UNITED STATES ENVIRONMENT AL PROTECTION AGENCY 

WASHINGTON D.C., 20460 

JUN 18 2015 

MEMORANDUM 

OFFICI:: OF CHEMICAL SAFETY 

AND POLLUTION PREVENTION 

SUBJECT: Isoxaben Qualitative Benefits Assessment (PC Code 125 851) 

FROM: 

THRU: 

Leonard Y ourman, Plant Pathologist ~ Biological Analysis Branch f . / z 

B1 ·11ch· ism, s· emor . B" 10 1 . og1st ~·~V//Ct /~ ~ >--- Biological Analysis Branch 

Derek Berwald, Economist 

Economic Analysis Branch 

Biological and Economic Analysis Division (7503P) 

Arnet Jones, Chief 

Biological Analysis Branc 

TO: Christina Scheltema, Chemical Review Manager 

Risk Management and Implementation Branch III 

Pesticide Reevaluation Division (7505P) 

BEAD Product Review Panel date: June 13, 2015 

SUMMARY 

Isoxaben is undergoing registration review. The Biological and Economic Analysis Division 

(BEAD) conducted a qualitative assessment of the benefits of isoxaben. Isoxaben is a 

benzamide chemical (WSSA Group 21) that inhibits cell wall biosynthesis. Isoxaben is the only 

herbicide ofbenzamide chemistry (Group 21). The largest use of isoxaben is to manage preemergent broadleaf weeds in ornamental nurseries and on golf course turf although there is use 

for managing pre-emergent broadleaf weeds in bearing and non-bearing fruit and nut orchards 

and vineyards. Much of the orchard and vineyard use is in California. It appears to be 

particularly effective for difficult to control nursery weeds such as bittercress, common 

groundsel, creeping woodsorrel, pearlwort, and spurge and appear to be effective for up to 18 
months in container plantings. Preemergence herbicides are also used to control annual weeds of 

turf. Isoxaben has been recommended on turf for managing black medic, cudweed, dandelion, 

henbit, knotweeds, and others. Isoxaben is primarily used on golf course roughs (it is not 

permitted to be used on golf course greens and tees). In orchards and vineyards, isoxaben is 

applied in the fall or winter and can provide 4-5 months of weed control. Isoxaben is effective 

on hairy fleabane, common mallow and marestail, which are important orchard weeds. 

BACKGROUND 

The Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) requires EPA to register 

pesticides before they can be distributed and used and to re-evaluate the registrations at least 

once every 15 years. FIFRA requires EPA to regulate the use of a pesticide to the extent 

necessary to prevent unreasonable adverse effects on the environment, including human health, 

where 'unreasonable' is defined as taking into account the economic, social, and environmental 

costs and benefits of the use of the pesticide. 

APPLICATION CONSIDERATIONS 

The primary concern for isoxaben use is risk to non-target plants, primarily from spray drift. 

Since spray drift after isoxaben application is a major consideration for risk boom height and 

droplet size are key factors to reduce the potential for drift. Isoxaben is absorbed by plant roots 

(very little is absorbed by leaves), which is optimally accomplished by a low boom height. 

Preemergence herbicides that are taken up by the plant roots should be applied with large 

droplets since they will roll off the leaves into the soil where they can be taken up by weed roots. 

Fine droplets can collect on leaf surfaces and never reach the soil or weed roots. Proper 

application and sprayer calibration of preemergence herbicides in orchards will maximize 

efficacy and reduce drift (Doll, 2013; Sumner, 2007). Preemergence herbicides often require 

0.5-1.0 inch of rain or irrigation for optimal incorporation. Some current labels indicate 

application of "medium to coarse" droplets. Mitigation of risk due to drift might be achieved 

through restrictions on boom height and droplet size. 

QUALITATIVE BENEFITS 

Golf Course Turf 

On turf, isoxaben is used for the preemergence control of a number of difficult to control annuals 

and perennial weeds (Yelverton, 2001). The unique mode of action and effectiveness of 

isoxaben may provide a particular benefit for the user in a weed control program. Isoxaben has 

been recommended for managing black medic, cudweed, dandelion, henbit, knotweeds, and 

others (e.g., McAfee and Baumann, 2012; Pfeiffer, 2011). It is primarily used on golf course 

roughs (the label prohibits its use on golf course greens and tees). Two other chemical groups 

can be used to target similar weeds. Alternative preemergence herbicides include Group 3 

herbicides dithiopyr, prodiamine, and pendimethalin (Reicher, 2013) and a Group 8 herbicide 

ethofumesate (McAfee and Baumann, 2012). 

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Nurseries 

Benefits of isoxaben are its effectiveness against difficult to control weeds and long-term weed 

control. For container-gro\\'.Il production isoxaben has been recommended for control of 

bittercress, common groundsel, creeping woodsorrel, pearlwort, and spurge (UC, 2009). It is 

considered an "excellent" material for broadleaf weed control and some weeds may be controlled 

for up to 18 months (UC, 2010). It is commonly combined with herbicides active against annual 

grasses. Disadvantages include possible injury to susceptible crops such as lilac, snapdragon, 

and Euonymus, and cost. There are limited chemistries used as preemergence herbicides in 

nurseries. Alternative preemergence herbicides that are used include oxyfluorfen (Group 14--

use only around conifers), oxadiazon (Group 14--residual activity of 12-16 weeks, although not 

very effective on pearlwort, ·chickweed, and horseweed), and oryzalin and pendimethalin (Group 

3-not highly effective on bittercress, fleabane, and groundsel) (UC, 2010). 

