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US EPA Conditional Registration of Isoxaflutole September 1998


Conditional Registration of Isoxaflutole

September 1998

1. What are the characteristics of isoxaflutole?
2. What are the environmental risks associated with isoxaflutole?
3. Does isoxaflutole pose any risks to human health?
4. What restrictions have been placed on isoxaflutole to minimize
phytotoxicity risks?
5. Which states are included in this conditional registration?
6. Why can isoxaflutole be used in some geographical areas and not in
others?
7. Can a state prohibit the use of isoxaflutole even if EPA registers
it for use in that state?
8. Where can I get more information about Isoxaflutole?


EPA has approved a three-year, geographically restricted registration for isoxaflutole use on field corn under Section 3 of the Federal Insecticide, Fungicide and Rodenticide Act (FIFRA). EPA has concluded that the use of isoxaflutole on field corn meets the legal standard of a reasonable certainty of no harm. However, to address potential ecological impacts from isoxaflutole use in certain areas, EPA has approved a time-limited registration with stringent use restrictions. EPA's decision is discussed below.

1. What are the characteristics of isoxaflutole?

Isoxaflutole is the first member of a new structural class of herbicides called the isoxazoles. It is the active ingredient in the product BALANCE, a soil-applied herbicide that is sprayed before field corn starts growing in a field. Isoxaflutole works by preventing the biosynthesis of carotenoid pigments, which protect a plant's chlorophyll from decomposition by sunlight. Without carotenoid pigments, chlorophyll pigments are damaged by the sun, and the plant eventually dies.

Isoxaflutole may be used instead of atrazine for pre-emergence weed control in corn with comparable efficacy. When substituted for atrazine in tank mixtures with other major corn herbicides, such as metolachlor, alachlor, acetochlor, dimethenamid and pendimethalin, the mixtures will be used at lower rates.

2. What are the environmental risks associated with isoxaflutole?

Isoxaflutole is mobile (that is, under certain conditions, it can easily travel from the application site to other areas) and degrades fairly rapidly. However, isoxaflutole's primary breakdown product is also mobile and breaks down more slowly than isoxaflutole itself, and may remain toxic to plants ("phytotoxic") at levels below which currently available test methods can measure (down to the parts per trillion level). Given this high level of phytotoxicity and mobility, EPA is particularly concerned about the risks posed to groundwater and surface water, and the potential impact on off-target plants and crops. Therefore, the Agency is requiring the manufacturer, Rhone-Poulenc, to conduct a number of field studies designed to characterize risks of leaching to groundwater, run-off to surface waters, and damage to non-target plants. If these studies indicate that use of isoxaflutole poses an unreasonable risk to human health and the environment, appropriate action will be taken. In addition, Rhone-Poulenc has agreed to provide analyses of up to 5,000 water samples per year from the states.

3. Does isoxaflutole pose any risks to human health?

EPA has concluded that use of isoxaflutole on field corn meets the FQPA standard of a "reasonable certainty of no harm" with respect to human health. The carcinogenic risk is estimated to be 9.3 x 10-8, which is below EPA's established level of concern for life-time cancer risk. EPA assessed the acute and chronic aggregate exposure levels and corresponding potential risk and concluded that residues of isoxaflutole and its metabolites in the diet will not exceed EPA's level of concern with respect to the aggregate acute or chronic human health risk associated with the proposed use on field corn. Field corn is used primarily for animal feed, not for human consumption.

4. What restrictions have been placed on isoxaflutole to minimize phytotoxicity risks?

BALANCE has been categorized as a "restricted use pesticide," which means that only trained, certified applicators (or people under the direct supervision of trained, certified applicators) may apply the product. In addition, isoxaflutole may not be applied from the air, which will help prevent plant damage resulting from spray drift (although this is still possible even with ground application methods). The label prohibits using the product on porous soils, including sandy soils, loamy sand soils, and sandy loam soils where the water table (i.e., level of saturation) is less than 25 feet from the surface and the organic matter is less than 2%.

5. Which states are included in this conditional registration?

The states in which isoxaflutole use is permitted are Arkansas, Illinois, Indiana, Iowa, Kansas, Kentucky, Missouri, Montana, Nebraska, North Dakota, Ohio, Oklahoma, South Dakota, Tennessee, Texas (panhandle region north of Interstate-20), and Wyoming. The registration will terminate on November 1, 2001. Prior to expiration, the Agency will re-evaluate the use of this compound based on information received from the field, as well as registrant-conducted studies, to determine if renewal of this registration is appropriate.

6. Why can isoxaflutole be used in some geographical areas and not in others?

Because isoxaflutole and, especially, its primary degradate (RPA-202248) are highly phytotoxic and mobile, EPA is concerned that use of the chemical in certain areas could contaminate groundwater and surface water, which in turn would pose potential risk to non-target plants or crops exposed to contaminated water.

7. Can a state prohibit the use of isoxaflutole even if EPA registers it for use in that state?

FIFRA allows states to make their own registration decisions and to be more restrictive than the Federal label.

8. Where can I get more information about Isoxaflutole?

An EPA-developed Pesticide Fact Sheet on isoxaflutole is also available for distribution. For a copy of this more technical Fact Sheet contact:

Communication Services
Office of Pesticide Programs
U.S. EPA
401 M Street, SW
Washington, DC 20460
(703) 305-5017