### UNITED STATESENVIRONMENTAL PROTECTIONAGENCY WASHINGTON D.C., 20460 

### FEB 25 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 

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# &.-£ Biological Analysis Branch Timothy Kiely, Chief 

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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 ProductReview Panel Date: February 17, 2016

Page 1Summary The Biological and Economic Analysis Division (BEAD) describes in this document information onuse parameters that may help to characterize the risks ofthidiazuron, a plant growth regulator (PGR), which is undergoing registration review in the Office ofPesticide Programs (OPP). The following information about thidiazuron is provided: the extent ofaerial applications, number of applications, and range ofapplicationrates commonly used by growers. BEAD also provides information about the benefits ofusing thidiazuron and the possible impacts ofpotential risk management options. Thidiazuron is appliedto a relatively large portion ofcotton acres grown with about 20 percent applied aerially. This PGRis mostly applied at rates belowthe maximum label rates. Thidiazuron may need up to three applications per season to achieve necessary defoliation when canopies are dense; nearly 20 percent oftreated acres receive multiple applications. Thidiazuron appears to be an importanttool in cottonproduction based onthe 30 percent ofcotton acres treated and maybe preferred by growers because it works as a hormonal defoliant. Background FIFRA Section 3(g) mandates that EPAperiodically review the registrations ofall pesticides to insure thatthey do notpose unreasonable adverse effectsto humanhealth and the environment. This periodic review is necessary in light ofscientific advancements, changes inpolicy, and changes in use patterns that may alter the conditions underpinning previous registration

_ decisions. In determining whether effects are unreasonable, FIFRArequires that the Agency considerthe risks and benefits ofany use ofthe 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 runoffand spray drift from ground and aerial applications. Aerial applications will typically pose the greatest risks since drift is more likely and more widespread. Othermain 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 ofendogenous ethylene. There are several benefits ofdefoliation in cotton including efficientharvesting and improved b<;lll quality. Hormonal defoliants such as thidiazuron have some advantages over herbicidal defoliants which may result in desiccation instead ofdefoliation (University ofArkansas, 2013). Desiccation may leave leaftrash in the harvested cotton. Thidiazuron is applied to about 30 percent ofcotton acres, indicating a relatively heavy reliance by growers onthis chemical.

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### Thidiazuron Usage Thidiazuron may he applied by ground or air. Approximately 20 percent ofthidiazuronis applied aerially (Table 1 ). On average, the applicationrates are similarbetween ground and aerial application methods, and are well below label maximum rates.



### ·Table 

### 1. Thidiazuron use in cotton.

Application Method AnnualAverage TotalAcres Treated Ground 3,455,000 Aerial 822,000 Total 4,277,000 Source: Market Research Data (MRD), 2009-2013 1Maximum 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 cottonproducing states inthe 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 bythe relatively highpercent ofacres treated by air shown in Table 2. Forexample, 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 maybe important for several reasons included: large size cotton farms withhigher acreages may rely on aerial applications; aerial applications canbe 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 bl 2 Th"d" t td "lib tt

a e . 1 1azuron acres rea e aer1a ty**~~v-** 

### s a e. Average Annual Average Annual PercentTreated State Acres Treated TotalAcres 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 of0.066 pounds per acre ofactive ingredient on average (Table 3). Nearly 98 percent ofall applications are made at a rate of0.15 orbelow, Page 3which is well belowthe maximum allowed application rate of0.20 lb ai/acre. Risks were assessed onthe basis ofthe maximum label rates, suggesting that the effects identified would be quite rare.



### T bl 3 Th"d" 

## r r t



### a e 

# . 1 1azuron app 1ca ion ra 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 1Market Research Data (MRD), 2009-2013 

### Multiple Applications Multiple applications ofthidiazuron are needed in dense canopies or ifa high number ofgreen bolls need to be opened (University of**Ark~sas,** 2013). Table 4 shows the number of application ofthidiazuron by state and the number ofacres treated with each application. Louisiana, Missouri, and Californiamay use up to three applications ofthidiazuron 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 of0.3 lb ail acre/year. Ifall labels limitthe maximum number ofapplications to two then cotton growers that use three applications may be impacted. 

### T bl 4 M If I 

## r r

a e . u •P 

### e app 1ca ions·ofth"d" b t t 

### 1 1azuron 1y 

### s a e. Base Acres Acres Treated Acres Treated PercentAcres 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

Page4Droplet Size The droplet size effects the efficacy and offsite drift ofa pesticide application and the optimum droplet size for any herbicide orplant growth regulator is rtot something that canbe generalized and it should be selected based onthe data and knowledge. Deposition ofthe most effective amount ofactive ingredient onplant foliage and reduction ofoff-site drift are the major objectives ofselecting 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 ofthe pesticide and other factors such as dryingofthe chemical priorto actual uptake, and air pressure used in the application. In addition, the interaction of

·concentration and plantuptake, and the interaction ofthe active ingredient with other chemicals in a tank mixture are some ofthe other factors that should be considered on selection ofoptimum droplet size. BEADdoes not have information specific to the relationship ofdroplet size and efficacy inthidiazuron.

Page 5REFERENCES Market Research Data (2009 -2013). Data collected and sold by a private market research firm. Data collected onpesticide 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 ofArkansas, 2013. 2013 Mid-South cotton defoliation guide. University ofArkansas Cooperative Extension Service. Online: http://www.uaex.edu/publications/pdf/MP503.pdf

. Verified: February 12, 2016.

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