Influence of water temperature on the efficacy of diquat and endothall versus curlyleaf pondweed

Studies were conducted in a greenhouse facility and a pond to determine the impact of water temperature on the efficacy of the contact herbicides diqu at (6,7-dihydrodipyrido [1,2-alpha :2',1'-c] pyrazinediium ion) and endotha ll (7-oxabicyclo [2.2.1] heptane-2,3-dicarboxylic acid) for control of the exotic nuisance species curlyleaf pondweed (Potamogeton crispus L.) across a range of water temperatures. Although P. crispus is reported to be highly susceptible to contact herbicides, treatments are usually conducted late i n the growing season after turion formation has occurred. These turions are the major source of reinfestation the following spring, and treatment stra tegies to prevent their production would require applications at water temp eratures which are generally thought to inhibit herbicide efficacy. Herbici de treatments were applied to 50-L aquaria containing curlyleaf pondweed gr owing at water temperatures of 10, 12.5, 15, 20, and 25C. Both diquat and e ndothall efficacy was inhibited as water temperature decreased; however, tr eatments at all temperatures significantly reduced biomass and turion forma tion. Although treatments conducted at 25C were the most efficacious, waiti ng until the water warms to this temperature limits the potential for reduc ing turion production. Endothall treatment of small plots in a pond infeste d with curlyleaf pondweed demonstrated that a late March treatment (18C wat er) reduced turion densities by 86%, whereas a mid-May treatment (25C water ) reduced turions by 40% compared to untreated plots. Based on results of t his study, it is recommended that early spring and fall treatment strategie s are rested to determine if curlyleaf pondweed can be more effectively man aged on a long-term basis by reducing turion populations.