UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON, D.C. 20460 

MAY 31 2012 

MEMORANDUM 

OFFICE OF CHEMICAL SAFETY 

AND POLLUTION PREVENTION 

SUBJECT: BEAD Chemical Profile for Registration Review: Methanearsonic acid, salts 

(MSMA) (013803) 

FROM: Bill Phillips, II, PhD AgrOfJ(Jmist~~---

THRU: 

:::::Y;:v:::w 

Science Information and Analysis 

Protection 

Branch 

Speciwist / if / ~ 

John Faulkner, Economist~ Economic Analysis Branch 

Biological and Economic ysis Division (7503P) 

Arnet Jones, Chief 

Biological Analysis Branch 

Diann Sims, Chief ~~' l...A-~~'V"""'­ Science Information and Analy . 

Timothy Kiely, Chief 

Economic Analysis Branch 

Biological and Economic Analysis 

TO: Tom Myers, Chemical Review Manager 

Risk Management and Implementation Branch 2 

Pesticide Re-evaluation Division (7508P) 

Product Review Panel date: May 16, 2012 

INTRODUCTION 

The purpose of this document is to convey usage information and a broad overview of the pest 

management roles of pesticides that are beginning to undergo registration review. The 

anticipated audience consists of the team of EPA (Agency) staff currently evaluating the 

1 
registration status of methane arsonic acid, salts (MSMA), and eventually the general public. This 

document builds upon the data already provided in the "Usage and Label Use Data Packages" 

that are also provided to Registration Review teams by BEAD. The document is based on 

information and data available to BEAD as of May 4, 2012. 

MSMA is an organic arsenical (OA) herbicide that is a broad-spectrum herbicide used to 

selectively control grasses and broadleaf weeds in cotton and turfgrass. Applications are made 

postemergence and post-directed to the weed and crop in cotton and are applied in turf 

postemergence to the target weed. This herbicide is classified by the Weed Science Society of 

America in the mechanism of action group 17; which is a class of potential nucleic acid 

inhibitors or non-de script mechanism of action herbicides. The mechanism of action by which 

MSMA controls weeds is unknown; however it is currently thought to be via cell membrane 

disruption and subsequent destruction. 

Currently there is registration follow up and evaluation work being conducted by the Agency on 

the MSMA registrations. Part of this work stems from the 2006 reregistration eligibility decision 

(RED) which found that none of the OA herbicides (CAMA, cacodylic acid, DSMA and 

MSMA)I were eligible for reregistration. In addition to the 2006 work, a post-RED (2009) 

follow up with the registrants resulted in an "agreement in principle" where CAMA, cacodylic 

acid, DSMA were voluntarily canceled and most of the uses ofMSMA were either voluntarily 

cancelled or scheduled for voluntary phased out. 2 It was determined that the use in cotton was 

one use with such high benefits that this single use of MSMA was determined to be eligible for 

reregistration.3 In addition, there were several turfgrass uses that remain labeled but are 

currently part of the phase-out program. Those uses which remain have had significant 

restrictions placed upon them (e.g., buffers, number of applications, spot treatments, etc.). 

The uses that are scheduled to be phased out, as a part of the agreement in principle, were 

divided into two phases. Phase one included the following uses that were removed in 2009, 

forestry, all fruit and nut crops, drainage ditch banks, grass seed production, non-crop areas, and 

residential lawns. Phase two uses were golf course, highway rights-of-way, and sod farms which 

are scheduled to be removed as of December 31, 2013, but as a condition of the continued use of 

MSMA in these sites, restrictions were placed on these uses to cover the period between 2009 

thru 2013 (e.g., buffers, spot treatments, etc.). As a part of phase two the Agency agreed to 

review any benefits information that would be submitted before December 31, 2011. If after the 

Agency's review of this information it is determined that the benefits are considered significant, 

with respect to human and environmental risks, these use sites would not be phased out; 

however, additional restrictions may be placed on the continued use. 

