Current and Future Chemical Management Practices for Hydrilla
The impetus for organizing the Hydrilla Issues Workshop revolved around the phenomenon of fluridone resistance. As a result much of the dialogue focused on the challenges associated with managing fluridone-resistant hydrilla (FRH). Discussion related to this issue included current and past chemical alternatives, as well as the potential for developing new products. In this section we address current and future chemical management practices, and identify the strengths, weaknesses, and issues associated with these management strategies. We conclude by making specific recommendations related to future chemical control efforts.
A Basic Understanding Fluridone Resistance
Within the aquatic plant and resource management communities the challenges associated with managing FRH have not been well understood. In order to provide background information to a broad range of resource managers a Symposium on Herbicide Resistance in Aquatics was held at the Aquatic Plant Management Society Meetings in Tampa, FL in July 2004. The recent Hydrilla Issues Workshop in December was organized to specifically focus on hydrilla management and fluridone resistance issues. While there remains much to learn regarding hydrilla and fluridone resistance, the following statements provide an overview of our current understanding of this issue:
1. Formerly sensitive populations of hydrilla have developed increased resistance to low levels of fluridone in numerous Florida lakes. Despite a similar genetic origin for hydrilla throughout Florida (Madiera et al. 1997), the majority of lakes with “resistance issues” have a history of fluridone management. While literature is sparse regarding hydrilla and fluridone resistance, recent publications describing this phenomenon have become available (MacDonald et. al 2002, Michel et al. 2004, Arias et al. 2005).
2. Fluridone resistance is caused by various point mutations at site 304 in the phytoene desaturase (PDS) gene, and each point mutation has a different impact on the response of hydrilla to fluridone (Michel et al. 2004). The fact that adjoining lakes support hydrilla with different mutations indicates that fluridone resistance has evolved independently at different sites.
3. The development of fluridone resistance was not predicted by the scientific community based on the asexual nature of dioecious female hydrilla, and the lack of previous evidence of resistance to PDS inhibitors such as fluridone. In fact, hydrilla is the first example of a somatic mutation leading to development of widespread resistance in the field.
4. FRH is currently widespread with populations dominating the Kissimmee Chain of Lakes, Lake Istokpoga, several Polk County Lakes, and numerous other public and private water bodies throughout the state. Resistant biotypes of hydrilla are present in lakes that contain well over 100,000 surface acres of water, and as the critical mass of FRH infestation increases the threat for continued inter-lake spread of these plants is greatly increased. 5. It has been suggested that it took approximately 14 years for hydrilla to develop resistance to fluridone. This scenario is based on the registration of fluridone occurring in 1986 and the documentation of resistance in the year 2000. Evidence from research ponds and various lakes suggest that selection of resistant biotypes can occur in a much shorter period of time. For example, large-scale fluridone treatments on Lake Cypress, FL were not initiated until 1996 and the entire 4000-acre lake contained a tolerant biotype by 2000. The complete dominance of resistant plants on Lake Cypress within 4 years of an initial treatment would suggest that fluridone resistant strains were established in the lake within a couple of years of the initial large-scale treatment. While rapid selection for resistant biotypes was inevitable, the subsequent dominance of these resistant biotypes was likely accelerated by successive treatments with sub-threshold fluridone concentrations.
6. There is currently no evidence of a “fitness penalty” for the development of fluridone resistance. This means that FRH is likely to be just as competitive and aggressive as the sensitive biotypes. Therefore, once resistant plants become established and produce tubers, it is unlikely that active management, lack of management, or intra-specific competition between the biotypes will remove the resistant plants from the system.
