Thanks to the information that the Army Corps Of Engineers provided us plus having witnessed the results from the recent high water on Lake Bistineau, we have taken a small step back and re-examined our position in regards to controlling salvinia on the lake.
While the recent flood waters on Lake Bistineau has had a tremendous impact in reducing the amount of salvinia that was on the lake, flushing is not the only method that should be used in an attempt to control this aquatic mess. Even though literally tons of salvinia was washed over the spillway and tons have been left stranded along the shoreline, thousands of acres still remain in the lake. The northern half of Bistineau is still besieged with salvinia, and the heavy stands of cypress still retain the plant. The high water did establish a good current, and today, salvinia continues to flow from the lake. But what does help or hinder this “flushing” action is wind direction. The wind plays a big role in moving the salvinia out of the woods and toward the spillway. Right now (and this is only a guess) we feel that about 15% to 20% of the plant has been removed or become stranded along the shoreline. This may sound like a small number, but actually this is a tremendous amount when compared to the total amount of salvinia that was on the lake.
We feel that a 4-point integrated management strategy is the only way to combat and reduce the amount of salvinia in Lake Bistineau. We believe that the following 4 items would be the most effective and least costly methods to employ.
1. Drawing the lake down ( inexpensive )
2. Spraying of herbicides (minimal cost with spraying being done only 2-months out of the year)
3. Flushing the lake (inexpensive)
4. Salvinia weevils (partnering with The Red River Waterway Commission could yield federal monies to help with this project)
Now before we even begin any type of program to combat the salvinia, we need to take away any consideration of how recreational aspects of the lake will be affected. That ship has already sailed when it comes to hunting, boating, skiing, fishing, swimming, etc. on the lake. The present amount of salvinia has killed those activities.
1. Drawing the lake down – Drawing the lake down can be effective. It hasn’t been in the past simply because it has been done the wrong time of the year. The draw-down should begin in April and continue until September. This would expose the salvinia to the hottest and driest time of the year. During this time normal evaporation of the lake would add an additional 12 to 18 inches to a normal 7-ft draw down exposing even more salvinia.
2. Herbicide Spraying – Spraying of herbicide should be done in May and June, during the early part of the draw-down before peak growing conditions are reached. Note – Spraying and drawing the lake down will add additional nutrients to an already nutrient rich lake in the form of decaying salvinia.
3. Flushing – In September close the gates and bring the water level to 2-ft above normal. This can be accomplished by sand-bagging the spillway. When the water level has reached the top of the sand bags, simply knock the barrier down allowing the lake to perform a flushing action. Mother nature will help during this phase being that it is the rainy time of the year and when we get the most north winds. We could possibly get two segments of flushing before it is time to open the gates again. A 2 to 3-ft increase in water level would not cause hardships on lake residents.
4. Salvinia Weevils - Cyrtobagous salviniae (commonly referred to as the salvinia weevil) is the only known successful biological control agent for salvinia. Flushing, drawing the lake down, spraying of chemicals, is not the answer to controlling the salvinia growth. Performing these three steps alone would insure that there would be a never ending battle. Without establishing a healthy weevil colony, the fight would continue from now until the end of time.
A density of 300 adults per square meter will control salvinia in most situations. There are 4046.86 sq. meters per acre. This equates to 1,214,058 weevils per acre. The LWFD estimated that there was 14,000 acres of salvinia on Lake Bistineau which means that 16,996,812,000 (just under 17 billion weevils are required). In order to put this number into a realistic and manageable figure, the amount of salvinia has to be reduced using the three methods listed. Even then, continued monitoring of the weevil colony would be required.
• Salvinia grows on still or slow-moving fresh water where nutrients are available.
• Growth rates decrease by 25% in water that is 10% as salty as seawater
• Growth is very slow in water that is 20% as salty as seawater
• plants die after 30 minutes in seawater
• A tertiary mat of salvinia can survive for up to 12 months on mud
• Under ideal conditions an infestation can double in size in less than 3 days
• Rates of growth vary according to climate zones, starting to increase as temperatures warm up, peaking in late summer, and slowing over the cooler months.
• There are no distinct seasonal periods for stages of plant development
• Permanent water bodies, and waterholes provide perfect conditions for salvinia growth, as temperatures remain ideal. Flushing associated with the wet season usually moves infestations downstream to estuarine waters where the plant does not survive
• There are very few situations where eradication is possible
• Carry out herbicide treatments as early as possible in the growth season (usually early spring). Starting herbicide treatments during peak growth periods may have little impact on an infestation
• Cyrtobagous salviniae (commonly referred to as the salvinia weevil) is the only known successful biological control agent for salvinia
• In tropical and subtropical climates weevils usually reduce an infestation in 2 years, sometimes less. In temperate climates it can take 3 or more years for weevil populations to increase enough to reduce an infestation
• A combination of biocontrol and seasonal flushing provides good ongoing control of salvinia, but re-releases can be necessary if whole infestations (and therefore populations of weevils) are dried or flushed
• Adensity of 300 adults per square meter will control salvinia in most situations. 4046.86 sq. meters per acre. This equates to 1,214,058 weevils per acre. The LWFD estimated that there was 14,000 acres of salvinia on Lake Bistineau which means that 16,996,812,000 (just under 17 billion weevils are required)
• Laboratory tests indicate that weevils cease feeding below 55°F, eggs fail to hatch at 63°F, and females stop laying eggs at 70°F. The lowest temperatures at which adults cease activity and die are currently under investigation, with preliminary findings that females will start laying eggs at 66°F.
• Salvinia highest growth rate occurs between 69°F and 86°F
• No salvinia growth occurs above 104°F
• Herbicides are used to their best advantage as part of an integrated management strategy Initial treatments will always need to be followed up with further treatments
• Trials showing that a good initial knockdown after herbicide application can be misleading, and that re-growth is likely to occur after treatment with any of the registered herbicides. The decaying biomass of sunken herbicide-treated salvinia will also return nutrients to the water, creating ideal conditions for re-growth of surviving plants