1 monosodium methanearsonate (MSMA), disodium methanearsonate (DSMA), calcium acid methanearsonate (CAMA), and 

cacodylic acid and its sodium salt. 

2 http://www.epagov/oppsrrdl/reregistration!organic_arsenicalsJs.html#msma 

3 PhiHips, B., D. Berwald, and E. Rim, 2006. Alternatives Assessment of the Organic Arsenical Herbicides Used in Residential 

and Golf Course Turfgrass, and Cotton (DP Barcode: 309117), Biological and Economic Analysis Division, U.S. EPA, April 12, 

2006, 27pp. 

2 
USE SITES 

Prior to the 2006 RED cancelations and the subsequent 2009 agreement in principle, the 

agricultural use sites for MSMA included citrus fruits, nut crops, pome fruits, stone fruits, cotton 

vineyards, and non-bearing fruit trees. In addition, there were non-agricultural use sites 

registered that included forest lands, omamentallawns and turf, athletic fields, parks, golf 

courses and storage yards. All of these uses still remain on many older labels as the existing 

stocks were allowed to be used. 

Current labeled uses include cotton and non-agricultural uses, golf course, highway rights-ofway, and sod farms. The non-agricultural uses are scheduled for cancellation in 2013. 

COMMON FORMULATIONS AND APPLICATION METHODS 

Methanearsonic acid, salts (MSMA) is formulated as emulsifiable concentrate, flowable 

concentrate, granular, and ready to use and can be applied as a foliar treatment, soil treatment 

and by aerial application. 

USAGE 

Agricultural Usage 

Based on private market usage data from 2000-2010, usage averaged approximately one million 

pounds active ingredient (lbs a.i.) for 850,000 acres treated. The average a.i. rate during this 

time period was approximately 1.14 pounds per acre. Between 2000 and 2005, the crops with 

the major use in terms of pounds a.i. applied were cotton (97%) and pasture (approximately 3%). 

The other crops that comprised less than one percent of usage included citrus (oranges and 

lemons), nuts (pistachios, walnuts and pecans) and apples. Between 2006-2010, cotton (89%) 

and pastureland (10%) still accounted for the major use in terms of pounds a.i. applied, while the 

remaining one percent of usage included citrus, nuts (pecans and almonds), wine grapes, and 

apples. Figure 1 presents usage of MSMA on cotton in pounds a.i. applied and total area treated, 

over the 2000-2010 time period. 

Because there has not been sufficient time between the 2009 agreement in principle and present 

cropping years, plus the continued use of existing stocks of the herbicide, there are insufficient 

data to quantitatively evaluate the use ofMSMA since this time. However, a cursory view of the 

data in Figure 1, demonstrates that there does not appear to be a dramatic change in the total area 

treated (TAT) and lbs a.i. ofMSMA between 2009 and 2010. As stated above, other than the 

cotton use data there does not appear to be a sizeable use in this herbicide. However, looking at 

the period between 2007 thru 2010 the TAT use over this period has increased in both cotton and 

the consolidated data but the actual amount of the a.i. applied for the same time period has begun 

to level off for the consolidated data (post 2009) and remains increasing for the cotton data 

3 
r' 

FIGURE 1. MSMA Use on Cotton in Pounds A.I. Applied and Total Area Treated in Acres 

(2000-2010 

2500000 ,...-----------

o 

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 

Years 

Source: Proprietary Data, 2000-2010 

TAT 

-a-lbsAI 

While the Agency was compiling infonnation regarding the use of MSMA 4, researchers in the 

state of Georgia, in 2005, confinned that there was a case of glyphosate resistant Palmer 

amaranth (Amaranthus palmeri) (GrAMAPA5

) established in a few counties in the state.6 Since 

that time, the occurrence of this resistant weed biotype has dramatically increased and spread; 

the results of which has lead some farmers to destroy entire crop stands in order to try to contain 

and eventually stop the spread of this problem. Quickly following the confirmation of resistance, 

weed control research provided evidence that the primary herbicide that controls the GrAMAP A 

was MSMA. Given the few herbicides, compared to other crops and the fact that this is a 

broadleaf weed in a broadleaf crop makes the choices that farmers have very limited. 

