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Increasing the Selectivity of Contact Herbicides on Hydrilla: Sensitivity Tests with American Lotus.

Madsen, J. D., Turnage, G., & Wersal, R. M. (2013). Increasing the Selectivity of Contact Herbicides on Hydrilla: Sensitivity Tests with American Lotus. Aquatic Plant Management Society 53rd Annual Meeting. San Antonio, TX.


Reservoirs in the southeastern United States have been particularly susceptible to invasion by both monoecious and dioecious hydrilla (Hydrilla verticillata) in the past three decades. Reservoirs often have areas of high water exchange or rapidly changing water flows that prevent the effective use of systemic herbicides for control of hydrilla. With the advent of some populations of hydrilla that have become resistant to fluridone, the switch to contact herbicides has accelerated. While contact herbicides are effective in rapid control of hydrilla biomass under conditions of high water exchange, the downside of many usage patterns is reduced selectivity. In many southeastern reservoirs, American lotus (Nelumbo lutea) is a common native plant that provides needed habitat and refuge for both fish and waterfowl. Our recent experience with using a tank mix of diquat with chelated copper in the Ross Barnett Reservoir (MS) was that, while it was effective in controlling hydrilla, it damaged adjacent American lotus populations. Therefore, we designed a study to examine the effectiveness of diquat and four chelated copper formulations to control hydrilla while minimizing damage on American lotus. This study was conducted at the R. R. Foil Plant Science Research Center at Mississippi State University, in 132, 100-gallon mesocosm tanks. Hydrilla was planted in five pots per tank, (66 tanks); while American lotus was planted in three pots per tank, in the remaining 66 tanks. Plants were allowed to grow for four weeks. Tanks were treated with diquat or one of four formulations of chelated copper, as well as combinations of diquat with each copper formulation; and an untreated reference for a total of 21 treatments. Each treatment was replicated three times. While several of these combinations provided better than 70% control of hydrilla, most also caused significant damage to American lotus. Copper ethylendiamine liquid alone at both 0.5ppm and 1.0 ppm provided 80% control of hydrilla, while American lotus actually increased biomass. Further research to demonstrate chelated copper formulations in the field are planned.

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