk assessment and agrees that the following 

errors should be corrected and incorporated into future risk assessments: 

 Change the chronic early life stage species from rainbow trout to fathead minnow. 

 Table 5 and Table 6 have been revised (see Appendix A) to incorporate Syngenta’s 

comments related to fate properties. 

 Table 7 has been revised to incorporate Syngenta’s comments on model inputs (see 

Appendix A). All future modeling exercises will be performed with the revised input 

values listed in the table in conjunction with current EFED guidance and models.

 Data used in generating half-lives for the total toxic residues (TTR) of paclobutrazol were 

included in the revised Appendix B. The half-life of Pappelacker soil was also modified 

(see Appendix B).

 The field study subsection of the “Environmental Fate and Transport” section has been 

revised to incorporate Syngenta’s comments and the mobility of paclobutrazol based on 

MRID 155854 (see Appendix B). The following field dissipation study citations will be 

incorporated into the fate data bibliography.

146597 Pearson, F.J. (1985) PP333 ketone residue dissipation in U.S. soils. Report –Series 

TMU1760/B. Unpublished study prepared by ICI Americas Inc., Goldsboro, NC, and 

Analytical biochemistry Laboratories, Columbia, MO; and submitted by ICI Americas 

Inc., Goldsboro, NC. 

155854 Hendley, P.; Francis, P. (1986) Paclobutrazol: Short Term Dissipation and Movement 

following a Broadcast Spray: TMU1953. Unpublished study prepared by ICI Americas, 

Inc. 
8 

References 

USEPA. 2013. Registration review – ecological risk assessment and effects determination of 

paclobutrazol. Environmental Fate and Effects Division, Office of Pesticide Programs, 

United States Environmental Protection Agency. 

USEPA. 2014. Review of nine ecotoxicity studies on ecological effects of paclobutrazol. 

Environmental Fate and Effects Division, Office of Pesticide Programs, United States 

Environmental Protection Agency. 
9 

Appendix A 

Tables 1 through 4 summarize the overall risk conclusions for listed and non-listed species. 

Tables 5 and 6 report revised RQs for a number of aquatic taxa and terrestrial plants, based on 

newly submitted toxicity data. 

Table 1. Revised summary table for potential risks to non-listed and listed species 

associated with direct or indirect effects from paclobutrazol (changes from the original 

assessment are in bold) 

Taxonomic Group Effects Endpoint1 Direct Effects Indirect Effects from Direct 

Effects to:1 Non-listed Listed 

Aquatic vascular 

plants 

Measures of biomass – 

EC50 and NOAEC (Lemna spp.) Yes 

May 

Affect/Likely 

to Adversely 

Affect 

Terrestrial plants 

Aquatic nonvascular plants Non-vascular plant –EC50

(green algae)

No; 

RQs<LOC 

No Effect; 

RQs<LOC 

Currently no listed non-vascular 

plants 

Terrestrial plants - 

dicots 

Measures of biomassVegetative vigor EC25 and 

NOAEC (tomato) 

Seedling emergence EC25 and 

EC05 (soybean)

Yes 

May 

Affect/Likely 

to Adversely 

Affect 

Mammals, birds, terrestrial 

invertebrates

Terrestrial plants - 

monocots 

Measures of biomassVegetative vigor EC25 (nondefinitive) and NOAEC (corn, 

onion, ryegrass, wheat) Seedling 

emergence EC25 NOAEC 

(ryegrass)

Yes 

May 

Affect/Likely 

to Adversely 

Affect 

Freshwater 

fish/amphibians 

Acute: Mortality- LC50 (grass 

carp) 

Mortality – LC50 (Asiatic toad) 

No; 

RQs<LOC 

No Effect; 

RQs<LOC Terrestrial invertebrates, aquatic 

invertebrates, terrestrial and 

Chronic: Chronic early life- aquatic plants 

stage-NOAEC (rainbow trout) Yes 

May 

Affect/Likely 

to Adversely 

Affect 

Estuarine/marine 

fish 

Acute: Mortality -LC50

(sheepshead minnow) (surrogate 

data)

No; 

RQs<LOC 

May 

Affect/Likely 

to Adversely 

Affect Terrestrial and aquatic plants, 

terrestrial invertebrates, birds 

Chronic: Early-life stage and/or mammals 

NOAEC (sheepshead minnow) 

(surrogate data)

