UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON D.C., 20460 

FEB 2 5 2016 · OFFICE OF CHEMICAL SAFETY 

AND POLLUTION PREVENTION 

MEMORANDUM 

SUBJECT: 

FROM: 

THRU: 

TO: 

Thidiazuron Usage and Benefits Information (PC# 120301). 

Sunil Ratnayake, Ph.D., Biologist 

Biological Analysis Branch 

Stephen Smearman, Economist 

Economic Analysis Branch 

~,~ 

Biological and Economic Analysis Division 

~~ 

(7503P) 

Colwell Cook, Acting Branch Chief Ufi ..9( &.-£ 

Biological Analysis Branch 

Timothy Kiely, Chief fr. 

Economic Analysis Branch " 'fl'-· 

Biological and Economic Analysis ivision (7503 

Khue Nguy en, Chemical Review Manager 

Neil Anderson, Chief 

Risk Management and Implementation Branch I · 

Pesticide Reevaluation Division (7505P) 

BEAD Product Review Panel Date: February 17, 2016 

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Summary 

The Biological and Economic Analysis Division (BEAD) describes in this document information 

on use parameters that may help to characterize the risks of thidiazuron, a plant growth regulator 

(PGR), which is undergoing registration review in the Office of Pesticide Programs (OPP). The 

following information about thidiazuron is provided: the extent of aerial applications, number of 

applications, and range of application rates commonly used by growers. BEAD also provides 

information about the benefits of using thidiazuron and the possible impacts of potential risk 

management options. 

Thidiazuron is applied to a relatively large portion of cotton acres grown with about 20 percent 

applied aerially. This PGR is mostly applied at rates below the maximum label rates. 

Thidiazuron may need up to three applications per season to achieve necessary defoliation when 

canopies are dense; nearly 20 percent of treated acres receive multiple applications. Thidiazuron 

appears to be an important tool in cotton production based on the 30 percent of cotton acres 

treated and may be preferred by growers because it works as a hormonal defoliant. 

Background 

FIFRA Section 3(g) mandates that EPA periodically review the registrations of all pesticides to 

insure that they do not pose unreasonable adverse effects to human health and the environment. 

This periodic review is necessary in light of scientific advancements, changes in policy, and 

changes in use patterns that may alter the conditions underpinning previous registration 

_decisions. In determining whether effects are unreasonable, FIFRA requires that the Agency 

consider the risks and benefits of any use of the pesticide. 

OPP has completed human health and ecological risk assessments for thidiazuron. No human 

health risks were identified. Ecological risks were found for non-target plant species in areas 

adjacent to treated fields via surface water runoff and spray drift from ground and aerial 

applications. Aerial applications will typically pose the greatest risks since drift is more likely 

and more widespread. Other main drivers in the risk assessment include application rates, 

droplet size, and boom height. 

Thidiazuron 

Thidiazuron is a plant growth regulator that is only registered for use on cotton. Thidiazuron is a 

cotton defoliant that acts by stimulating the production of endogenous ethylene. There are 

several benefits of defoliation in cotton including efficient harvesting and improved b<;lll quality. 

Hormonal defoliants such as thidiazuron have some advantages over herbicidal defoliants which 

may result in desiccation instead of defoliation (University of Arkansas, 2013). Desiccation may 

leave leaf trash in the harvested cotton. Thidiazuron is applied to about 30 percent of cotton 

acres, indicating a relatively heavy reliance by growers on this chemical. 

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Thidiazuron Usage 

Thidiazuron may he applied by ground or air. Approximately 20 percent ofthidiazuron is 

applied aerially (Table 1 ). On average, the application rates are similar between ground and 

aerial application methods, and are well below label maximum rates. 

· Table 1. Thidiazuron use in cotton. 

Application Method Annual Average 

Total Acres Treated 

Ground 3,455,000 

Aerial 822,000 

Total 4,277,000 

Source: Market Research Data (MRD), 2009-2013 

1 Maximum application rate is 0.20 lbs a.i./acre. 

Annual Average Average 

Pounds Applied Application Rate 1 

231,000 0.067 

52,700 0.064 

284,000 0.066 

Texas and Georgia are the top cotton producing states in the US and the largest users of 

thidiazuron (Table 2). Aerial applications appear to be important iri states along the Mississippi 

river and i.n the Southwest as reflected by the relatively high percent of acres treated by air 

shown in Table 2. For example, the acres treated aerially in states such as Arizona,' Louisiana, 

California, and Mississippi, are 57, 42, 38, and 35 percent, respectively (Table 2). Aerial 

applications may be important for several reasons included: large size cotton farms with higher 

acreages may rely on aerial applications; aerial applications can be less expensive than ground 

applications; growers may be concerned that ground applications will damage the cotton crop; or 

ground application equipment cannot be used due to wet field conditions. 