Use of Fluridone for Control of Resistant Biotypes of Hydrilla
For those individuals with a peripheral involvement in hydrilla management, one of the more difficult questions to address is why aquatic plant managers continue to use fluridone after resistance has developed. First and foremost, the number of tools available for hydrilla control is quite limited, and fluridone and triploid grass carp remain the only tools currently proven to provide cost-effective control of large-scale hydrilla infestations. In addition, the slow plant death following a fluridone application is a desirable trait that is often overlooked, and yet from a wildlife and fisheries, and water quality standpoint, the incremental loss of dense hydrilla over several months may be one of the most important characteristics of any fluridone treatment, whether for susceptible or resistant biotypes.
The level of fluridone resistance can vary considerably and therefore, while the term resistance connotes a qualitative character (i.e. a population is or is not resistant), it is the quantitative determination of the level of resistance that is more critical to issues such as treatment cost, non-target plant impacts, and long-term efficacy. Quantifying the sensitivity of a hydrilla population to fluridone can be determined through intensive sampling and laboratory assay to assist aquatic managers in determining treatment strategies.
The most resistant strain of hydrilla characterized to date, remains sensitive to fluridone well within the label’s maximum allowable use rate of 150 ppb. One of the key attributes of fluridone has been its specificity for plants and hence its wide margin of safety for non-target organisms (invertebrates, fish, and wildlife) up to the maximum use rate. There are also no fishing, swimming, or potable water use restrictions associated with the use of fluridone up to the maximum rate of 150 ppb. Therefore, increasing traditional use rates of fluridone from 5 to 10 ppb by 3 to 5 times, while certainly changing the economics of hydrilla control, is not expected to have a direct adverse impact on invertebrates, fish, wildlife, or recreational uses of the water body. There will, however, still be indirect impacts because of loss of fish and wildlife habitat.
While resistance has greatly impacted the economics of fluridone treatments, fluridone costs remain competitive and are generally lower than costs of other chemical techniques when one considers control on a per vegetated acre basis. Nonetheless, this comparison may not be appropriate as contact herbicides tend to be applied directly to a densely vegetated priority area, and therefore control is generally limited to the immediate vicinity of the treatment zone. In contrast, fluridone tends to be quite long-lived in the water column and will disperse well beyond the treatment site. This has allowed the use of fluridone to control hydrilla in large areas throughout a water body. The need to maintain a long-term concentration in the water column regardless of the plant distribution or density is both a strength and weakness of fluridone. Fluridone is the only chemical tool that can be used in a cost-effective manner to prevent hydrilla expansion when plants are widely distributed on a large water body. In contrast, when resistant hydrilla is concentrated within an area of a large water body, dispersion of residues to areas without hydrilla or into sites dominated by native vegetation reduces either the cost-effectiveness or selectivity of the treatment.
The reality of managing resistant hydrilla has greatly reduced the expected longevity of control following a fluridone treatment. As a result, yearly applications of fluridone have been conducted since the late 1990s to suppress hydrilla growth during the spring and summer months in the Kissimmee Chain of Lakes. The greatest challenge in developing treatment strategies is related to the need to maintain a fluridone concentration above a target threshold for an extended period of time (Netherland and Getsinger 1995). Initial water levels and water level schedules can have a dramatic impact on the cost and effectiveness of treatments in large flowing lakes such as the Kissimmee Chain or Lake Istokpoga. Under the best of conditions (sites with limited or no flow) the ability to maintain fluridone residue thresholds for over 100 days when targeting a resistant biotype has proven challenging (see the following section).
Selectivity Concerns
In addition to the significant cost increase and reduced longevity of control created by hydrilla resistance, adverse impacts to non-target vegetation have also been noted as the use rates have increased. While the lower use rates of fluridone for control of susceptible hydrilla had minimal impacts to important native submersed (e.g. vallisneria, Illinois pondweed) and emergent (e.g. bulrush, knotgrass, maidencane, water lily, spatterdock) vegetation, the higher use rates necessary to control FRH have shown potential to result in significant injury to several native species. Unfortunately, the vast majority of published literature regarding the selectivity of fluridone has been conducted in the northern tier states with an emphasis on submersed aquatic species (Getsinger et al. 2002). Due to the whole-lake use patterns of fluridone, non-target plant injury is a significant issue, and the desire to protect native vegetation will likely result in a practical upper limit on fluridone residues depending on the native communities present in a given water body. The availability of an accurate immunoassay test for fluridone analysis (Netherland et al. 2002) has greatly facilitated hydrilla management, and similar sampling protocols could be used to determine the relation between field residues and non-target impacts.