Yes 

May 

Affect/Likely 

to Adversely 

Affect 

Freshwater 

invertebrates 

Acute: Mortality- EC50

(D.magna) 

Mortality- LC50 (Chinese 

mystery snail) 

No; 

RQs<LOC 

May 

Affect/Likely 

to Adversely 

Affect; 

No Effect 

(freshwater 

mollusks only) 

Terrestrial and aquatic plants 

Chronic: Growth and 

Reproduction- NOAEC (D. 

magna) (surrogate data)

Yes 

May 

Affect/Likely 

to Adversely 

Affect 

Estuarine/marine 

invertebrates 

Acute: EC50 (Eastern oyster and 

mysid shrimp) (surrogate data) 

No; 

RQs<LOC No Effect Terrestrial and aquatic plants 
10 

Taxonomic Group Effects Endpoint1 Direct Effects Indirect Effects from Direct 

Effects to:1 Non-listed Listed 

Chronic: chronic toxicity 

NOAEC (mysid shrimp) 

(surrogate data)

Yes 

May 

Affect/Likely 

to Adversely 

Affect 

Mammals 

Acute: Oral LD50 (rat) Yes

May Affect, 

Likely to 

Adversely 

Affect; Terrestrial invertebrates, birds, 

reptiles, land-phase amphibians; 

Chronic: Growth, survival, and terrestrial plants 

reproduction - Multi-gen rat 

study 

Yes 

May Affect, 

Likely to 

Adversely 

Affect 

Birds 

(surrogate for 

reptiles and landphase amphibians) 

Acute: Oral LD50 (mallard duck) 

Acute: Dietary LC50 (bobwhite 

quail) 

No; 

RQs<LOC 

No; 

RQs<LOC 

Terrestrial invertebrates, 

mammals, terrestrial plants Chronic: Growth, survival, and 

reproduction- (bobwhite quail)

Chronic 

dietary - Yes 

May Affect, 

Likely to 

Adversely 

Affect 

Terrestrial 

invertebrates 

Acute: Contact and oral LD50

(honeybee) No No Terrestrial plants

1 Direct effects to species may result in indirect effects to other species by changing availability of prey and modification to 

habitat. 

Table 2. Revised summary of potential direct effects to listed and non-listed terrestrial 

organisms from exposure to paclobutrazol1 (changes from the original assessment are in 

bold) 

Paclobutrazol Listed Terrestrial Species Non-listed Terrestrial Species 

Birds2 Mammals Plants Inverts. Birds2 Mammals Plants Inverts. 

Acute Chronic Acute Chronic Acute Chronic Acute Chronic 

Turf N N Y 

(S,M) 

Y 

(S,M,L) 

Y N N N N Y 

(S.M.L) 

Y N 

Potted 

ornamental 

spray 

N N N Y 

(S,M,L) 

N N N N N Y 

(S,M,L) 

Y N 

Tree injection N N N Y 

(S,M,L) 

N/A N N N N Y 

(S,M,L) 

N/A N 

Soil injection/ 

basal drench 

N Y Y 

(S,M,

L) 

Y 

(S,M,L) 

Y N N Y N Y 

(S,M,L) 

Y N 

Potted 

ornamental 

drench 

N N N Y 

(S,M,L) 

Y N N N N Y 

(S,M,L) 

Y N 

Tomato seed 

treatment 

N N N N N N N N N N N N 

Pepper seed 

treatment 

N N N N N N N N N N N N 

Cucurbit seed 

treatment 

N N N N N N N N N N N N 
11 

Paclobutrazol Listed Terrestrial Species Non-listed Terrestrial Species 

Birds2 Mammals Plants Inverts. Birds2 Mammals Plants Inverts. 

Acute Chronic Acute Chronic Acute Chronic Acute Chronic 

Broccoli seed 

treatment 

N N N N N N N N N N N N 

Cauliflower 

seed 

treatment 

N N N N N N N N N N N N 

Cabbage seed 

treatment 

N N N N N N N N N N N N 

1

 Y= Potential direct effects (RQ ≥ LOC); N = Potential direct effects not expected (RQ < LOC); N/A = not 

available (incomplete exposure pathway, thus it was not analyzed); S, M or L indicate which size class is potentially 

affected; S = small bird (20 g) or mammal (15 g); M = medium bird (100 g) or mammal (35 g); L = large bird (1000 

g) or mammal (1000 g). For mammals, chronic size classes are for dose-based exposure only. Dietary-based acute 

and chronic exposure for birds and mammals is not evaluated using body weights. RQ values were determined for 

these specific weights only, therefore, it is expected that potential direct effects may exist between two size classes 

in which the lower size class exceeded the LOC but the next size class did not. Therefore, for example, if small 

birds (20 g) exceeded the LOC but medium birds (100 g) did not, then potential direct effects may exist for all birds 

less than 100 g at any life stage until can be shown otherwise. 

