The results of 116 aquatic weed control experiments using 10 herbicides and their combination are discussed. Included are data for pre-emergent soil application during winter draw-down, pre-emergent total pond treatments and post-emergent applications. Pre-emergent soil application during winter drawdown was comprised of 31 experiments, testing varying concentrations of eight different soil sterilants for the control of rooted aquatic weeds. D.M.A. (disodium monomethylarsonate) showed promise as a pre-emergent control of southern watergrass (Hydrochloa carolinensis). Also, fenac (2,3,6 trichloro phenyl acetic acid) was effective in clearing fishing areas in ponds heavily infested with a variety of aquatic plants. Both liquid and granules were used at concentrations of 10 Ibs. and 20 Ibs, active ingredients per acre. Best results were obtained when even distribution of the herbicides was accomplished. Other soil sterilants tested gave varying degrees of control but were not considered effective enough to be recommended. Fourteen pre-emergent total pond treatments indicated that simazine (2-chloro-4, 6-bis (ethylamino) -s-triazine), atrazine (2-chloro-4ethylamino- 6-isopropylamino-s-triazine) and prometryne (2,4-bis (isopropylamino) -6-methylthio-s-triazine) effectively inhibited the development of filamentous algae at concentrations of .5 and 1.0 ppm. However, in each case the development of phytoplankton blooms were adversely affected for several weeks. Each herbicide was applied by broadcasting the material mixed with water over the surface of the pond. The more soluble atrazine and prometryne appeared to be better adapted to this simple application technique than did simazine. Post-emergent applications included the results of 51 field experiments using six herbicides and their combinations. Diquat(1:lethylene2:2-' dipyridylium dibromide» and paraquat (1:1-ethylene4,4'-dipyridylimn dichloride) were found to be effective non-selective herbicides at concentrations ranging from .20 to 10.0 ppm. Duckweed was found to be the most susceptible plant to these herbicides, while members of the lily family appeared to have the greatest tolerance. Spot applications indicated that these contact herbicides are suitable for clearing fishing areas of certain species of aquatic vegetation within a two-week period. The triazine herbicides simazine (1), atrazine (2) and prometryne (3) effectively controlled filamentous algae and showed promise for controlling several species of vascular plants. At concentrations used for filamentous algae control, virtually all phytoplankton was removed and did not develop properly for varying periods of time, depending on the concentrations used and the amount of water exchange. Two tests using atrazine at .08 and .16 ppm satisfactorily removed heavy Microcrysti.3 blooms which were immediately replaced by more desirable plankton algae.