4 Phillips, B., D. Berwald, and E. Rim, 2006. Alternatives Assessment of the Organic Arsenical Herbicides Used in 

Residential and Golf Course Turfgrass, and Cotton (DP Barcode: 309117), Biological and Economic Analysis 

Division, U.S. EPA, April 12, 2006, 27 pp. 

5 AMAP A is the official WSSA computer code for Amaranthus palmeri. "Gr" is the prefix to the WSSA code that 

is used only in this document to denote the glyphosate resistant biotype of this particular genus and species. 

6 A. Stanley Culpepper, Timothy L. Grey, William K. Vencill, Jeremy M. Kichler, Theodore M. Webster, Steve M. 

Brown, Alan C. York, Jerry W. Davis, and Wayne W. Hanna. 2006. Glyphosate-resistant Palmer amaranth 

(Amaranthus palmeri) confinned in Georgia. Weed Science 54(4):620-626. Weed Sci. Soc. of America 

4 
r' 

This lack of tools and the increase in the amount of the a.i. applied between 2007 and 2010 

coincides with the 2005 discovery and subsequent spread of the glyphosate resistant biotype of 

GrAMAP A, as well as the subsequent research that demonstrated the high efficacy of MSMA on 

GrAMAP A. This is indicative of the need to increase the application rate of an he:r:bicide given 

control problems with difficult-to-control weed species. Table 1 demonstrates the high use of 

MSMA in the state of Georgia for the period of2006 to 2010. From this very limited use data it 

would seem that the use of MSMA will at least remain consistent, if not increase, in the near 

future. 

Over the years MSMA use in cotton has declined dramatically and was on its way to being 

relegated to the occasional use. Due to the establishment of this pernicious weed biotype in 

Georgia, and now in other cotton producing states (e.g., North Carolina), coupled with the early 

research conducted at the University of Georgia, MSMA has again been adopted as what may 

soon be a commonly used herbicide throughout the cotton growing region (Figure 2). However, 

even with the potential increase in the use of MSMA, it is doubtful that the lbs a.i. and the TAT 

of this herbicide will return to the level of use of2000. 

Table 1 compares the states with the most agricultural usage between the years of 2000 and 2005 

and the years between 2006 and 2010. The state with. the most pounds a.i. applied in both 

timeframes was Georgia (21 % oftotallbs applied in 2000-2005, and 59% oftotallbs applied in 

2006-2010), where both pounds applied and acres treated increased in the state during the 2006-

2010 timeframe. This timeframe coincides with the establishment of GrAMAP A. The "other" 

states accounting for approximately 13% of usage in the 2000-2005 timeframe included South 

Carolina, California, Tennessee, Missouri, Virginia, and Florida. In the 2006-2010 timeframe, 

the "other states" accounting for approximately 9% of usage included California, Arkansas, 

Louisiana, Arizona, Oklahoma, and South Dakota. While use in other cotton producing states 

have not increased, and in some cases have decreased, the current trend of increasing 

confIrmation of cases of GrAMAP A in many of these states where cotton is produced will lead 

to an increase in use given that some of these states have cropping systems where MSMA will be 

a good fit. 

5 
TABLE 1. Comparison of States with Highest Use of MSMA- Percent of Total Lbs. 

Applied Between the 2000-2005 and the 2006-2010 Timeframes 

2000-2005 

Average = 1.27 million lbs/year 

State %Lbs 

Georgia 21 

Mississippi 18 

N.Carolina 13 

Arkansas 12 

Louisiana 10 

Alabama 7 

Texas 6 

Other 13 

Source: Proprietary Data, 2000-2010 

Non-Agricultural Usage 

2006-2010 

Average = 621,000 lbs/year 

State %Lbs 

Georgia 59 

N. Carolina 12 

Mississippi 7 

Alabama 6 

S. Carolina 4 

Texas 3 

Other 9 

In tenns of non-agricultural usage, data are available only for use prior to the restrictions and 

cancellations imposed as a result of the 2009 RED and the 2006 AGREEMENT IN PRINCIPLE. 