2

 Birds are surrogates for terrestrial reptiles and terrestrial-phase amphibians. 

Table 3. Revised summary of potential direct effects to listed aquatic organisms from 

exposure to paclobutrazol1 (changes from the original assessment are in bold) 

Paclobutrazol 

Freshwater Fish Estuarine/Marine 

Fish 

Vascular 

Aquatic 

Plants 

Freshwater 

Invertebrates 

Estuarine/Marine 

Invertebrates 

Acute Chronic Acute Chronic Acute Chronic Acute Chronic 

Turf N Y N Y Y Y N N Y 

Potted 

ornamental 

drench 

N Y Y Y Y Y Y N Y 

1

 Y= Potential direct effects; N = Potential direct effects not expected

Table 4. Revised summary of potential direct effects to non-listed aquatic organisms from 

exposure to paclobutrazol1 (changes from the original assessment are in bold) 

Paclobutrazol 

Freshwater Fish Estuarine/Marine 

Fish 

Vascular 

Aquatic 

Plants 

Non-vascular 

Aquatic 

Plants 

Freshwater 

Invertebrates 

Estuarine/Marine 

Invertebrates 

Acute Chronic Acute Chronic Acute Chronic Acute Chronic 

Turf N Y N Y Y Y N N N Y 

Potted 

ornamental 

drench 

N Y N Y Y Y Y Y N Y 
12 

1

 Y= Potential direct effects; N = Potential direct effects not expected

Table 5. Revised aquatic RQs for select groups 

Aquatic 

Exposure 

(model) 

Use Scenario 

(modeled rate) 

Freshwater 

Invertebrate 

Chronic RQ 

(21-Day EEC/ 

NOAEC) 

NOAEC = 

320 μg ai/L 

Non-Listed 

Aquatic 

Vascular Plant 

RQ (Peak 

EEC/EC50) 

EC50 = 4.61 μg 

ai/L 

Listed Aquatic 

Vascular Plant 

RQ (Peak 

EEC/ NOAEC) 

NOAEC = 1.65 

μg ai/L 

Non-Listed 

Aquatic NonVascular Plant 

RQ (Peak 

EEC/EC50) EC50

= 6400 μg ai/L 

Total Toxic Residue (Paclobutrazol plus CGA 149907and NOA 457654)1,2 

Surface Water 

(Tier 2 PRZM/ 

EXAMS) 

Ornamentals: 

CA Ornamentals STD 

FL Ornamentals STD 

MI Ornamentals STD 

NJ Ornamentals STD 

OR Ornamentals STD 

TN Ornamentals 

STD(1 app. X 2.5 lb 

ai/A) 

<0.01-0.04 

0.46-1.04*** 

0.62-0.94 

0.45-0.80 

0.16-0.32 

0.45-0.80

<0.01-2.8*** 

33-72*** 

43-65*** 

31-54*** 

11-22*** 

32-56*** 

0.02-7.9*** 

88-202*** 

120-182*** 

87-150*** 

32-62*** 

88-156***

<0.01-<0.01 

0.02-0.05 

0.03-0.05 

0.02-0.04 

0.01-0.02 

0.02-0.04 

Ornamentals: 

CA Ornamentals STD 

FL Ornamentals STD 

MI Ornamentals STD 

NJ Ornamentals STD 

OR Ornamentals STD 

TN Ornamentals STD 

(1 app. X 0.56 lb ai/A) 

<0.01-<0.01 

0.10-0.23 

0.14-0.21 

0.10-0.18 

0.04-0.07 

0.10-0.18 

<0.01-0.61 

6.9-16*** 

9.5-15*** 

6.9-12*** 

2.6-5.0*** 

6.9-12***

<0.01-1.7***

19-45*** 

27-41*** 

19-35*** 

7.3-14*** 

19-35***

<0.01-<0.01 

0.01-0.01 

0.01-0.01 

0.01-0.01 

<0.01-<0.01 

0.01-0.01 

Turf: 

