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Sustainable Approach for Integrated Management of Herbicide Resistant Hydrilla in the U.S.


GOALS: Our long-term goal is to reduce the threat that the spread of fluridone resistant hydrilla biotypes represents to aquaculture and irrigated agriculture in the U.S. The overall objective of this application, which is the next step toward attainment of our long-term goal, is to develop and demonstrate an integrated reduced risk solution for hydrilla control. Our central hypothesis is that integrating herbivory by a naturalized meristem mining midge Cricotopus lebetis Sublette (Diptera: Chironomidae) with the native fungal pathogen Mycoleptodiscus terrestris (Gerd.) Ostazeski (hereafter Mt), and/or low doses of a new acetolactate synthase (ALS) inhibiting herbicide (imazamox)is a viable strategy for long-term sustainable management of hydrilla. We expect our IPM strategy will safely control fluridone resistant and susceptible hydrilla biotypes in Florida watersheds and in other locations in the US where the resistant biotypes are expected to become established. <P>OBJECTIVES: (a) Establish the biotic and abiotic limits for optimal survival and reproduction of the hydrilla midge C. lebetis; (b) Evaluate the compatibility of C. lebetis with new hydrilla control technologies; and (c) Demonstration of a novel integrated strategy for controlling hydrilla. <P>EXPECTED OUTPUTS: First, because of the midge's unique larval feeding habits, we expect our research to show that the insect is host specific by developing only on hydrilla. This is important because by demonstrating the host specificity (or safety) of the midge, we will be in position to readily obtain appropriate federal/state permits for interstate movement of the insect to new sites where hydrilla resistant biotypes are likely to become established. <P>Second, we expect our research to show that the water temperature tolerances of the midge will facilitate its establishment in hydrilla-infested water bodies across a wide geographic area. This also is important because we expect the resistant hydrilla biotypes to eventually invade the same tropical to temperate geographical localities in the US currently infested by susceptible hydrilla. <P>Third, we expect to show that the midge C. lebetis is compatible with low doses of Mt and imazamox, which will lower the contact time needed for the pathogen and herbicide, respectively, to impact hydrilla.<P> Fourth,a variety of outreach methods will be used to educate water body managers about hydrilla IPM methodologies.<P> To reach the greatest target audiences and to accom-modate a variety of learning styles, educational programs will consist of documents to help facilitate the planning of guided field tours of hydrilla IPM demonstration sites, regional workshops on hydrilla IPM, web-based video clips with how-to instructions and informational brochures and publications. Educational programs will be developed pending the outcome of the needs assessment survey and direction provided by the Hydrilla IPM Program Advisory Committee.<P> Collectively, these outcomes are expected to have a positive impact by demonstrating the potential for integrating these reduced risk tactics for controlling hydrilla in public and agriculturally relevant waterbodies.

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Non-Technical Summary: In 2000, aquatic plant researchers in Florida discovered the aquatic weed hydrilla (Hydrilla verticillata (L.f.) Royle) was developing resistance to fluridone, the only EPA registered herbicide approved for use only in aquatic systems. This finding confirmed field observations of declining hydrilla control by public and private aquatic plant managers even though the same procedures from previously successful fluridone treatments were used. The fluridone resistance was unexpected because hydrilla reproduces asexually in Florida. At least six clones have been identified with a two- to sevenfold increased resistance to fluridone, and the resistance is stable over time, even in the absence of fluridone selection pressure. This herbicide resistance problem is cause for concern because the spread of resistant hydrilla is inevitable, and the higher fluridone concentrations required to control it will adversely affect on our country's hydrologic system, especially water supplies used for crop irrigation and organic or conventional aquaculture. Our objective is to develop and demonstrate an integrated reduced risk solution for hydrilla control by integrating selective insect herbivory and a disease with low concentrations of a new herbicide recently registered for aquatic use. We expect our research and demonstration project to show that these different low risk control tactics are compatible with each other, and that by integrating them, we can achieve safe and cost-effective control of both susceptible and resistant hydrilla. <P> Approach: OBJECTIVE (a): Larval feeding tests will be conducted initially under no-choice conditions with naive neonates using a culture tube procedure & individual sprigs of the test plants. Adult oviposition/larval development tests (Test 2) under close confinement (no-choice) will be performed in glass jars in which mated female midges will be confined with each test plant species supporting larval development in Test 1. Finally, loose confinement adult oviposition/larval development tests (Test 3) will be conducted in the same glass jars except the jars will be placed inside screen cages where females will have equal access to the jars. Jars containing hydrilla (susceptible and fluridone-resistant) will be used as controls in Tests 2 and 3. Three replicates of each type of host specificity test will be performed. Number of adult midges produced on hydrilla (susceptible & resistant) as well as other plant species exposed to the insect in each phase of the testing sequence will be transformed using ? x + 0.5. Means will be analyzed with ANOVA & separated by LSD or other appropriate MCP at p=0.5. In order to establish lower & upper temperature thresholds for adult survival & larval development, laboratory studies will be conducted in environmental chambers at 11 different temperatures between 10 & 35 degrees centigrade under cool white fluorescent lamps at 14:10 (L:D) photoperiod. Neonates will be placed singly in culture tubes (n=40 per temperature treatment) containing a single sprig of hydrilla and well water. Larval development at each temperature will be recorded every other day until adult emergence. The linear portion of the developmental rate curve [R(T) = a + bT] will be modeled using least squares linear regression. The developmental zero & degree-day requirements for will be calculated from the data. OBJECTIVE (b): Procedures for evaluating the compatibility of C. lebetis with the fungal pathogen Mt & the ALS herbicide imazamox will follow established procedures. Two rates of Mt (low & high), imazamox (low & high) & two densities of the insect (low & high) alone & in combination will be randomly applied to aquaria containing established hydrilla plants & replicated three times. Hydrilla shoot biomass will be harvested 30 & 90 days after treatment & biomass data (grams dry weight) will be subjected to repeated measures ANOVA. Data will be transformed, if necessary, to meet normality & equal variance assumptions. We expect feeding damage by midge larvae to increase the susceptibility of hydrilla to infection by Mt, & that midge densities will be higher in the aquaria treated with low rates of imazamox due to the new hydrilla shoot tips produced by the plants following exposure to the herbicide. OBJECTIVE (c): Field days & a needs assessment survey will be developed to analyze end users' perceived knowledge of hydrilla & role of IPM in the plant's management, analyze end users' preferred methods of obtaining information on hydrilla IPM strategies, determine the characteristics, needs & priorities of the target audience & determine types & numbers of educational resources currently being used by end users to manage hydrilla.

Cuda, James
University of Florida
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