Giant Salvinia

Christopher R. Mudge
Research Biologist, U.S. Army Engineer Research & Development Center
Adjunct Professor, LSU School of Plant, Environmental & Soil Sciences


Chemical Management of Giant Salvinia

Aquatic herbicides are one of the most commonly used tools to manage giant salvinia (Salvinia molesta).Prior to using an herbicide, or deploying salvinia weevils in an integrated pest management approach, proper plant identification will ensure the correct weed management technology is being utilized.Giant salvinia is often confused with its close relative present in Louisiana: common salvinia (Salvinia minima).Both plant species are covered with small hair-like structures called trichomes on the surface of the leaf (i.e., frond). But the difference is that the trichomes of giant salvinia are closed at the tips and resemble an “egg-beater”, whereas the trichomes on common salvinia are branched and look like fingers. For more information, please see this video about giant versus common salvinia identification.

Once the plant is identified as giant salvinia, herbicides can be applied to the foliage or subsurface.Foliar applications involve spraying the fronds of the plants with the herbicide plus water solution, while subsurface (or submersed) applications involve injecting the herbicide treatment directly into the water column for product uptake through the fronds and submersed fronds (i.e., roots).Foliar applications consist of adding water, herbicide, and an adjuvant (surfactant) to a tank (typically 25 to 200 gallons in size) and spraying the mixture out of a boat, ground rig, or helicopter.These treatments will deliver 1 ounce (oz) all the way up to 1 gallon (128 oz) of herbicide per surface acre.Conversely, treating the plants as a subsurface application requires the same setup, but the mixture is directed into the water column to achieve a target concentration in parts per billion (ppb, µg/L) or parts per million (ppm, mg/L).Regardless of application technique, one of the primary goals is to evenly distribute the herbicide solution to the foliage (foliar treatment) of the plants or apply evenly throughout the water column (subsurface treatment) to allow maximum coverage and uptake of the product by the plants.

Aquatic Herbicides

To date, there are 15 Section 3 herbicides registered for aquatic use by the United States Environmental Protection Agency (USEPA) under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA). In particular, carfentrazone-ethyl, diquat dibromide, endothall (dipotassium salt), flumioxazin, fluridone, glyphosate, and penoxsulam have demonstrated high levels of giant salvinia control based on research conducted by scientists throughout the U.S. In addition, a Section 24(c) registration [Special Local Need (SLN) label] was granted in Louisiana (and Texas) to use metsulfuron-methyl for control of giant salvinia in public waterbodies (see metsulfuron details below). Prior to applying aquatic herbicides in the state of Louisiana, an individual must possess a commercial pesticide applicator’s license, which can be obtained by passing the general standards and category 5a (Aquatic Pest Control) exams administered by the Louisiana Department of Agriculture & Forestry (LDAF).

Herbicides can be classified into various groups, but the most common is by speed of activity: contact vs. systemic.Contact herbicides such as carfentrazone, diquat, endothall, and flumioxazin rapidly injure plants (browning/necrosis) a few hours to a couple of days after treatment.Conversely, plants treated with fluridone, glyphosate, and penoxsulam can take one or two weeks to show injury symptoms.Simply put, contact herbicides are rapid-acting, whereas, systemic products are slow to control.Regardless of type of product you choose, it is important to apply herbicides early in the growing season prior to the formation of a thick surface mat. Application of herbicides early in the growing season allows for better control and prevents plants from forming multiple layers.Mid- and late season applications are often less effective since plants are mature and reach the tertiary, or mat forming growth stage.Once the plants reaches multiple layers multiple herbicide applications are often required to achieve control. However, during the winter, giant salvinia is often reduced to a single layer of plants and some products work better (see details below).

For each herbicide (i.e., active ingredient), at least one proprietary and possibly several generic options are available for purchase.Note: consult the product label to verify it is labeled/registered for aquatic use in Louisiana or another state.Herbicides used for the management of giant salvinia and other aquatic weeds must be approved for aquatic use since some products can be toxic to aquatic invertebrates, fish, wildlife, etc.Do not use products registered for use on row crops (rice, soybean, cotton, corn, sugarcane, etc.), right-of-way, turf, etc. unless aquatic use is permitted.The pesticide label contains information on personal protection equipment (PPE), use rates, restrictions, and other important details for the end user that must be strictly followed.The label is the law.

Although there are additional herbicides registered for aquatic use that have shown some activity on giant salvinia, the active ingredients listed below are more effective and are recommended based on published data from replicated research.General details are provided for each herbicide below, but the pesticide label must be consulted for specific details on rates, use patterns, restrictions, etc. to ensure proper and safe use prior to product application since labels can vary within a particular active ingredient/herbicide.The herbicides listed below are in alphabetical order and not listed according to preference or efficacy.Use of common names does not constitute endorsement or approval of the use of such commercial products.

Carfentrazone

Carfentrazone is a contact herbicide that results in rapid injury to the fronds of giant salvinia when applied to the foliage.Although carfentrazone results in quick injury when applied alone (up to 13.5 fluid oz per surface acre), especially at higher rates, it is often tank mixed at 2 to 6 fluid oz per surface acre) with the systemic herbicides glyphosate (96 to 128 oz per surface acre) or penoxsulam (2 to 4 fluid oz per surface acre) due to the plant’s ability to recover rapidly when applied alone.This product is more effective (alone or tank mixed) against plants growing in a single layer.

