Effects of the Aquatic Herbicide Diquat on Non-Target Aquatic Biota: A Mesocosm Study

Invasive aquatic plants threaten the health of aquatic ecosystems, and demand for chemical control is likely to increase as nuisance levels are reached. Diquat is a contact herbicide registered in many countries to control invasive aquatic plants. The objective of our study was to assess the effects of the aquatic herbicide diquat (Reward®) on North American native and non-native plants, algal communities, an amphipod and an amphibian using outdoor mesocosms to simulate natural systems. Our experimental design included a control and five nominal concentrations of diquat ranging from 100% (18.3 L/ha; 1153 µg/L) to 6.4% (1.2 L/ha; 74 µg/L) of the label rate of a single diquat application. Effects of diquat were found to vary among study organisms. All four plant species were negatively affected at all concentrations, exhibiting either mortality or severe reductions in dry biomass (< 1% the biomass of the controls). In contrast, phytoplankton biomass increased 7 d following diquat application concomitant with significant changes in algal community structure. A concentration-response relationship was observed for amphipod survival (LC₅₀ at 6 weeks = 155 µg/L) with 100% mortality in the highest treatment after two weeks. In contrast, diquat had a significant positive effect on tadpole survival, growth and development, possibly because of the higher algal biomass and decaying plant tissues. A lower label rate than currently recommended, at least in waterbodies with low turbidity, could provide effective control of target species while reducing effects on non-target biota.