Some of the historic data is available for sites that are no longer on the MSMA label (e.g., 

nursery/greenhouse) (Table 2). Site such as lawn care operators (LCO's) and golf courses, landscape, 

nursery/greenhouse, and turffanns will continue to be available for at least until 2013. The data 

suggest that prior to the 2009 label changes the lbs a.i. for non-agricultural usage surpassed the 

agricultural use. Data for sites where MSMA has been cancelled will continue to show use while 

existing stocks are exhausted. 

Table 2. MSMA Select Non-Agricultural Usage (Pounds A.I.) (20 02-2006) 

Site 2002 2004 2006 

Golf Courses 482,284 206,203 311,537 

Institutional Turf 67,691 299,950 211,700 

Landscape 15,123 37,461 306,069 

Lawn Care Operators 299,875 104,862 94,420 

TurfFanns 375,690 186,636 182,348 

Nursery/Greenhouse N/A 43,104 N/A 

TOTAL Reported (lbs. a.i.) 1,240,633 878,216 1,106,074 

N/A=not avruJable 

Source: Kline & Co., 2002, 2004, and 2006 

6 
REFERENCES 

Kline & Co., 2002, 2004, and 2006 

Proprietary Data, 2000-2010 

Senseman, S.A. Herbicide Handbook, Ninth Edition. 2007. Weed Sci. Soc. Of America. 

Lawrence, Kansas 66044-8897 

USDA, 2006-2010. NASS Crop Reporting Districts. Online: 

http://www.ers.usda.govibriefing/arms/resourceregions/resourceregions.htm#nass. 

USDA, 2007. Census of Agriculture. Online: 

http://www .agcensus. usda.gov !Publications/2007 /index.asp 

7 
Appendix 

Provided for the Environmental Fate and Effect Division Risk Assessment: 

MSMA Usage by Crop Reporting District (2006-2010) 

Another measure of usage is the use intensity. In this analysis, the use intensity is 

expressed as the pounds a.i. applied per acre of land in farms. This differs from the 

application rate, which is expressed as the pounds a.i. applied per treated acre. Figure 2 

spatially represents MSMA use intensity over the period of 2006 thru 2010 in the US 

with use in Georgia showing the highest intensity compared to other states where 

MSMA is used. Again, this information supports the current use and increases in the 

amount ofMSMA, with respect to lbs a.i. applied, in Georgia where the GrAMAPA 

issues have arisen. 

FIGURE 2. MSMA Usage by Crop Reporting District (2006-2010) 

Average Pounds Per 

1,000 Acres of Farmland 

. 0-3 

4-10 

D 11-24 

25- 55 

58 - lOB 

8 
I 

Sources: 

Proprietary Data. 2006-2010 

USDA, 2006-2010. NASS Crop Reporting Districts. 

USDA, 2007. Census of Agriculture. 

This is a map of agricultural pesticide usage at the Crop Reporting District (CRD) level. CRDs are 

boundaries created by USDA NASS which are aggregates of counties (USDA, 2010). Pesticide usage is 

displayed as average pounds (for the years 2006-2010) per 1,000 acres offarmland in a CRD to normalize for 

the variation in farmland between CRDs. Farmland acreage was obtained from USDA (2007). 

Usage is based on private market surveys of pesticide use in agriculture (Proprietary Data, 2006-2010). Th~ 

survey data are limited to the states that represent the top 80-90% of acreage for the individual crops, 

therefore, use may be occurring in regions outside the scope of the survey. CRDs showing no usage of 

pesticides may be due to either the lack of pesticide use in the region or non-participation in the agricultural 

surveys. In addition, across the years, there may be variations in the specific crops included in the CRD 

survey. This may result in a lower annual average for the CRD. 

9