One of the major concerns regarding fluridone selectivity is based on repeated applications and increasing use rates of fluridone. Fluridone impacts plants in a unique way, and the bleaching symptoms tend to be highly visual for the more sensitive emergent species. Intense bleaching at the new growing points is a symptom of exposure, yet these symptoms do not indicate that plant death is imminent. While fluridone is often referred to as a systemic herbicide, it is not translocated in the phloem like glyphosate or 2,4-D (e.g. Rodeo, Weedar), and therefore a long-term aqueous exposure to fluridone is required to kill even the most sensitive emergent plants. A short-term exposure to high fluridone residues can result in a severe but temporary bleaching of new growth that is followed by rapid recovery. In contrast, long-term exposure to near threshold concentrations can greatly reduce the growth of non-target plants and may eventually result in death of established emergent plants. While investigations are ongoing, the area of fluridone selectivity definitely requires further research attention.
It should be noted that regardless of the fluridone use rate, it is unlikely that treatments would result in a complete loss of vegetation. Higher use rates would likely alter plant competition and result in a significant shift in the vegetation community from a dominance of native grasses, lilies, or submersed macrophytes to more tolerant plants such as pickerelweed, smartweed, or the macrophytic algae Chara or Nitella.
Endothall as a Chemical Alternative
With the onset of fluridone resistance, use of the aquatic herbicide endothall has significantly increased over the past several years. Endothall has been used for hydrilla control for over 30 years, and unlike fluridone it requires only hours to a few days of exposure to provide control of hydrilla. Typical use rates for endothall are in the range of 3000 ppb and, therefore, while the exposure time is greatly reduced, use volumes are in the range of 100 times greater than fluridone on a per acre basis. The increased volume requirement presents practical limitations to the amount of Endothall that can be applied. Product volumes, product costs, rapid control of vegetation, and a 3-day fishing restriction have typically limited the use patterns of endothall to smaller treatment areas. While endothall provides a strong benefit in small treatment blocks, there is ongoing discussion of increasing the size of treatment blocks to manage larger contiguous areas of hydrilla. Regardless of the size of the treatment block, the concentration and exposure time of endothall in the treatment area remains a critical element in achieving the desired level and longevity of hydrilla control (Netherland et al. 1991). Endothall currently represents the key chemical alternative to fluridone in preventing the establishment of new infestations of hydrilla. In addition, the practice of integrating endothall into fluridone treatments for control of FRH has enhanced the efficacy of both compounds.
At the present time endothall is typically viewed as a product that provides several weeks to a few months of relief from hydrilla. Research is ongoing to improve the longevity of endothall treatments for use on shoreline strips, boat trails, and large contiguous blocks of hydrilla. With the spread of FRH, endothall is an indispensable tool for creating access or open water in the midst of large hydrilla infestations. Despite a significant increase in endothall use for hydrilla control during the past several years, the literature is sparse regarding improving methods, treatment timing, or use rates for hydrilla control. The recent development of an immunoassay for endothall detection in the water column has greatly facilitated both laboratory and field research efforts. Monitoring treatment timing, formulation differences, and various use rates in the field is currently ongoing.