FL Turf STD 

PA Turf STD 

(3 app. [2 app. X 0.75 

lb and 1 app. X 0.5 lb] 

ai/A) 

0.07-0.12 

0.11-0.21

4.6-8.5*** 

7.6-15***

13-24*** 

21-41***

<0.01-0.01 

0.01-0.01 

 Parent Paclobutrazol Only3

FL Ornamentals STD 

(1 app. X 2.5 lb ai/A) 

0.58 41*** 113*** 0.03 

PA Turf STD 

(3 app. [2 app. X 0.75 

and 1 app. X 0.5 lb] 

ai/A 

0.16 11*** 31*** 0.01 

1

Multiple months were modeled; the range of RQs represents the highest and lowest EECs. 2

Total toxic residue simulation includes two metabolites (CGA 149907and NOA 457654) 3

Exposure of parent only based on highest exposure scenarios (FL Ornamentals STD and PA Turf STD) identified 

from total toxic residue simulations. 

***Exceeds LOC (1) 
13 

Table 6. Revised terrestrial plant RQs, based on new seedling emergence data (MRID 

49300408) 

Plant Type Listed Status Dry Semi-Aquatic Spray Drift 

Turf (1 app at 0.75 lb ai/A) 

Monocot Non-listed 0.27 1.91*** <0.1 

Monocot Listed 2.30*** 16.07*** 0.77 

Dicot Non-listed 3.57*** 24.96*** 1.19*** 

Dicot Listed 3.97*** 27.78*** 1.32*** 

Potted ornamental spray (1 app at 0.26 lb ai/A) 

Monocot Non-listed <0.1 0.66 <0.1 

Monocot Listed 0.80 5.57*** 0.27 

Dicot Non-listed 1.24*** 8.65*** 0.41 

Dicot Listed 1.38*** 9.63*** 0.46 

Soil injection/basal drench (1 app at 1.9 lb ai/A) 

Monocot Non-listed 0.69 4.84*** N/A 

Monocot Listed 5.82*** 40.71*** N/A 

Dicot Non-listed 9.03*** 63.23*** N/A 

Dicot Listed 10.05*** 70.37*** N/A 

Potted ornamental drench (1 app at 6.1 lb ai/A) 

Monocot Non-listed 2.22*** 15.55*** N/A 

Monocot Listed 18.67*** 130.71*** N/A 

Dicot Non-listed 29.00*** 203.01*** N/A 

Dicot Listed 32.28*** 225.93*** N/A 

Tomato seed treatment (1 app at 0.0004 lb ai/A) 

Monocot Non-listed <0.1 <0.1 N/A 

Monocot Listed <0.1 <0.1 N/A 

Dicot Non-listed <0.1 <0.1 N/A 

Dicot Listed <0.1 <0.1 N/A 

Pepper seed treatment (1 app at 0.00079 lb ai/A) 

Monocot Non-listed <0.1 <0.1 N/A 

Monocot Listed <0.1 <0.1 N/A 

Dicot Non-listed <0.1 <0.1 N/A 

Dicot Listed <0.1 <0.1 N/A 

Cucurbit seed treatment (1 app at 0.00029 lb ai/A) 

Monocot Non-listed <0.1 <0.1 N/A 

Monocot Listed <0.1 <0.1 N/A 

Dicot Non-listed <0.1 <0.1 N/A 

Dicot Listed <0.1 <0.1 N/A 

Broccoli seed treatment (1 app at 0.000096 lb ai/) 

Monocot Non-listed <0.1 <0.1 N/A 

Monocot Listed <0.1 <0.1 N/A 

Dicot Non-listed <0.1 <0.1 N/A 

Dicot Listed <0.1 <0.1 N/A 
14 

Plant Type Listed Status Dry Semi-Aquatic Spray Drift 

Cauliflower seed treatment (1 app at 0.000064 lb ai/A) 

Monocot Non-listed <0.1 <0.1 N/A 

Monocot Listed <0.1 <0.1 N/A 

Dicot Non-listed <0.1 <0.1 N/A 

Dicot Listed <0.1 <0.1 N/A 

Cabbage seed treatment (1 app at 0.00012 lb ai/A) 

Monocot Non-listed <0.1 <0.1 N/A 

Monocot Listed <0.1 <0.1 N/A 

Dicot Non-listed <0.1 <0.1 N/A 

Dicot Listed <0.1 <0.1 N/A 

N/A – not applicable because spray drift does not occur with seed treatment/drench/soil injection 

uses 

*** – exceeds the LOC of 1 
15 

Revised Table 5. Summary of physical, chemical, and fate properties of paclobutrazol 

