Center For Aquatic And Invasive Plants

Diquat

*Important: See Reference Guide Beforehand

No single herbicide is appropriate for controlling all invasive aquatic plants (or nuisance growths of native aquatic plants), in all situations. A herbicide may perform differently depending on the waterbody, its use, the time of year—or even the time of day. Therefore, aquatic plant managers must have a thorough understanding of how each herbicide acts in Florida aquatic systems. The following parameters are evaluated when considering this herbicide to manage aquatic plants in a specific waterbody. Each parameter is linked to an explanation and examples are provided to demonstrate their relevance to developing comprehensive aquatic plant management strategies.

These considerations are not meant to replace or contradict the EPA approved label.
Always read and follow the pesticide label for each product used.

Table A: Herbicide Use Patterns for Diquat

Target Plant	Scientific Name	Use Pattern	Compatible Herbicides
Hydrilla	Hydrilla verticillata	Frequent	Mostly used in combination with potassium endothall; occasionally used alone or in combination with flumioxazin, bispyribac, or topramezone
Water lettuce	Pistia stratiotes	Frequent	Used alone
Water hyacinth	Eichhornia crassipes	Frequent	Used alone or in combination with 2,4-D
Cuban club-rush	Cyperus blepharoleptos	Frequent	Usually in combination with 2,4-D
Wright’s nutrush	Scleria lacustris	Occasional	Used alone
Feathered mosquitofern	Azolla pinnata	Occasional	Used alone
Giant salvinia	Salvinia molesta	Spot / eradication	Used alone

Table B: Water Uses and Functions

Water Use Parameters	Management Considerations
Downstream Uses and Needs	Water-use restrictions depend on rate 1-3 day restriction for ornamental plant watering 1-5 day restriction for irrigating crops
Fish and Wildlife Mgmt.
Vegetation planting	Avoid applications within newly installed aquatic revegetation sites
Forage and prey	Diquat dibromide has a low bioaccumulation potential
Fisheries	Slight to moderate toxicity to fish 8-hour LC₅₀ is 12.3 mg/L for rainbow trout and 28.5 mg/L for Chinook salmon Do not use in waters used for fish protein concentrate or fish meal production
Non-game wildlife	No issues related to this tool
Endangered species	No issues related to this tool
Waterfowl	Moderately toxic to birds Mallard duck oral LD₅₀ is 564 mg/kg
Flood Control	No issues related to this tool
Navigation and Access	No issues related to this tool
Irrigation	Water-use restrictions depend on rate 1-3 day restriction for ornamental plant watering 1-5 day restriction for irrigating crops
Livestock Consumption	1-day water-use restriction if treated waterbody is the sole source for watering
Potable Water	Rate-dependent setbacks of 350-1600 feet from functioning potable water intake See label for specific information Within these distances, shut off water intake for 1-3 days depending on rate, or Until concentration falls below 0.02 ppm Coordinate with water facility manager for applications within 1600 feet of an active potable-water intake
Recreation
Boating	No issues related to this tool
Fishing	No fish consumption restriction Does not bioaccumulate in fish
Hunting	No issues related to this tool
Swimming	No swimming restriction