Two of the major concerns associated with endothall use are the requirement for posting a 3-day fishing restriction (fluridone has no use restrictions), and the potential for a rapid decay of plant biomass that could reduce dissolved oxygen to levels that could result in a fish kill. In December 2004, the registrant submitted data to the USEPA requesting that the 3-day fishing restriction for Aquathol be removed from the label. While fish kills in association with herbicide applications on large public water bodies have been rare, a cautious approach is still warranted. The timing of application (e.g. water temperature) and the density of the target vegetation at the time of treatment are likely the most significant variables that will determine if there is a potential for significant oxygen depletion. Reduced efficacy of endothall over time has also been noted as a significant concern. While these observations remain anecdotal research into potential tolerant strains of hydrilla and enhanced environmental degradation are ongoing.
In regards to plant selectivity, the nature of endothall degradation and the use patterns favor the selective use of this product. While endothall use rates are typically in the range of 3000 ppb, the molecule is rapidly degraded by microbial action. In addition, use patterns typically result in the treatment of smaller areas within large lake systems. Rapid dispersion and degradation result in minimal exposure of plants outside of the treatment zone. Submersed applications of endothall do not tend to have impacts on emergent vegetation (Skogerboe and Getsinger 2001). Vallisneria and macrophytic algae tend to thrive following endothall applications, while Illinois pondweed and naiads tend to be fairly sensitive.
Contact Treatment Strategies
Within the Harris Chain of Lakes (Griffin, Eustis, Harris), the Saint Johns Water Management District, Lake County Mosquito and Aquatic Plant Management, and the FDEP have implemented an intensive surveillance and spot treatment strategy with endothall to prevent hydrilla from expanding within these systems. The recent expansion of hydrilla in these large lakes has been attributed to improved water clarity. The management response has been to greatly increase surveillance using a variety of people (agency personnel, contractors, and the public) to locate and map new hydrilla infestations. Spot treatment strategies are rapidly implemented to prevent further spread, and follow up visits are made to insure treatment efficacy. This approach has been effective for these systems; however, there may be unique features of these lakes that favor this strategy.
Hydrilla within the Harris Chain of Lakes represents small pioneer infestations that may be deterred from rapid expansion by the shading activities of intense phytoplankton blooms. Recovery of hydrilla following the small contact herbicide operations is likely hindered by the reduced water clarity provided by these blooms. Furthermore, the plants in the Harris Chain of Lakes are susceptible to fluridone, so any large-scale recovery of hydrilla could be set back using a low rate and cost-effective fluridone treatment. In contrast, hydrilla growing in the Kissimmee Chain of Lakes or Lake Istokpoga is widespread and past growth has not generally been constrained by phytoplankton blooms. In these systems water quality conditions following a contact treatment are often favorable for rapid recovery of any plant tissue that survives the initial herbicide application. In addition, this hydrilla has an increased resistance to fluridone, and the inability to manage growth with contacts through the spring could lead to a situation of widespread hydrilla coverage and inability to implement whole-lake management strategies under conditions of high biomass and high precipitation and flow during the summer months.
It is interesting to note that through the spring of 2005 hydrilla infestations on the Kissimmee Chain of Lakes, Lake Istokpoga, and Lake Weohyakapka are greatly reduced following the intense disturbance from hurricanes in the summer of 2004. This has led to the suggestion that a greater emphasis should be placed on surveillance and spot control for hydrilla management on these sites. In addition, the current situation of highly colored water on these lakes is a strong deterrent to hydrilla recovery due to markedly reduced light penetration into the water column. The combination of highly stained water and low hydrilla biomass in the spring of 2005 may allow for the successful implementation of the surveillance/spot treatment strategy through the summer and fall of 2005. While a surveillance and spot treatment strategy may represent a sound management program as long as current low biomass and water clarity conditions exist, most aquatic managers recognize that these conditions can change rapidly. As water clarity improves and sprouting hydrilla tubers survive over thousands of contiguous acres, maintaining hydrilla control with a surveillance/spot treatment strategy will be greatly complicated. The current example illustrates that need for aquatic plant managers to use prevailing conditions to their advantage when implementing hydrilla control strategies.
