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The Practical Guide to Lake Management in Massachusetts
TREATMENT WITH DIQUAT
How it Works
Diquat is a fast acting contact herbicide, producing results within 2 weeks of application through disruption of photosynthesis. It is a broad-spectrum herbicide with potential risks to aquatic fauna, but laboratory indications of invertebrate toxicity have not been clearly documented in the field. A domestic water use restriction of 3 days is normally applied. Irrigation restrictions of 2 to 5 days are applied, depending on dose and crop to be irrigated. Regrowth of some species has been rapid (often within the same year) after treatment with diquat, but two years of control have been achieved in some instances. Concentrations in treated water should not exceed 2 mg/L, and are usually no more than half that dose in Massachusetts.
Diquat is used as a general purpose aquatic herbicide, both as a primary control agent for a broad range of macrophytes and as a follow-up treatment chemical for control of plants (especially milfoil) missed by other herbicides or physical control techniques. Treatment with diquat is recommended early in the season to impact early growth stages, but can be applied any time. Usage in Massachusetts has shown that the effects of diquat are generally visible after 2-3 days and plants are controlled within 7-10 days. Diquat is less effective in turbid, muddy water due to adsorbance onto sediments and other particles. Since diquat is a broad spectrum herbicide, it can be expected to impact non-target plants when they are present. Loss of vegetative cover may have some impact on aquatic animals, but short-term effects are not expected. The acute toxicity of diquat for fish is highly variable depending on species, age, and hardness of water. Young fish are more sensitive than older fish. Toxicity is decreased as water hardness increases. Toxicity is rare at doses applied in Massachusetts.
Field concentrations of diquat are hard to maintain because diquat rapidly sorbs to the sediments. Maximum concentrations based on the Reward brand label are currently 0.72 ppm as the cation, based on the maximum rate of 2 gallons per acre in areas deeper than 2 feet. For water less than or equal to 2 feet in average depth, a maximum of 1 gallon of Reward per acre is allowed. Normally Reward is used at a rate of 1 gallon per surface area in Massachusetts waters with an average depth of 4 feet. This typically renders a concentration of 0.1 ppm of active ingredient. Treatment doses are therefore not expected to exceed thresholds for potential toxicity. Other formulations of diquat may have different dose restrictions, but concentrations tend to be low relative to maximum allowable rates.
Benefits
Effective against a wide variety of species root systems that may
Relatively rapid kill of targeted vegetation generate regrowth. It is
Can be used for spot treatments; limited drift or impact outside often used for spot
target area treatment of limited
areas as a follow-up to
Detriments
Not very selective; kills most species contacted treatment with another
herbicide, but is also
Does not damage portions of plants with which it does not contact; used where other
regrowth from roots is common herbicides are less
Potential for toxicity to fauna, but uncommon in practice effective.
Information for Proper Application
Knowledge of lake and downstream water uses
Inventory of aquatic biota with emphasis on sensitive species
Mapping of aquatic vegetation with accurate identification of all species and general appraisal of
relative abundance and overall cover/biomass
Water quality data that facilitate dose planning and evaluation of effectiveness and
suspended solids/water clarity, hardness, dissolved oxygen and temperature should be
at a minimum
Treatment plan to include dose, areas treated, expected alteration of plant community, ar
up activities
Knowledge of use restrictions after treatment
Monitoring program for assessing effectiveness and impacts
Factors Favoring the Use of this Technique
An invasive plant species has been detected as patches of dense growth but is not am(
physical control techniques
Overall vegetative density is excessive over a large portion of the lake, negatively affect
and water uses, and is not amenable to alternative control methods
Localized control of plants is needed either to support localized use (e.g., swimming an
follow-up to alternative controls
Performance Guidelines
Map plant community and note density and distribution of target and non-target species;
of protected species may prevent treatment
Application must be performed by licensed applicators
Apply diquat product in accordance with label instructions and restrictions; justify dose,
and timing of treatment
Where a large portion of the lake is treated, apply diquat in strips or zones to provic
refuges
Monitor water quality before and after treatment, with emphasis on-oxygen and nutrient
more than 10% of lake is treated
Monitor plant community features before and after treatment
Possible Permits
WPA permit through local Conservation Commission/DEP
Review by NHESP (further action if protected species are present)
License to Apply Chemicals from DEP
Impacts Specific to the Wetlands Protection Act
Protection of public and private water supply Neutral
e Protection of groundwater supply Neutral no interaction as diquat is adsorbed to soil par
Storm damage prevention Neutral (no significant interaction)
Prevention of pollution Generally neutral (no significant interaction), but could be a del
plant die-off causes low oxygen at the lake bottom
Protection of land containing shellfish Generally neutral (no significant interaction), but
algae might reduce food resources for shellfish, and direct toxicity is possible under
circumstances
Protection of fisheries Possible benefit (habitat enhancement) and possible detrime
source alteration, loss of cover)
Protection of wildlife habitat Possible benefit (habitat enhancement) and possible detrim(
Cost Considerations
Diquat treatments typically cost $200 to $500 per acre.
Management Techniques: Diquat
As a contact herbicide, diquat is relatively non-selective and will leave root systems that may generate regrowth. It is often used for spot treatment of limited areas as a follow-up to more selective lakewide treatment with another herbicide, but is also used where other herbicides are less effective.
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