Diquat Considerations

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.

Table A: Herbicide Use Patterns for Diquat

Target Plant Scientific Name Use Pattern Herbicides
Water lettuce Pistia stratiotes Frequent diquat
Water hyacinth Eichhornia crassipes Frequent diquat, or diquat + 2, 4-D
Hydrilla Hydrilla verticillata Small-scale or spot control diquat + endothall
Duckweed Spirodela and Lemna species Frequent diquat
Salvinia Salvinia minima and molesta Frequent diquat

Table B: Water Uses and Functions

Water Use Parameters Management Considerations
Downstream Uses and Needs Rate depends on water-use restrictions (1 to 3 day restriction for ornamental plant watering, 1 to 5 day restriction for crops)
Fish and Wildlife Mgmt.  
Vegetation planting Avoid applications within newly planted aquatic revegetation sites
Forage and prey No issues related to this tool
Fisheries 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 No issues related to this tool
Flood Control No issues related to this tool
Navigation and Access No issues related to this tool
Irrigation Rate depends on water-use restrictions (1 to 3 day restriction for ornamental plant watering, 1 to 5 day restriction for crops)
Livestock Consumption 1 day water-use restriction
Potable Water Rate-dependent setbacks of 350 to 1600 feet from functioning potable water intake: see label for specific information
Within these distances, shut off water intake for 1 to 3 days depending on rate, or
Until concentration falls below 0.02 ppm
Coordinate with the water facility manager for all applications within 1600 feet of an active potable-water intake
Boating No issues related to this tool
Fishing No fishing restrictions
Hunting No issues related to this tool
Swimming No swimming restrictions

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 hyrilla control
Breakdown / Inactivation Less than 48 hours half life in water reported
Adsorption Binds tightly to clay: do not use in muddy or turbid waters for submersed plant control
Microbial Broken down microbially
Photolysis Broken down by sunlight
Dissipation Highly soluble, but rapidly taken up by plants or adsorbed to clay and organic particles, so dissipation is slow
Liquid Available in liquid formulations only
Mode of Action
Contact Fast acting, contact-type herbicide: rapidly absorbed by plant leaves. Interferes with plant cell respiration
Systemic Little to none: injures plants too rapidly to be translocated within plant tissues
Plant Growth Regulator Not used as a plant growth regulator
Herbicide resistance Resistance confirmed in some species of duckweed in Florida
Rotate other compounds where possible
Used alone for most floating plant applications. Therefore, consider rotating with other herbicides
Used in combination with endothall or occasionally copper compounds  for submersed plant control
Microbial degradation Taken up rapidly by plants and broken down slowly by microbes. Repeated use does not affect enhanced microbial degradation.
Waterbody Parameters Management Considerations
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 or copper for submersed plant control
Use caution in large-scale treatments in warm water to avoid dissolved oxygen depletion
Alkalinity, ph, hardness No issues
Nutrient content
Fast-acting 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  
Sand/clay: binds tightly to, and is inactivated in waters with suspended clay particles
Organic: binds tightly to suspended organic particles
Potential impacts from sediment resuspension Avoid stirring flocculent sediments during submersed plant applications, especially in shallow water
Avoid drawing turbid water for tank mixes using diquat
Plant Physiology Parameters Management Considerations
Plant origin/ growth potential  
Native Frequent use for water lettuce and water meal control: infrequent for cattail control
Non-native Frequent use for salvinia control
Invasive   Frequent use for water lettuce and water hyacinth control
Occasional use for small-scale or spot hydrilla control
Plant growth stage (target/non-target) Lower rates may be applicable for young, actively growing plants  
Plant susceptibility (target/non-target)
Apply to actively growing target plants: resistance confirmed in some duckweed species. Good efficacy for water hyacinth/water lettuce control.
Used alone or in combination with 2,4-D to control water hyacinth mixed with water lettuce
Used in place of 2,4-D to control water hyacinth when mixed with bulrush (Scirpus spp.)
Bulrush provides valuable cover and substrate for fisheries: the seeds are used by waterfowl
Repeated use or higher doses of 2,4-D are toxic to bulrush
Diquat may spot or brown bulrush stems, but they quickly recover
Poor or inconsistent control of hydrilla when used alone
Used in combination with copper herbicides to eradicate new patches of hydrilla, especially at boat ramps
Used in combination with endothall to control small to moderate areas of hydrilla in quiescent waters
Diquat rates of 0.37 ppm; endothall rates of 1 to 3 ppm
Control is relatively fast
Faster than with endothall alone
Hydrilla mats begin to collapse in as little as 24 hours
Controls some beneficial submersed plants like southern naiad, Illinois pondweed, and strapleaf sagittaria
Minimize non-target effects by applying when native plants are dormant or before hydrilla covers large areas
Potential for regrowth (target/non-target) Very effective for long-term control of water hyacinth and water lettuce
Regrowth is mostly from seeds and new plants flushed into the area
Hydrilla control varies with season and plant growth stage
Regrowth from turions, tubers, and surviving root crowns: generally 3 to 6 months
Commingled non-target plants may be spotted or browned during floating plant control, but usually recover
Climate Parameters Management Considerations
Weather Daily
Need at least 30 minutes of contact for floating or emergent plants
Several months of hydrilla control when applied in fall or winter
1 to 3 months of control when applied to mature hydrilla in warm water
Light intensity Low intensity reduces submersed-plant ability to recover
Water temperature Apply with water temperature generally above 50oF 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 available
Anticipated Control Amount  
Spatial Acres
Contact-type herbicide generally does not disperse outside treatment area
Acres of submersed plants controlled generally equals acres of plants to which diquat (+ endothall) is applied
Percent of Water Column: need to treat the entire water column
Duration Provides good water hyacinth/water lettuce control
Hydrilla control varies (1 to 6 months) based on water temperature and plant growth stage
Time to Achieve Control Fairly fast-acting for floating plants (a few days) and submersed plants (several days)
Contractor/Equipment Apply by hand gun and 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 small acreages of hydrilla (1 to 200 acres)
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