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 PlantScientific NameUse PatternHerbicides
Water lettucePistia stratiotesFrequentdiquat
Water hyacinthEichhornia crassipesFrequentdiquat, or diquat + 2, 4-D
HydrillaHydrilla verticillataSmall-scale  or spot controldiquat + endothall
DuckweedSpirodela and Lemna speciesFrequentdiquat
SalviniaSalvinia minima and molestaFrequentdiquat

Table B:  Water Uses and Functions

Water Use ParametersManagement Considerations
Downstream Uses and NeedsRate 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 plantingAvoid applications within newly planted aquatic revegetation sites
Forage and preyNo issues related to this tool
FisheriesDo not use in waters used for fish protein concentrate or fish meal production
Non-game wildlifeNo issues related to this tool
Endangered speciesNo issues related to this tool
WaterfowlNo issues related to this tool
Flood ControlNo issues related to this tool
Navigation and AccessNo issues related to this tool
IrrigationRate depends on water-use restrictions (1 to 3 day restriction for ornamental plant watering, 1 to 5 day restriction for crops)
Livestock Consumption1 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
Recreation 
BoatingNo issues related to this tool
FishingNo fishing restrictions
HuntingNo issues related to this tool
SwimmingNo swimming restrictions

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

Herbicide ParametersManagement 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 / InactivationLess than 48 hours half life in water reported
AdsorptionBinds tightly to clay: do not use in muddy or turbid waters for submersed plant control
MicrobialBroken down microbially
Photolysis  Broken down by sunlight
DissipationHighly soluble, but rapidly taken up by plants or adsorbed to clay and organic particles, so dissipation is slow
Formulation 
LiquidAvailable in liquid formulations only
Mode of Action
 
ContactFast acting, contact-type herbicide: rapidly absorbed by plant leaves. Interferes with plant cell respiration
SystemicLittle to none: injures plants too rapidly to be translocated within plant tissues
Plant Growth RegulatorNot used as a plant growth regulator
Stewardship 
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 degradationTaken up rapidly by plants and broken down slowly by microbes. Repeated use does not affect enhanced microbial degradation.
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 movementAt 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, hardnessNo 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 
Composition
  • 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
  • Seasonal
    • 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 intensityLow 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

ParameterManagement Considerations
CostGenerics 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 ControlFairly 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)

Last updated: 03 October 2012