Current State Policy on the Use of Copper-based Products
In the late 1980s and early 1990s the State of Florida began moving away from the use of copper-based products for hydrilla control in natural water bodies throughout the state. A policy was implemented regarding copper use in manatee aggregation areas, but this soon became the standard for the majority of natural water bodies. While copper use has not been banned as part of the state program, its use is generally limited to urban and artificial water bodies that are considered degraded. Within the state program, copper is sometimes used in combination with diquat in areas around boat ramps where retention time is often very limited due to the small size of the treatment. Despite considerable debate regarding the fate and toxicology of copper that is bound to the bottom sediments, it is not disputed that each application results in further accumulation of copper in the sediment. A 1992 workshop held at the University of Florida was organized to present information on the bioavailability and toxicity of copper (Joyce 1992). Several opposing viewpoints were expressed regarding copper toxicity to non-target organisms and the bioavailability of copper bound to sediments. Despite the passage of over 12 years, the major issues associated with copper use remain largely unchanged.
FDEP has taken the stance that copper should not be used if reasonable alternatives exist. The major concern expressed over the use of copper was the need for multiple treatments and hence an impact on the accumulation of copper in the sediments. Given the recognition that FRH has become widespread, there is some question as to whether the state should consider the use of copper on a limited basis as a rotation tool for endothall. The combination of diquat and copper has been a standard treatment for hydrilla in other parts of the country, and the efficacy of this combination is not in question. The amount of copper necessary to enhance diquat activity is currently not well defined, and studies along these lines could prove valuable in allowing recommendations of reduced copper rates for improved hydrilla control. This approach would seek to use the minimal amount of copper necessary to control hydrilla. It should be noted that low rates of copper might also enhance the activity of endothall or fluridone.
Potential Development of Alternative modes of action
During the course of the Hydrilla Issues Workshop, the need for development of alternative modes of action was discussed. While many in attendance viewed this as an industry responsibility, two alternatives were presented that would allow state and federal agencies and university personnel to become more integrally involved in the process of registering products for aquatic use. The two alternatives proposed included cooperation with the USDA IR-4 program and the potential for a state agency within Florida to explore a third party registration.
The USDA IR-4 program was initiated in response to the loss and lack of products available for minor use markets (e.g. vegetables, horticulture). While industry was generally averse to investing extra money to develop products in these niche markets, they were willing to move into these markets. With the consent of an industry partner, the IR-4 program helped to develop data requirements that would support a new product use in minor markets. These data were then submitted to the Environmental Protection Agency for registration of the product in the minor use market. The end-user benefited as a new tool became available for their industry. It is worth noting that in several cases, IR-4 has helped to develop new tools in response to resistance issues in minor use markets. Current discussions are ongoing within the IR-4 program on how best to include the field of aquatic plant control.
One area where IR-4 could be particularly helpful would be in the development of food crop tolerances (i.e. the maximum amount allowed in commodities) that would reduce the restrictions on new and existing aquatic products. For example the irrigation restrictions on the recently registered products triclopyr and imazapyr are largely driven by a lack of tolerances in food crops. Development of these tolerances would be aligned with the IR-4 mission, and would provide greater flexibility for use of chemical tools in aquatics.
Third party registration would involve the FDEP or another governmental agency becoming involved in registering a product for their exclusive use for hydrilla control. While close cooperation with Industry would be required, precedent for this arrangement has been set by the Florida Fruit and Vegetable Growers Association (they have several third party registrations), and by the Florida Tropical Fish Farmers Association. This approach could provide an incentive for evaluating off-patent compounds. At the present time there is little incentive for researchers to screen off-patent herbicides, as it is very unlikely that industry would be willing to consider investing several million dollars in such a project. While this would be a non-traditional approach, there are numerous scenarios where private industry and government could work together to develop new tools for the aquatic market.