Parameter Value Reference Comments 

Chemical Paclobutrazol MRID 456625-01 --- 

Chemical Class Triazole ---

CAS No. 76738-62-0 --- 

Empirical Formula C15H20ClN3O MRID 456625-01 --- 

IUPAC (2RS,3RS)-1-(4-

chlorophenyl)-4,4-dimethyl2-(1H-1,2,4-triazol-1-

yl)pentan-3-ol 

MRID 456625-01

--- 

CAS Name (αR,βR)-rel-β-[(4-

chlorophenyl)methyl]-α-

(1,1-dimethylethyl)-1H1,2,4-triazole-1-ethanol

MRID 456625-01

--- 

Structure MRID 456625-01

--- 

Selected Physical/Chemical Parameters

Molecular mass 293.8 g/mol MRID 46180501 ---

Solubility (20.0°C) 2.29 x 10-2 (water) 

5.67 g/L (xylene) 

0.166 g/L (n-Heptane) 

72.4 g/L (Acetone) 

45.1 g/L (Ethyl Acetate) 

29.4 g /L (n-Octanol) 

115 g/L (methanol) 

51.9 g/l (1,2-

Dichloromethane) 

MRID 468342-02 

---

Vapor pressure (20.0°C) 1.9 x 10-6 Pa 

(1.43 x 10-8 mm Hg) 

MRID 468342-02 ---

Henry’s law constant (20°C) 2.36 x 10-10 atm M3 mol-1) MRID 468342-02 ---

Octanol-water partition coefficient 

(LogKOW) (25°C) 

3.11 MRID 468342-02 ---

Persistence

Hydrolysis 

pH 4: stable 

pH 7: stable 

pH 9: stable 

MRID 00132698 ---

Aqueous Photolysis (t 1⁄2 ) Stable MRID 00132699 ---

Soil Photolysis (t 1⁄2 ) Stable MRID 40685002 After 33 days 

paclobutrazol decrease 

to 86.4 % of applied 

radioactivity 

Aerobic Soil Metabolism 

(t 1⁄2 ) 

Sandy clay loam 

19 (21 and 17) months1,2

[47 (58 and 36) months]3

MRID 40685003 Major metabolite 

(CGA 149907 (≈18.7%)
16 

Parameter Value Reference Comments 

Sandy clay loam 

301 days 

[385 days] 

MRID 47338504 Supplemental 

No major metabolites 

detected above 10%. 

Sandy loam 

26 days4

[38.9 days]

Major metabolites 

NOA 457654(25.1%) 

CGA 149907(13.3%) 

Sandy loam 

25 days4

[36 days] 

Major metabolites 

NOA 457654 (20.5%) 

CGA 149907(26.3%) 

Anaerobic Soil Metabolism No data 

Anaerobic Aquatic Metabolism (t 1⁄2 ) Stable MRID 40685003 After a year of 

incubation, 

paclobutrazol decrease 

to ≤79% of applied 

radioactivity 

Major transformation 

product: 

(CGA 149907 (≈16.8%)

Aerobic Aquatic Metabolism 

No data -- 

Mobility

Soil Partition Coefficient Adsorption Kd (mL/g) 

9.5 (Clay loam) 

10.2 (Sandy Clay loam) 

7.6 (Silty clay loam) 

21.3 (Silt loam)5

4.1 (sandy loam) 

2.4 (loamy sand) 

5.8 (Silty clay) 

1.4 (Sand) 

3.8 (Silt loam) 

MRID 40685005 There was no significant 

correlation (r2

 = 0.12) 

between Kd and % 

organic carbon. 

Soil Partition Coefficient Desorption Kdes (mL/g)6

14.8 (Clay loam) 

17.4 (Sandy Clay loam) 

13.5 (Silty clay loam) 

33.4 (Silt loam)5

8.6 (sandy loam) 

4.9 (loamy sand) 

9.2 (Silty clay) 

2.7 (Sand) 

5.8 (Silt loam) 

MRID 40685005 

---
17 

Parameter Value Reference Comments 

Adsorption Kd (mL/g) 

1.7 (Sandy loam) 5

0.8 (Sandy loam) 

2.7 (Silty clay loam) 

2.1 (Clay loam) 

Desorption Kdes (mL/g)6

10.9 (Sandy loam) 5

3.81 (Sandy loam) 

