HYDRILLA TUBER FORMATION IN RESPONSE TO SINGLE AND SEQUENTIAL BENSULFURON METHYL EXPOSURES AT DIFFERENT TIMES

Hydrilla (Hydrilla verticillata (L.f.) Royle) grown in outdoor tanks w as exposed to bensulfuron methyl concentrations of 25, 50, or 100 ppb on June 16, August 20 or October 15, 50 ppb June 16 and August 20, or 25 ppb on June 16, July 21, August 20, and October 15, 1990, with a 35 -day contact time. Hydrilla was also exposed to the compound on August 9, 1991 at concentrations of 10, 20, 30, 40, or 50 ppb. In 1990, the August 20 exposure resulted in the greatest inhibition of tuber produc tion for a single application. Exposure in June caused hydrilla to pro duce at least twice as many tubers as unexposed plants by April 10, 19 91. Exposure in October arrested tuber production, which had already b egun. Exposure in June and August delayed tuber formation until after February 9, 1991. Exposure in June, July, August, and October inhibite d tuber formation for the entire growing season. Hydrilla treated with all concentrations of bensulfuron methyl on August 9, 1991 produced t ubers only sporadically through March 16, 1992, compared to unexposed hydrilla, which produced an average of 48 tubers/531 sq cm by January 4, 1992. With the onset of warmer weather after March 16, tubers produ ced by unexposed hydrilla more than doubled, and comparable numbers of new tubers were produced by plants that were exposed to 10 or 20 ppb. Tuber production was inhibited for the entire growing season by expos ure to 50 ppb on August 9, 1991. In spite of the promise that bensulfu ron methyl showed for use in aquatic plant management, the Experimenta l Use Permit was not renewed in 1992 and efforts to register the compo und were discontinued.