Endothal concentration and exposure time relationships for the control of Eurasian watermilfoil and hydrilla.

Herbicide concentration and exposure time relation­ ships were determined for endothall (the dipotassium salt of 7-oxabicyclo [2,2,1] heptane-2,3dicarboxylic acid) and control of Eurasian watermilfoil (Myriopyllum spicatum L.) and dioecious hydrilla (Hydrilla verticillata L. F. Royle) under controlled-environment conditions. Sixteen en­ dothall concentration and exposure time combinations were tested for Eurasian watermilfoil: concentrations ranged from 0.5 to 5.0 mg acid equivalent (ae)/l; exposure times ranged from 2 to 72 hr. Twenty-seven endothall con­ centration and exposure time combinations were tested for hydrilla: concentrations ranged from 1.0 to 5.0 mg ae/l; exposure times ranged from 6 to 72 hr. Plant control was based on shoot and root biomass harvested at the end of the experiments. Weekly visual injury ratings were used to characterize efficacy during the course of the experiments. Plant control increased (biomass decreased), as either con­ centration or exposure time increased. A threshold level was reached in which a concentration/exposure time com­ bination provided satisfactory control. Severe Eurasian watermilfoil injury (> 85% biomass reduction) occurred when exposed to 0.5 mg ae/l for 48 hr, 1.0 mg ae/l for 36 hr, 3.0 mg ae/l for 18 hr, and 5.0 mg aell for 12 hr. Severe hydrilla injury (> 85% biomass reduction) occurred when exposed to 2.0 mg ae/l for 48 hr, and 3.0, 4.0 and 5.0 mg aell for 24 hr. The 1.0 mg ae/l treatment failed to produce severe hydrilla injury at the maximum exposure time tested of 72 hr. Increased control of Eurasian watermilfoil and hydrilla is likely for treatments in systems where plants remain in contact with endothall concentrations and expo­ sure times greater than the developed threshold levels.