https://doi.org/10.1016/j.aquatox.2026.107901

Mitrovic, D., Bragg, L. M., Servos, M. R., & Wilkie, M. P. (2026). Greater than additive interactions between two phenolic pesticides used to control invasive sea lamprey (Petromyzon marinus): Underlying mechanisms and implications. Aquatic Toxicology, 298, Article 107901. https://doi.org/10.1016/j.aquatox.2026.107901

The pesticide TFM (3-trifluoromethyl-4′-nitrophenol) is applied to rivers and streams that drain into the Laurentian Great Lakes of North America, the nearby Finger Lakes, New York, and Lake Champlain, (New York/Vermont) to selectively target the larval sea lamprey (Petromyzon marinus). When used in combination with niclosamide at 0.5–2.0%, also a molluscicide and anthelmintic, the TFM concentration required to eliminate 99.9% of the larval sea lamprey (MLC – minimal lethal concentration) is reduced by up to 40%. The concentration addition model was used to determine the nature of the interactions between TFM and niclosamide using the toxic unit (TU) concept, where 1 TU is defined as the 12-h LC50 for each chemical. When larval sea lamprey were exposed to TFM mixed with niclosamide (TFM/1% Nic), at greater than 1 TU of total exposure (> 1 TUTotal Exp) observed mortality (%) was greater than 50%, indicating synergistic interactions. However, when combined TFM/1% Nic concentrations were below 1 TUTotal Exp, the interactions were antagonistic, suggesting that TFM/1% Nic mixture interactions are concentration dependent. Greater accumulation of TFM in muscle at > 1TUTotal Exp, but not in the liver, suggests possible competition between TFM and niclosamide for detoxification enzymes in the liver. The concentrations at which TFM and niclosamide interacted synergistically approximated those typically used in field lampricide treatments in waters of similar pH and alkalinity, shedding new insights into why niclosamide enhances TFM toxicity in larval sea lamprey.

This work was funded by a Great Lakes Fishery Commission research contract (Contract 2018-WIL-54,074) to M.P.W., NSERC Discovery (RGPIN2017–03,816) to M.R.S. and Global Water Futures Observatories to M.R.S.
Additional data is included in the supplementary material file or available upon request.

https://doi-org.cyber.usask.ca/10.1016/j.aquatox.2026.107901