Aquatic Plant Control Research Program

Susan L. Sprecher, Kurt D. Getsinger, Jan Sharp

ABSTRACT

The contact-type membrane -active herbicide endothall (7 -oxabicyclo(2.2.1)heptane -2,3-dicarboxylic acid; C8H10O5) has been widely used in aquatic sites for nearly 50 years. (The herbicide is also known as endothal. The term 3,6 -endoxohexahydrophthalic acid has been applied to endothall but is not regarded as the proper nomenclature.) For use as an aquatic herbicide, the free organic diacid endothall is formulated as two of its salts and provided as water -based concentrates and dry granular materials or granules. The currently available aquatic endothall formulations are registered for application to water by the U.S. Environmental Protection Agency (USEPA). The inorganic dipotassium salt is the active ingredient of the Aquathol K herbicides, used predominantly for lake and static water treatments to control aquatic macrophytes. They are available as Aquathol K Aquatic Herbicide, Aquathol K Granular Aquatic Herbicide, and, since 1998, as the Aquathol Super K Granular Aquatic Herbicide, a concentrated granule formulated in the Culigel super absorbent polymer. The mono (NN-dimethylalkylamine) endothall salt is registered as the liquid Hydrothol 191 Aquatic Algicide and Herbicide, and as Hydrothol 191 Granular Aquatic Algicide and Herbicide. Hydrothol products are used predominantly for canal treatments to control algae and submerged macrophytes.

This body of research, often undertaken in cooperation with other Federal and state agencies, universities, and industry, has provided valuable information on the ability of this herbicide to control and mange nuisance aquatic vegetation under static and flowing water conditions and on the fate of endothall in the aquatic environment. Data generated in some of these studies have been used by industry to fulfill requirements for U.S. and state registration or reregistration of specific herbicide formulations.

The objective of this report is to provide a review and summary of CE studies on the aquatic uses of endothall over the past three decades. This summary includes a discussion on the efficacy of endothall against invasive weeds, as well as selected nontarget plants, and how efficacy is based upon application rates and techniques, water exchange characteristics, and herbicide exposure time mechanisms. Information from growth chamber, mesocosm, and field studies will be provided to link effective plant control to endothall concentration/exposure time relationships. In addition, the degradation and dissipation of endothall under field conditions and how those processes relate to selective plant control and potential impacts to the aquatic environment are reviewed.

Numerous studies and evaluations over two decades have confirmed that endothall is a highly effective and environmentally safe tool for management of submersed aquatic vegetation. Furthermore, it is effective with relatively short contact times, and it is rapidly and completely decomposed in aqueous systems without accumulation or formation of toxic intermediate compounds. Manipulation of endothall dosage rates for effective control of hydrilla and Eurasian watermilfoil is well understood. The CE has identified those minimal rates of the dipotassium salt as low as 0.5 mg ae L -1 for 48 hr for Eurasian watermilfoil and 48 -hr exposure to 2.0 mg ae L -1 for hydrilla that can provide cost -effective and environmentally sound control. Minimal effective dosage rates are becoming better known for locally important weed species, such as sago pondweed and curlyleaf pondweed. Application techniques suited to the various herbicide formulations are available to provide control under a wide range of water exchange conditions, including canals, rivers, lakes, and reservoirs.

Recent findings on the ability of endothall to control target plants selectively, through manipulation of dosage rate or application timing, show that this herbicide also has a role in the restoration of aquatic ecosystems that have been degraded by invasions of nuisance weed species. The suitability of endothall as a tool for aquatic vegetation management is firmly established.