14.2 (Silty clay loam) 

11.7 (Clay loam) 

MRID 47338503 Supplemental 

Leaching Aged methine 14C- labeled 

paclobutrazol residues were 

relatively immobile 

Aged trizole 14C-labeled 

paclobutrazol residues were 

mobile in loamy sand and 

clay loam soils and low 

mobility in sands and sandy 

loam soils 

MRID 40685004 Major transformation 

product 

CGA 149907 (≈12.6%) 

Field Dissipation

Terrestrial Field Dissipation (t 1⁄2 ) 

 450 to 973 days 

 (CA, WV, FL) 

MRID 

40685006-

406850012 

Supplemental 

Major transformation 

product 

 CGA 149907 

88 days (CA) 

133 days (NY) MRID 47338505 

Supplemental 

Major transformation 

product - CGA 149907 

(40%) 

175 to 224 days 

(MS, NC, IL) MRID 00146597 

Supplemental 

Transformation product

 CGA 149907 

1

 Calculated half-lives are based on 14C labeled triazole and methane of paclobutrazol 2

 Extrapolated half-lives exceeded beyond the study period except soils with pH > 7.0 3

 Italic half-lives are calculated based on paclobutrazol and its major degradates (CGA 149907 and NOA 457654). 

Calculation can be found in Appendix B. 

4

 High Soil pH (≥ 7.0) may have contributed faster degradation rates for aerobic soil metabolism 5

 Japanese soils, excluded during exposure assessment 6

 Reported values are based on second desorption step 
18 

Table 6. Selected physical, chemical, and fate properties of NOA-457654 

Parameter Value Reference Comments 

Chemical NOA-457654 MRID 473385-01 ---

Chemical Class Triazole MRID 473385-01 ---

CAS No. 122442-66-4 MRID 473385-01 ---

CAS Name 4H-1,2,4-Triazol-3-ol MRID 473385-01 ---

IUPAC Name 1H-1,2,4-Triazol-3-ol. MRID 473385-01 ---

Synonyms Hydroxytriazole, Triazole 

hydroxy, J7140/45/2 

MRID 473385-01 ---

Empirical Formula C2H3N3O MRID 473385-01 ---

Molecular Weight 85.07 g/mole MRID 473385-01 ---

Aerobic Soil Metabolism 

(t 1⁄2 ) 

Sandy clay loam soil 

8.2 days 

MRID 473385-01 No major degradate 

except CO2

Silt loam soil 

3.8 days 

Loamy sand soil 

10.8 days 

Sandy loam soil 

3.4 days 

Soil Partition Coefficient 

Adsorption Kads (mL/g) 

0.13 (Loamy Sand) 

0.25 (Loam) 

0.19 (Sandy loam) 

0.30 (Sandy loam) 

Adsorption KOC (mL/g) 

8.9 (Loamy Sand) 

10.7 (Loam) 

8.1 (Sandy loam) 

13.0 (Sandy loam)

MRID 47338502 Calculated r2

 value for 

Kads vs. % organic 

carbon content was 

0.93and Kads vs. % clay 

was 0.8 

Desorption Kdes (mL/g) 

1.0 (Loamy Sand) 

1.7 (Loam) 

1.5 (Sandy loam) 

1.8 (Sandy loam) 

Adsorption KOC (mL/g) 

72,7 (Loamy Sand) 

74.6 (Loam) 

72.0 (Sandy loam) 

66.5 (Sandy loam)

--- 
19 

Revised Table 7. PRZM-EXAMS input parameter values for paclobutrazol 

Parameter Value Source Comments

Application Rate 

(lb ai/A) 

2.50 lb ai/A (based 

on 59% retention of 

parent in soil 

column)1 

0.55 lbs/A (based on 

91% retention of 

parent in soil column 

2.0 lbs/A for turf 

Memo from the 

Biological and Economic 

Analysis Division 

(BEAD) (US EPA, 2011) 

Product Label 

Maximum application rate was estimated 

based on information provided by BEAD 

(US EPA 2011) and adjusted with the 

retention of paclobutrazol in a soil column 

leaching study (MRID 40685004) 

EPA Reg No. 100-1079 

Number of 

Applications 

1 (Nursery) 

3 (Turf) 

Product Label EPA Reg No. 100-1079 

Application 

frequency (days) 

