Laboratory and field comparisons of TFM bar formulations used to treat small streams for larval sea lamprey

A solid formulation of the pesticide TFM (4-nitro-3-(trifluoromethyl)-phenol) was developed in the 1980s for application in small tributaries during treatments to control invasive sea lamprey (Petromyzon marinus Linnaeus, 1758). Several initial inert ingredients were discontinued and substituted, culminating with an interim formulation that unacceptably softens and rapidly decays in warm conditions. A new TFM bar formulation was developed to resolve poor thermal stability and it was registered with the U.S. Environmental Protection Agency and Health Canada Pesticide Management Regulatory Agency in 2020. Laboratory studies compared the thermostability and dissolution (i.e., TFM release) of the interim and new formulation of TFM bars that were held at 20 C or 45 C for 24 hours prior to evaluation. Field tests compared the dissolution of the interim and new formulation of TFM bars when applied in three small tributaries in Michigan. Laboratory tests show that the new formulation bars remain usable when held at 45 C for 24 hours; whereas, the interim formulation bars partially liquify and are not usable. Field tests indicate the new formulation bars have superior characteristics including a near consistent release of TFM for 1013 hours when applied in waters with a velocity of < 0.06 m/sec. A near consistent release of TFM was observed for a maximum of about 6 hours in one field application of the interim formulation bars. Water temperature and water velocity influenced both formulations; however, the greatest effects were observed with interim formulation bars where higher initial TFM concentrations were followed by precipitous TFM concentration decreases in tributaries with the highest water temperature or velocity. Field treatment applications will provide data for refining application parameters such as the number of bars required per unit discharge at various water temperatures and the acceptable water velocity range for applications.