Comparative Efficacy of Diquat for Control of Two Members of the Hydrocharitaceae: Elodea and Hydrilla

The submersed plants hydrilla (Hydrilla verticillata (L.f.) Royle) and elodea (Elodea canadensis Rich.) are both members of the Hydrocharitaceae family and cause problems in waterways throughout the world. Diquat (6,7-dihydrodipyrido[1,2-α:2’,1’-c]pyrazinediium dibromide) is a contact herbicide used to control nuisance submersed and floating aquatic macrophytes. There is no readily available information in the literature on the control of elodea under various diquat concentration and exposure times (CET) and other than a study by Van et. al 1987, little on hydrilla. Since CET relationships are critical in controlling submersed plants in areas influenced by water exchange, this study was designed to evaluate the efficacy of diquat on hydrilla and elodea under various CET scenarios. (PDF has 3 pages.

Plant Manage

ABSTRACT The contact aquatic herbicide, diquat (6,7-dihydrodipyrido[1,2-α :2',1'-c]pyrazinediium ion) was evaluated under simulated flowing water conditions in an outdoor mesocosm facility for efficacy on five submersed aquatic plants: hydrilla ( Hydrilla verticillata (L.f. Royle), Eurasian watermilfoil ( Myriophyllum spicatum L.), sago pondweed ( Stuckenia pectinata (L.) Boerner) , American pondweed ( Potamogeton nodosus Poiret) , and egeria ( Egeria densa Planchon). Diquat was applied at concentrations of 0.37 mg/L ai and 0.185 mg/L ai (cation) under flow-through conditions to provide theoretical 3 and 6 hr herbicide half-lives that produced observed herbicide half-lives of 2.5 and 4.5 hr, respectively. An additional treatment included 0.37 mg/L ai applied under static conditions (no water exchange). Results showed that diquat applications significantly inhibited shoot biomass production from 42 to 100 percent at all application concentrations and exposure times for all species, except hydrilla. Diquat resulted in no measurable control of hydrilla, except under static conditions. Results suggest that Eurasian watermilfoil, egeria, and sago pondweed are highly susceptible to diquat even in areas where herbicide dilution may occur in less than three hours

Aquatic dissipation of the herbicide triclopyr in Lake Minnetonka, Minnesota

A study of the aquatic fate of the triethylamine salt of triclopyr (3,5,6-trichloro-2- pyridinyloxyacetic acid) was conducted in three bays of Lake Minnetonka, Minnesota. Triclopyr is under review by the US Environmental Protection Agency as a selective aquatic herbicide. The primary purpose of this study was to determine dissipation rates of the parent active ingredient, triclopyr, and its major metabolites, 3,5,6-trichloropyridinol (TCP) and 3,5,6-trichloro-2-methoxypyridine (TMP) in selected matrices including water, sediment, plants, finfish and shellfish. Two 6.5- ha plots dominated by the weedy species Eurasian watermilfoil (Myriophyllum spicatum L) were treated with triclopyr-triethylammonum at a rate of 2.5mg AE liter ÿ1 (2.5ppm) on 21±23 June 1994. A third 6.5-ha plot was established as an untreated reference. Water and sediment samples were collected from within the plots and at selected locations up to 1600m outside of the plots through six weeks post-treatment for chemical residue analysis. In addition, residue samples were collected from the target and non-target plants and other non-target matrices, including game and rough fish, clams and crayfish. All test animals were sequestered in cages located in the center of each plot and samples were collected through four weeks post-treatment. Half-lives for dissipation of triclopyr and TCP in water ranged from 3.7 to 4.7 days and from 4.2 to 7.9 days, respectively, with trace amounts of TMP found. Peak triclopyr sediment values ranged from 257 to 335ng gram ÿ1, with a mean half-life of 5.4 days, while peak TCP sediment levels ranged from 27 to 65ng gram ÿ1 (mean halfÿlife=11.0 days). Trace levels of TMP were detected at one treatment site at one sampling event. Triclopyr and TCP accumulated and cleared from animal tissues proportionately to concentrations in the water (triclopyr dissipation half-lives \u3c11 days, TCP\u3c14 days). TMP levels were two to three times higher than those of the other compounds, particularly in visceral tissue. In all cases, residues of these compounds were higher in the inedible portions of the animals, and were usually higher in bottom-feeding fish species