One of the greatest challenges to the research community and industry is identifying desirable attributes for a new product in the aquatic market. Despite the large number of products available in terrestrial agriculture, there are actually very few herbicides that would have a good fit within the aquatic market. Some of the key attributes that have been identified include the following:
1. A compound must have a very strong toxicology package for mammalian, bird, fish, and invertebrate organisms. This requirement eliminates many families of herbicides from consideration. Water use restrictions (e.g. fishing, drinking water, etc.) are generally undesirable and are largely based on the toxicological properties of the compound.
2. A compound must provide a high level of selective control for hydrilla.
3. A compound with a long residual in the water column and an alternate site of action compared to fluridone represents a strong candidate.
4. A compound with a short residual in the water column and systemic activity on hydrilla would be highly desirable.
5. Products that result in a slow pattern of plant death (e.g. like fluridone) would be beneficial from a water quality standpoint.
6. The product should be easy to apply, cost-effective, safe for applicator handling, and preferably used at low volumes.
The Development of Acetolactate Synthesis inhibitors as an alternative to Fluridone
There was considerable interest expressed at the workshop regarding the development of new herbicides for aquatic use. Acetolactase synthesis (ALS) inhibitors currently show strong potential to control hydrilla at low use rates. These compounds block the formation of essential branched-chain amino acids in plants. The ALS enzyme system is unique to plants, therefore non-target toxicity of these compounds is very low. Preliminary evaluations indicate that several ALS inhibitors are active on hydrilla at rates in the low part per billion ranges. Due to the low use rates, activity on a specific plant enzyme, and impact on the new growth of hydrilla it is likely that ALS chemistry would have a very similar use pattern to fluridone. The maintenance of low concentrations over a long period of time will be critical to the efficacy of an ALS inhibitor. While the issues of potential resistance development and use of water for irrigation have been discussed, the need to evaluate new modes of action is deemed the more critical issue at this point in time. In regards to the selectivity of ALS chemistry, research is ongoing at the greenhouse and field scale. The herbicides penoxsulam and imazamox recently received Experimental Use Permits from the EPA for up to 500 acres of water in Florida. Both compounds are expected to provide good control of hydrilla at low use rates. Several use sites have been identified for evaluation in the spring and summer of 2005.
Recommendations
Recommendation 3: Based on the extent of Fluridone resistant hydrilla (FRH), the identification and development of new herbicides for hydrilla control is critical. FDEP should immediately re-invigorate Florida’s chemical research programs for aquatic plant management programs. FDEP should lead by obtaining needed state and federal funding (goal 10% of State of Florida’s existing activities budget), and entering into agreements with universities, federal agencies or private entities for research and the development of new or improved aquatic plant control methods. In addition to the USEPA data requirements for the registration of a new product, a thorough evaluation of the efficacy and selectivity of a new herbicide will be critical prior to recommending its use on large public water bodies.
Justification: The inability to develop new tools for hydrilla control will result in further spread of FRH and this will greatly compromise the ability of the FDEP and its cooperators to manage hydrilla throughout the state. The best strategy for resistance management is the development of multiple tools that can be rotated. To conduct the appropriate research, funding is needed. FDEP provided research funding in the 1980s (FS 369.20(4)(b)) and the State of Florida got a good return on investment. Therefore, a good argument can be made to the Florida Legislature for increased research funding. As the largest purchaser of aquatic herbicides in the world, the FDEP and other end-users should make it clear to Industry that new tools would be welcomed and integrated in to their existing program. The increased reliance on endothall as the sole chemical alternative to fluridone may result in future problems with endothall efficacy. Finally, the addition of new aquatic products could provide enhanced benefits to the state for control of aquatic invasive species other than hydrilla.
Recommendation 4: There is a strong need to improve our ability to quantify the impact that fluridone or other lake management techniques are having on key non-target plant species. Methodologies for collecting reliable and useful field data need to be worked out between responsible agencies so results can be compared across both managed and unmanaged water bodies and sites treated at different fluridone use rates.
