Evaluation of Selected Herbicides for the Control of Exotic Submerged Weeds in New Zealand: II. The Effects of Turbidity on Diquat and Endothall Efficacy

Diquat is the only herbicide registered in New Zealand for the control of submerged invasive weeds such as hornwort or coontail ( Ceratophyllum demersum L) . However, under turbid conditions, binding to clay and charged particles in the water may deactivate diquat. To determine whether endothall achieved better control of Coontail under turbid conditions, both diquat and endothall were applied at maximum label rates, for three different exposure periods (1, 2 and 8 days) to tanks of varying turbidity (0 to 40 NTU) . All plants exposed to endothall were killed irrespective of turbidity or exposure period, whereas diquat only achieved complete control (kill) at the maximum exposure when there was no turbidity.