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Title: Influence of biological control damage on efficacy of penoxsulam and two other herbicides on waterhyacinth

item Moran, Patrick

Submitted to: Journal of Aquatic Plant Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/22/2012
Publication Date: N/A
Citation: N/A

Interpretive Summary: Waterhyacinth is a non-native floating aquatic plant that was introduced to the U.S. from South America over 100 years ago for its pretty flowers and odd, bulb-like leaves, but it is now one of the most widespread and damaging aquatic weeds in the southern U.S., invading over 50 lakes in Texas alone. Along the Rio Grande in deep south Texas and other warm areas, floating mats of waterhyacinth can block river flows and water intake pumps that supply cities and farms. The USDA-ARS released two weevils from the plant’s native territory into North America to control waterhyacinth biologically in the 1970s, and several fungi native to the U.S infect waterhyacinth leaves and make black spots. However, chemical herbicides are still often needed to control waterhyacinth. This study examined the ability of the herbicide known as penoxsulam to interact with weevil damage to kill plants. The combination of a lower-than recommended dose of penoxsulam, applied to water or to plants, and waterhyacinth weevils released at a rate of about one weevil per plant, killed plants up to two weeks faster than did the herbicide alone. This result was achieved in both summer and winter tests. Colonies of waterhyacinth plants exposed to both the penoxsulam herbicide and weevils tended to produce fewer offshoots from ‘mother’ plants. Two other herbicides, triclopyr and glyphosate, also killed waterhyacinth plants, but kill rate was not influenced by the biological control of weevils. The results of this study could be used to select lower doses of penoxsulam against waterhyacinth in the presence of the biological control weevils than when weevils are absent, reducing the amount of herbicide used and thus the cost of waterhyacinth control.

Technical Abstract: Populations of waterhyacinth (Eichhornia crassipes (Mart.) Solms.) in the southeastern U.S. have been reduced by widespread herbicidal control and by introduced waterhyacinth weevils (Neochetina spp) and native pathogens. However, damaging populations of this weed persist and integrated approaches are needed. In this study, the effect of combined application of penoxulam (2-(2,2-difluoroethoxy)-6-(trifluoromethyl)-N-(5,8-dimethoxy[1,2,4]triazolo-[1,5c]pyrimidin-2-yl)-benzenesulfonamide) and biological control (BC) agents (weevils and a conidial suspension of the fungus Cercospora piaropi Tharp) on short mortality was examined in outdoor tanks. Penxosulam alone killed plants at below-label concentrations as low as 5 to 10 ug ai L-1 as a water application or 17.5 g to 35 g ai ha-1 foliar application, but combined nonlethal BC damage and penoxsulam reduced time to plant death by one to two weeks. At 10 ug ai L-1 or 35 g ai ha-1, this combination also caused a net decline in shoot density prior to colony death. Below-label penoxsulam + BC killed plants 3 weeks earlier than did triclopyr (3,5,6,-trichloro-2-pyrinyloxyacetic acid, triethylamine salt) (0.42 kg ae ha-1) or glyphosate (N-(phosphonomethyl)glycine, isopropylamine salt) (0.56 kg ae ha-1) combined with BC in a winter test. Triclopyr, with or without BC, killed plants 3 weeks earlier in a summer test. Biological control alone had little or no effect in all tests. The efficacy of penoxsulam, but not triclopyr and glyphosate were positively influenced by weevil damage at low doses.