T a e . bl 2 Th"d" 1 1azuron acres t rea td e aer1a "lib ty ~~v- s a tt e. 

Average Annual Average Annual Percent Treated State Acres Treated Total Acres Treated Aerially Aerially 

Georgia 1,045,000· 171,000 16 

Texas 1,009,000 99,500 10 

Mississippi 437,000 154,000 35 

Arkansas 314,000 78,800 25 

California 309,000 118,000 38 

Louisiana 265,000 111,000 42 

Arizona 140,000 79,700 57 

Other States 758,000 10,056 1 

Total 4,277,000 822,000 19 

Source: Market Research Data (MRD), 2009-2013 

Application Rates 

Thidiazuron is applied at an application rate of 0.066 pounds per acre of active ingredient on 

average (Table 3). Nearly 98 percent of all applications are made at a rate of 0.15 or below, 

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which is well below the maximum allowed application rate of 0.20 lb ai/acre. Risks were 

assessed on the basis of the maximum label rates, suggesting that the effects identified would be 

quite rare. 

T a bl e 3 . Th"d" 1 1azuron app r 1ca r ion ra t es. 

Maximum Single Application 

Rate from Label Average Single Application Rate at Crop (lb ai/A) Application Rate the 90th Percentile1 

(maximum seasonal rate for (lbs ai/A) 1 

all crops is 2 lbs ail A) 

Cotton 0.20 0.066 0.10 

1

Market Research Data (MRD), 2009-2013 

Multiple Applications 

Multiple applications of thidiazuron are needed in dense canopies or if a high number of green 

bolls need to be opened (University of Ark~sas, 2013). Table 4 shows the number of 

application of thidiazuron by state and the number of acres treated with each application. 

Louisiana, Missouri, and California may use up to three applications of thidiazuron in one year. 

The seasonal rate on acres treated twice averages 0.126 lb a.i./acre/year and on acres treated 

three times averages 0.145 lb a.i./acre/year (Market Research Data, 2009-2013). Certain labels 

restrict use to two applications and a maximum annual rate of 0.3 lb ail acre/year. If all labels 

limit the maximum number of applications to two then cotton growers that use three applications 

may be impacted. 

T a e . u bl 4 M If •P I e app r 1ca r ions· o f th"d" 1 1azuron b 1y s a t t e. 

Base Acres Acres Treated Acres Treated Percent Acres 

State Treated more Treated Twice Three Times than Once 

Georgia 919,000 116,000 0 13 

Texas 723,000 205,000 0 28 

Mississippi 359,000 74,800 0 21 

Arkansas 272,000 32,600 0 12 

California 219,ooo 89,300 300 41 

Louisiana 178,000 59,000 5,400 36 

Missouri 130,000 36,800 2,000 30 

Other States 693,000 8,700 700 1 

Total . 3,493,000 622,000 8,400 18 

Source: Market Research Data (MRD), 2009-2013 

Page4 
Droplet Size 

The droplet size effects the efficacy and off site drift of a pesticide application and the optimum 

droplet size for any herbicide or plant growth regulator is rtot something that can be generalized 

and it should be selected based on the data and knowledge. Deposition of the most effective 

amount of active ingredient on plant foliage and reduction of off-site drift are the major 

objectives of selecting an optimum droplet size. In some cases, droplet size and application rates 

are correlated; that is, coarser droplets are associated with higher application rates to maintain 

efficacy. However, in some systemic herbicides or defoliants, coarse droplet size with less 

coverage will still allow effective control. The optimum droplet size is impacted by 

chemical/physical parameters of the pesticide and other factors such as drying of the chemical 

prior to actual uptake, and air pressure used in the application. In addition, the interaction of 

· concentration and plant uptake, and the interaction of the active ingredient with other chemicals 

in a tank mixture are some of the other factors that should be considered on selection of optimum 

droplet size. BEAD does not have information specific to the relationship of droplet size and 

efficacy in thidiazuron. 

Page 5 
REFERENCES 

Market Research Data (2009 -2013). Data collected and sold by a private market 

research firm. Data collected on pesticide use for about 60 crops by annual surveys of 

agricultural users in the continental United States. Survey methodology provides statistically 

valid results, typically at the state level. 

University of Arkansas, 2013. 2013 Mid-South cotton defoliation guide. University of Arkansas 

Cooperative Extension Service. Online: http://www.uaex.edu/publications/pdf/MP503.pdf. 

Verified: February 12, 2016. 

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