Orchards and Vineyards 

In orchards and vineyards isoxaben appears to be particularly effective for managing weeds that 

have been difficult to control with other chemical groups. Fleabane is difficult to control with 

the available preemergence herbicides registered for almonds and is susceptible to 

postemergence herbicides when weeds are small (UC Pest Management Guidelines, 2009). 

Applied in the fall or winter isoxaben can provide 4-5 months of weed control. Because 

isoxaben has a unique mode of action it a good rotational herbicide with other herbicide modes 

of action to manage weed resistance (Canevari et al., 2014). Isoxaben targets a wide range of 

weed pests (e.g., UC Pest Management Guidelines, 2012; UC Pest Management Guidelines, 

2014). Limited preemergence chemistries are available for orchards and vineyards. Alternatives 

that target a similar range of weeds include Group 14 herbicides flumioxazin and oxyfluorfen. 

Weeds such as hairy fleabane, common mallow and marestail are important orchard weeds 

effectively managed by isoxaben. For example, the prolific production of windborne hairy 

fleabane seeds allows this weed to quickly grow in orchard tree rows. Integrated weed 

management programs that include preemergence herbicides can be effective for orchard and 

vineyard cropping systems (Hanson, 2014). 

CONCLUSION 

Isoxaben provides a unique chemistry as a preemergence herbicide where there are limited 

modes of action available for controlling more problematic weeds. For difficult to control weeds 

of the three main use sites (nurseries, turf, and orchards/vineyards) extension information 

suggests that isoxaben is among the most effective. Isoxaben is optimally applied with larger 

droplets and low boom height to maximize exposure of the chemical into the root zone. This 

would have the advantage of reducing drift. 

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REFERENCES 

Canevari, M., Holtz, B., and Hanson, B. 2014. Dormant weed control in tree nut crops 2014. The 

Almond Doctor, Jan. 27, 2014. http://thealmonddoctor.com/2014/01/27/dormant-weedcontrol-tree-nut-crops-2014/ 

Doll, D. 2013. Pre-emergent herbicide use in perennial crop orchards. The Almond Doctor, Dec. 

14, 2013. http://thealmonddoctor.com/2013/12/14/pre-emergent-herbicide-use-perennialcrop-orchards/ 

Hanson, B. 2014. Preemergence herbicide concepts for orchard crops. The Almond Doctor. 

http://thealmonddoctor.com/2014/ 11 /24/preemergence-herbicide-concepts-orchard-crops/ 

McAfee, J. and Baumann, P.A. 2012. Herbicides for weed control in turfgrass. AgriLife 

Extension, Texas A&M University, Publ. SCS-2007-13. 

http:/ !publications. tamu.edu/TURF LANDSCAPE/PUB turf Herbicides%20for%20 We 

ed%20Control%20in%20Turfgrass.pdf 

Pfeiffer, M. 2011. Control of weeds in turf. Pesticide Training Resources, Arizona. 

http://ptrpest.com/pdf/turf weed. pdf 

Reicher, Z. 2013. Pre-emergent herbicides for golf turf. Golfdom, Jan. 6, 2013. 

http ://www.golfdom.com/pre-emergent-herbicides-for-golf-turf/ 

Sumner, P. E. 2007. Weed control sprayers. University of Georgia. 

http://www.ent. uga. edu/peach/peachhbk/pdf/weedcontro I. pdf 

UC. 2009. Special weed problems for container nurseries. Floriculture and Ornamental 

Nurseries, UC Pest Management Guidelines, University of California Extension. 

http://www.ipm.ucdavis.edu/PMG/r2807004 l l .html 

UC. 2010. Treatment for container nurseries. Floriculture and Ornamental Nurseries, UC Pest 

Management Guidelines, University of California Extension. 

http://www.ipm.ucdavis.edu/PMG/r280700511.html 

UC Pest Management Guidelines. 2014. Susceptibility of weeds to herbicide control-pistachio. 

University of California Extension. 

http://www.ipm.ucdavis.edu/PMG/r6057003 l 1.html#SPRING 

UC Pest Management Guidelines. 2012. Susceptibility of weeds in almond to herbicide control. 

University of California Extension. 

http://www.ipm.ucdavis.edu/PM G/r3 700411.html#WINTER 

UC Pest Management Guidelines. 2009. Integrated weed management-almond. University of 

California Extension. http://www.ipm.ucdavis.edu/PMG/r3700111.html 

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WSSA. 2014. Herbicide Handbook, Weed Science Society of America, Tenth Edition, 2014. 

Yelverton, F. 2001. Strategies for turfgrass weed control with preemergence herbicides. North 

Carolina State University. 

http://www.turffiles.ncsu.edu/PDFFiles/000071 /Preemergence Herbicide Strategies for 

Turfgrass Weeds.pdf 

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