Diquat

Diquat is a rapid-acting contact herbicide that is effective alone (96 to 128 fluid oz per surface acre) when used during the non-growing season – typically late fall (November) through late winter (February/March) – when plant growth is slow and minimal.Diquat sprayed onto giant salvinia during the growing season (March/April to October) provides less control and plants often recover; therefore, diquat is more commonly used at 12 to 32 fluid oz per surface acre with a tank mix partner such as glyphosate (96 to 128 fluid oz per surface acre) during the warmer months.If the water used to fill the tanks and treat plants is turbid (muddy in appearance), avoid using diquat because the herbicide will become inactive and lose effectiveness under these conditions.

Flumioxazin

Flumioxazin also provides rapid plant injury and is more effective when applied to plants growing in a single layer.However, high product rates (up to 12 dry oz per surface acre) are needed to control giant salvinia because plants can recover in 2 to 3 weeks after application when applied alone at lower rates.Therefore, this product is commonly tank mixed at 2 to 6 dry oz per surface acre with glyphosate (96 to 128 fluid oz per surface acre) or penoxsulam (2 to 4 fluid oz per surface acre) during the spring and summer months for maximum activity.Also, similar to diquat, flumioxazin is active on plants during the winter time when growth is slow.

Fluridone

Fluridone is a slow-acting systemic herbicide that is more effective when applied as a subsurface treatment (i.e., herbicide applied directly into the water column) at 20 to 40 ppb to actively growing giant salvinia during the spring and summer months. Refer to product label on how to calculate ppb based on waterbody size using the number of surface acres and average depth.Ideally, plants should be exposed to fluridone for at least 10 weeks to achieve control.Due to the difficulty of maintaining herbicide exposures in the water column, repeat applications (also called bump treatments) are often necessary to keep herbicide concentrations at lethal doses throughout the exposure period.If water flows in and/or out of the system due to rivers/bayous or frequent rain events, maintaining lethal herbicide concentrations will be difficult.The entire waterbody must be treated to reach the necessary herbicide concentrations; therefore, treating parts of the lake/pond will result in rapid dilution of the product when applied subsurface.

Glyphosate

The non-selective, systemic herbicide glyphosate is applied to the foliage of giant salvinia at 96 to 128 fluid oz per surface acre.It does not control plants when applied subsurface because the herbicide is inactive when it reaches the water column.Although this herbicide can control giant salvinia alone at higher rates, tank mixing with low to moderate rates of carfentrazone (2 to 4 fluid oz per surface acre), diquat (12 to 32 fluid oz per surface acre), or flumioxazin (2 to 4 dry oz per surface acre) is an effective option to speed up plant injury and control.

Metsulfuron

Metsulfuron is a slow acting, systemic herbicide that is restricted in the state of Louisiana.It can only be applied by government applicators (local, state, and federal agencies) or their hired contractors to manage only giant salvinia (NO other aquatic plants can be targeted) within public waterbodies (e.g., state and federal parks, conservation areas, refuges, etc.) and cannot be applied to private lands.It can only be applied to the foliage of giant salvinia at 0.5 to 1.0 dry oz per surface acre alone or tank mixed with other aquatic herbicides, including but not limited to glyphosate, diquat, carfentrazone, or flumioxazin.

Penoxsulam

Penoxsulam is a slow-acting systemic herbicide that can be used alone to treat the foliage (up to 5.6 fluid oz per acre) or applied subsurface at 10 to 20 ppb.Similar to fluridone, in-water treatments should be maintained for at least 10 weeks to ensure plant control.When applied as a foliar treatment, penoxsulam can also be tank mixed at 2 to 4 fluid oz per surface acre with carfentrazone (2 to 4 fluid oz per surface acre) or flumioxazin (2 to 4 dry oz per surface acre) to speed up plant injury and overall control when used to treat the foliage (fronds).If applied subsurface, repeat/bump applications are often necessary to keep herbicide concentrations at lethal doses.Similar to fluridone, rivers/bayous that add extra water as well as frequent rain events can dilute the herbicide.As a result, maintaining lethal herbicide concentrations will be difficult over an extended period of time.Treating small sections of the lake/pond will result in rapid dilution of the product; therefore, treating the entire waterbody (subsurface) is necessary to reach herbicide concentrations that will control the plants.

Adjuvants/Surfactants

Adjuvants, also known as surfactants, are not required when managing giant salvinia, but are strongly recommended to increase herbicide uptake and speed up plant control.Commonly used adjuvants include non-ionic surfactants (NIS), methylated seed oils (MSO), methylated vegetable oils (MVO), organosilicone surfactants, and blends of these products.Adjuvants are commonly mixed at 0.25% v/v, but can be used at rates up to 1% v/v.For example, if an adjuvant label recommends mixing the product at 0.25% v/v, add 32 fluid oz of adjuvant into a tank that holds 100 gallons of water; 0.25% for 25 gallon spray tank = 8 fluid oz.Similar to aquatic herbicides, adjuvants must be approved for aquatic use since non-aquatic formulations can be toxic to aquatic organisms (e.g., fish, wildlife, invertebrates, etc.).


Dr. Christopher R. Mudge is a Research Biologist for the U.S. Army Engineer Research & Development Center in Vicksburg, MS.Since 2013, he has served as an Adjunct Professor in the LSU School of Plant, Environmental & Soil Sciences in Baton Rouge, LA.Use of common names does not constitute endorsement or approval of the use of such commercial products.The author declares no conflicts of interest.

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