Justification: While increasing fluridone use rates does not pose a direct threat to non-plant organisms, the potential loss or severe reduction of a key individual plant species is a legitimate concern that requires improved data collection to support future decision-making. The bleaching symptoms following a fluridone application are quite visual, and conclusions on the ultimate impact to these native plants are often anecdotal and based on a bias regarding fluridone use for whole-lake management. There has been little or no quantitative assessment of the impact to native submersed and emergent vegetation following increased use rates of fluridone. While laboratory and mesocosm data for non-target native plants are currently being generated, these data need to be put in the context of actual field results. The FWC has conducted some initial field monitoring, but these efforts have generally been limited and have remained internal.
Recommendation 5: For sites where the hydrilla remains susceptible to fluridone, consecutive year applications are discouraged. It is also crucial that resistance management strategies be developed to prevent FRH from developing a dual resistance to another mode of action.
Justification: Fluridone has proven its utility in providing large-scale hydrilla control, and a successful treatment should greatly reduce the need to conduct an application the following the year. In situations where adequate control is not achieved, aquatic managers need to determine the basis for this reduced efficacy (e.g. increased herbicide resistance, loss of residues to flow, enhanced degradation). Based on the widespread coverage of FRH on the Kissimmee Chain of Lakes and several other large lake systems, it is apparent that sequential applications of fluridone can ultimately facilitate the lake-wide expansion of resistant biotypes.
ALS chemistry represents a potential new tool that could be rotated with fluridone for control of susceptible hydrilla. In the case of FRH, management with an ALS herbicide will be complicated the fact that managers will be treating plants that have already developed a resistance to one mode of action. For sites already dominated by FRH, management strategies need to be considered to prevent development of a dual resistance to both fluridone and ALS inhibitors. This issue suggests that more than one new mode of action is needed for the long-term control of hydrilla.
Recommendation 6: In addition to considering rotation schemes with fluridone, aquatic managers also need a contact product that can be rotated with Aquathol. There are currently no new contact products being considered for registration. In order to provide a new tool that would be available for immediate use of combinations of products should be further evaluated. . We recommend that copper only be considered for hydrilla control when used in combination with the herbicide diquat or other registered herbicides. Research should be conducted to determine if low rates of products such as the dimethyalklyamine formulation of endothall or hydrogen peroxide can enhance the activity of diquat or endothall for spot control of hydrilla.. As the treatment of new infestations is the top FDEP priority for hydrilla control, addition of a new contact product would provide a highly useful tool to address this priority.
Justification: Endothall is the only contact product in wide-scale use in Florida public waters, and this complete reliance on a single contact herbicide does not represent a good resistance management strategy. There are many cases where multiple applications of endothall are being applied in the same areas. In lieu of waiting for a new contact herbicide registration (this could be years away), aquatic managers are encouraged to support research that evaluates the use of combination products to provide enhanced control and the ability to rotate products.
Recommendation 7: When possible, intense but small-scale management of hydrilla is preferable to large-scale whole-lake management efforts. In the case of larger lakes, this requires a considerable commitment to surveillance, sound reporting of the exact locations and size of hydrilla infestations, rapid action, and aquatic managers who can make decisions on the optimal treatment recommendations for insuring that small infestations are not allowed to spread. This recommendation fits with the current priority list of the FDEP regarding intense management of new finds, and this strategy should be employed to delay the spread of hydrilla, especially resistant strains.
Justification: When practiced properly, this form of management most resembles the highly successful water hyacinth maintenance control program and it represents the best use of limited state resources and manpower. Preventing the establishment and dominance of hydrilla in water bodies with abundant native vegetation is the best management practice both in terms of cost-effectiveness and selectivity. If hydrilla can no longer be controlled in this manner, then whole-lake options should be considered. Experience suggests that once hydrilla has been allowed to cover a water body, it is likely that whole-lake management will be required for multiple years to keep the plants under control. This increases both the long-term cost and the likelihood of resistance development.