Herbicides for management of Waterhyacinth in the Sacramento/San Joaquin River Delta, California

Authors: Madsen, John; KYSER, GUY - University Of California

Submitted to: Journal of Aquatic Plant Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/2/2019
Publication Date: 7/1/2020
Citation: Madsen, J.D., Kyser, G. 2020. Herbicides for management of Waterhyacinth in the Sacramento/San Joaquin River Delta, California. Journal of Aquatic Plant Management. 58:98-104.

Interpretive Summary: Two new aquatic herbicides, imazamox and penoxsulam, were tested for control of waterhyacinth and compared to the current standard herbicides of 2,4-D and glyphosate for control of waterhyacinth in the Sacramento/San Joaquin River Delta. Both imazamox and penoxsulam provided good to excellent control of waterhyacinth, at use rates that reduce the total loading of herbicide into the Delta simply from switching active ingredients. However, both imazamox and penoxsulam are ALS inhibitors, and as such as highly susceptible to the development of herbicide-resistant populations of waterhyacinth. Therefore, implementation of those herbicides into an integrated pest management program will require stringent adherence to an effective resistance management plan.

Technical Abstract: Waterhyacinth (Eichhornia crassipes (Mart.) Solms) is a worldwide aquatic weed. While a number of herbicides such as 2,4-D and glyphosate have been, and remain, effective for control, additional herbicides need to be evaluated to address concerns for herbicide resistance management and environmental restrictions on the use of herbicides in particular areas. Waterhyacinth has become a significant nuisance in the Sacramento/San Joaquin River Delta (hereafter the Delta). In the past, 2,4-D and glyphosate have been the predominant herbicides used for management. However, environmental restrictions related to irrigation water residues and limitations to accommodate endangered species under Endangered Species Act reviews are prompting consideration of the new reduced risk herbicides imazamox and penoxsulam. Two trials were performed in floating quadrats in the Delta. In the first, two rates each of 2,4-D, glyphosate, imazamox and penoxsulam were treated in four replicate quadrats. In this trial, the highest rates of all four herbicides provided satisfactory control (2,4-D, 82%; glyphosate, 87%; imazamox, 93%; and penoxsulam, 94%). In the second trial, the lower rate of glyphosate (1681 g ae ha-1) was compared to four rates each of imazamox (187 to 1494 g ai ha-1) and penoxsulam (12 to 98 g ai ha-1). In this trial, the highest rates of imazamox and penoxsulam provided excellent control (96% and 95%, respectively) compared to both the untreated reference and the glyphosate treatment. Imazamox and penoxsulam will provide suitable control of waterhyacinth as part of an operational program, but will need to be used as part of an integrated pest management program with considerations of herbicide resistance management.