Drench –None 

Foliar-14 

Product Label 

EPA Reg No. 100-1079 

Scenarios Ornamentals 

CA Ornamentals 

STD 

FL Ornamentals STD 

MI Ornamentals STD 

 NJ Ornamentals 

STD 

OR Ornamentals 

STD 

TN Ornamentals STD 

Turf 

FL Turf STD 

 PA Turf STD 

Selected ornamental scenarios were chosen 

to represent various regions of the 

Continental United States 

Application Date Ornamentals–01 

(Mar –Sept) 2

Turf 

FL Turf -15 (FebSept)3

PA Turf 15 (AprSept)4

--- 

Spray Drift 

Fraction 

Drench - 0 

Ground foliar-0.1 Model input guidance 

Application (US EPA, 2010) 

Efficiency 

Drench- 1.0 

Ground Foliar -0.99 
20 

Parameter Value Source Comments

Soil Partitioning 

Coefficient (Koc

ml/g) 

144.0 (clay loam) 

338.9 (sandy clay 

loam) 

232.8 (silty clay 

loam) 

233.3 (sandy loam) 

262.9 (loamy sand) 

413.8 (silty clay) 

204.0 (sand) 

429.9 (silt loam) 

263.0 (sandy loam) 

106.0 (silty clay 

loam) 

40.4 (clay loam) 

242.6 (Mean value)5 

MRIDs 4188113 and 

47338503 

Mean value: 242.6 

Aerobic Soil 

Metabolism t1/2

(d) 

999.2 (Total residue)6 

444.2 (Parent)6 

MRIDs 40685003 and 

47338504 

90th percentile upper confidence bound on 

the mean. 

Method of 

Application 

Drench 

Ground Paclobutrazol labels --- 

Solubility in 

Water (mg/L) 

22.9 MRID 468342-02 --- 

Aerobic Aquatic 

Metabolism t1/2

(days) 

1998.4 (Total 

Residue) 

888.4 (Parent) 

2X of Aerobic soil 

metabolism half-life 

Model input guidance 

(US EPA, 2010) 

Hydrolysis t1/2 0 MRID 00132698 Stable to hydrolysis 

Aquatic 

Photolysis t1/2

(days) 

0 MRID 0013299 Stable to aquatic photolysis 

1

 Example of application rate calculation: 6.1 lbs/A- (6.1lbs/A*0.59) 

 Where 

 6.1 lbs/A is application rate for drench application for 8-inch container grown ornamentals (US EPA, 2011) 

 0.59 is the minimum fraction of retention of parent (paclobutrazol) in column leaching study (MRID 40685004) 2 In absence of application date from the label, model simulations were performed from March to September with 

date of application first of the month 

3

In absence of application date in label, model simulations were performed from February to September with date of 

application 15th of the month 4

 In absence of application date in label, model simulations were performed from April to September with date of 

application 15th of the month 5

 Coefficients of variations (CV) were lower for organic carbon normalized solid-water distribution coefficients 

(KOC; CV=50%) as compared to solid-water distribution coefficients (Kd; CV=73%). Therefore, and consistent with 

the input parameter guidance, the mean Kd of 4.5 L/Kg will be revised with mean Koc of 142.6 L/Kg to estimate 

paclobutrazol concentrations for future aquatic modeling. 

6

 Some of the estimated half-lives (Table 5) are beyond the study period and can be considered as stable. However, 

to use 90th percentile of the degradation rate in the modeling, all estimated half-lives were considered 
21 