Table C: Herbicide, Waterbody, Plant, and Climate Parameters

Herbicide Parameters	Management Considerations
Herbicide Rate	Maximum label rate for controlling floating plants is 0.75 gpa Generally applied at 0.25 to 0.5 gpa Applied at concentrations up to 0.37 ppm for submersed plant (hydrilla) control
Breakdown / Inactivation	Almost immediately bound to soil or vegetation Tightly bound residues are not biologically available, so the herbicide is deactivated on soil Bound residues are resistant to microbial degradation Soil dissipation half-life exceeds 3 years
Microbial	Broken down slowly by microbes if not bound to sediments
Adsorption	Binds tightly to clay and organic material Do not apply in muddy or turbid waters for submersed plant control Avoid tank mixing with muddy or turbid water before applying to emergent foliage
Photolysis	Broken down slowly by sunlight if not bound to sediments
Hydrolysis	Not sensitive to hydrolysis
Dissipation	Dissipation half-life in water is 1-2 days as diquat is bound to sediment and deactivated
Formulation
Liquid	Available in liquid formulation
Solid	Not available in solid formulation
Mechanism of Action	Classified in WSSA Resistance Grouping #22 Photosystem I inhibitor
Mode of Action
Contact	Fast acting, contact-type herbicide: rapidly absorbed by plant leaves Interferes with cell respiration, the process by which plants take in oxygen Plant tissues are killed too quickly to allow translocation to other parts of the plant
Plant Growth Regulator	Used as herbicide for aquatic plant control applications in FWC programs Functions as growth inhibitor or growth regulator at lower rates
Stewardship
Herbicide resistance	Resistance confirmed in Landoltia species of duckweed in Florida Rotate or combine with other compounds where possible Used in combination with endothall for submersed plant control Combine with 2,4-D for mixed water hyacinth / water lettuce populations
Waterbody Parameters	Management Considerations
Hydrology
Water depth	Avoid disturbing organic or clay sediments in shallow waters during submersed plant control (binds tightly and is inactivated in turbid waters)
Water volume	Use at rates up to 0.37 ppm for submersed plant control Amounts used are depth-dependent – see label for chart
Water movement	At least 24 hours of contact are required for submersed plant control
Water chemistry
Dissolved oxygen (DO)	Fast-acting when used in combination with endothall for submersed plant control Use caution in large-scale treatments in warm water to avoid dissolved oxygen depletion Treat only 1/3 to 1/2 of the water body at one time – wait 14 days between treatments
pH, alkalinity, hardness	No issues related to this tool
Nutrient content	Fast-acting contact type herbicide Nutrients may be released from decomposing plants in large-scale treatments
Water transparency	Color/tannic content: no issues Turbidity: avoid mixing with, or applying to turbid waters Binds tightly to clay and organic particles
Sediment characteristics
Composition	Sand: no issues related to this tool Clay: binds tightly to, and is inactivated in waters with suspended clay particles Organic: binds tightly to suspended organic particles and organic sediments
Potential for re-suspension	Avoid stirring flocculent sediments during submersed plant applications Especially in shallow water Avoid drawing clay or organic turbid water for tank mixes using diquat
Plant Physiology Parameters	Management Considerations
Plant origin / growth potential
Native	Illinois pondweed (Potamogeton illinoensis), southern naiad (Najas guadalupensis) Occasionally controlled for access or navigation 1.0 ppm potassium endothall + 0.37 ppm diquat
Non-native	Water fern (Salvinia minima) Occasional control for access or navigation in confined areas 0.5-0.75 gpa
Invasive	Hydrilla – where suspended organics or clay are not a concern Poor or inconsistent control of hydrilla when used alone 0.37 ppm diquat + 1 ppm potassium endothall (2 ppm endothall for mature hydrilla) Rapid knockdown of hydrilla mat Requires shorter retention time and faster control than endothall alone Resistance management strategy Cutting trails or small exposed bands 0.37 ppm diquat + 0.10 ppm flumioxazin Use in clear, low pH waters due to short flumioxazin half-life in pH > 7 Very active on water lilies Resistance management strategy Floating plants (water hyacinth / water lettuce) 1-2 qt/ac for smaller water hyacinth – up to 3 qt/ac for mature “Bull” hyacinth Spray to wet – need complete coverage 1-2 qt/ac for water lettuce 1 qt diquat + 2 qt 2,4-D/ac for hyacinth/lettuce mix and resistance management Cuban club-rush and Wright’s nutrush 2 qt/ac diquat or 1 qt diquat + 2 qt 2,4D/ac Giant salvinia (Salvinia molesta) and Feathered mosquitofern (Azolla pinnata) 2 qt/ac
Plant growth stage (target/non-target)	Apply to actively growing target plants Lower rates may be applicable for young, actively growing plants
Plant susceptibility (target/non-target)	Resistance confirmed in Landoltia species of duckweed in Florida Good efficacy alone or combined with 2,4-D for water hyacinth/water lettuce control Use in place of 2,4-D to control water hyacinth mixed with bulrush (Schoenoplectus spp.) Repeated use or higher doses of 2,4-D are toxic to bulrush Diquat may spot or brown bulrush stems, but they quickly recover Controls some beneficial plants like southern naiad, Illinois pondweed, strapleaf sagittaria (Sagittaria kurziana) Spatter-dock (Nuphar spp.), jointed spikerush (Eleocharis interstincta) Minimize non-target effects by applying when native plants are dormant Or before target plants mature or covers large areas
Potential for regrowth (target/non-target)	Longer term control when applied to actively growing immature hydrilla Temporary injury reported for soft-stem bulrush in greenhouse studies
Climate Parameters	Management Considerations
Weather	Daily Need at least 30 minutes of exposure for floating or emergent plants Need at least 24 hours of contact time for submersed plant control with endothall Seasonally Several months of hydrilla control when applied in fall or winter 1-3 months of control when applied to mature hydrilla in warm water
Light intensity	Low intensity may reduce submersed plant ability to recover after application
Water temperature	Apply with water temperature above 50o F to ensure active growth of target plants Plants must be actively growing for plant to take up herbicide Do not apply for hydrilla control if strong thermocline exists Or use deep and shallow trailing hoses to treat the entire water column

Table D: Other Parameters

Parameter	Management Considerations
Cost	Generics are available Alternate herbicides with equivalent efficacy may be available for some species
Anticipated Control Amount
Spatial	Contact-type herbicide – generally does not disperse far outside submersed plant treatment area Acres of submersed plants controlled equals area to which diquat (+ endothall) is applied Need to treat the entire water column
Duration	Provides good water hyacinth/water lettuce control Regrowth is generally from seed or new plants drifting from outside control zone Hydrilla control varies (1 to 6 months) based on water temperature and plant growth stage
Time to Achieve Control	Fast-acting Floating plant symptoms within hours after application – control in several days to one week Submersed plant control in 1-2 weeks after application
Contractor/Equipment	Apply by hand gun from airboat for small acreages of floating plants Apply aerially by helicopter for larger acreages of submersed plants Apply with endothall by trailing hoses from airboat for submersed-plant control, especially smaller acreages of hydrilla (1 to 200 acres)