Revised Appendix B – Half-Life Calculations of Parent and Total 

Toxic Residues 

Aerobic Soil Metabolism 

Chemical: Paclobutrazol 

MRID: 40685003 

PC Code: 125601 

Guideline: 835.4100 

Mass Balance- 18 Acres loam 

Posttreatment Paclobutrazol CGA 149907 

Total Toxic 

Residue 

Months % of Applied 

Triazole 14C 

Label 

0 87.5 0.1 87.6 

1 87.0 5.2 92.2 

4 78.7 10.1 88.8 

8 66.9 14.1 81.0 

12 60.0 18.7 78.7 

Methane 14C 

Label 

0 96.0 0.1 96.1 

1 88.0 3.6 91.6 

4 74.8 9.8 84.6 

8 67.9 12.2 80.1 

12 57.9 17.2 75.1 

Chemical: Paclobutrazol 

MRID: 47338504 

PC Code: 125601 

Guideline: 835.4100 

Mass Balance- 18 Acres Sandy clay loam 

Posttreatment Paclobutrazol CGA 149907 

Total Toxic 

Residue 

Days % of Applied 

0 86.6 1.0 87.6 

0 86.2 1.0 87.2 

7 88.8 3.0 91.8 

7 84.1 3.1 87.2 

22 87.8 4.0 91.8 

22 90.5 3.6 94.1 
22 

35 68.9 4.1 73.0 

35 72.5 3.9 76.4 

50 74.4 3.1 77.5 

50 64.8 5.0 69.8 

70 69.0 3.7 72.7 

70 63.3 3.4 66.7 

91 73.3 4.1 77.4 

91 83.3 4.2 87.5 

120 64.9 4.5 69.4 

120 63.2 3.8 67.0 

Mass Balance- Pappelacker Sandy loam 

Posttreatment Paclobutrazol CGA 149907 

Total Toxic 

Residue 

Days % of Applied

0 90.1 0.0 90.1 

0 80.4 0.0 80.4 

7 81.9 9.1 91.0 

7 83.1 8.6 91.7 

22 58.9 24.8 83.7 

22 60.3 24.0 84.3 

35 36.0 26.6 62.6 

35 36.5 25.9 62.4 

50 21.6 24.5 46.1 

50 22.4 27.3 49.7 

70 11.8 20.9 32.7 

70 12.7 23.2 35.9 

91 7.8 13.6 21.4 

91 6.2 11.1 17.3 

120 3.8 9.1 12.9 

120 3.7 6.9 10.6 

Mass Balance- Gartenacker Sandy loam 

Posttreatment Paclobutrazol CGA 149907 Total Toxic Residue 

Days % of Applied

0 85 0 85

0 93.4 0 93.4

7 85.4 5.7 91.1

7 86.2 5.5 91.7

22 67.3 13.5 80.8

22 66.5 13.1 84.9

35 41.5 12.3 53.8

35 41.2 12.6 53.8

50 30.5 11.8 42.3

50 26.1 11.9 38

70 16.1 10.7 26.8
23 

70 17.1 11.1 28.2

91 12.9 10.7 23.6

91 9 9.1 18.1

120 3.8 3.7 7.5

120 3.9 6.8 10.7

Calculations of Half lives 

 (MRID 40685003) 

Parent 

DT50 = 21 months 

DT50 = 17 months 
24 

Total Toxic Residue 

DT50 = 58 months 

DT50 = 36 months 
25 

MRID 47338504 

Total Toxic Residue 

Half-life = 385 days 
26 

Dissipation of paclobutrazol in Gartenacker sandy loam soil system 

(0-120 days) 

5.00 -,------------------"'f!"="-=A-f)'l-q.,!l-lr-F--;H;~ 

R2 = 0.9676 j 4.00 -1-------="'"'-<!1~------------------- ii Half life = 36 days 

~ 3.00 

~ 2.00 -1----------------------~~--­

'*- -

z 

_. 1.00 +------------------------------------------------

0.00 -1------,-----,-----,-----,-----,------,-----, 

0 20 40 60 80 100 120 140 

Days 
27 

Revised section for field studies 

c. Field Studies

Paclobutrazol residues were persistent and mobile in terrestrial field dissipation studies in 

California, West Virginia, Florida, North Carolina, Illinois, and Mississippi (Supplemental 

MRID 00155854, 40685007, 40685009, 40685010, 40685011, 4068512 and 40685006- 

and146597). Paclobutrazol residues include paclobutrazol and its ketone metabolite. Half-lives 

of paclobutrazol residues ranged from 450-973 days for orchard soils in California, West 

Virginia, and Florida, and 25 weeks to 32 weeks in agricultural soils in Mississippi, North 

Carolina, and Illinois. Although it was characterized as mobile in some soils in laboratory 

studies, no significant movement of paclobutrazol was detected in the agricultural soils. In the 

orchard studies, total paclobutrazol residues were detected at 0.01 to 1.9 ppm in the deepest soil 

samples that were collected, depths of 48 inches in the California and Florida and 24 inches in 

the West Virginia study. Note that these depths are the maximum depths sampled at each study. 

The CGA149907 metabolite was predominately detected in the subsurface soil layers, also at 

insignificant levels. In addition, there is no evidence of paclobutrazol and its ketone analogue 

movement was observed in an irrigated turf-covered sandy soil plot (MRID 155854). However, 

the study was terminated after only 30 days, before the pattern of decline of paclobutrazol was 

established.