Overview
Herbicides are chemicals that adversely affect the physiological activity and development of plants, and are used to control vegetation by causing death or suppressing growth. These compounds, as active ingredients, are incorporated into a wide variety of commercial herbicide formulations, for application to plants and to surrounding soil or water as sprays, granules, and liquid concentrates.
Today, herbicides are the major method of weed control in noncrop situations as well as in agriculture because they are effective, reliable, cost-efficient, safe and easy to use. They are applied in thousands of formulations with a wide variety of equipment, ranging from airplanes to hand sprayers.
Most herbicides do not have as much potential to be toxic to humans and animals as other pesticides, but all must be used with care and with full awareness of the problems they may cause if applied improperly.
Herbicide Registration and Label Precautions
Herbicides sold in the United States must be registered with the Federal government,
and in some cases by state regulatory agencies. They are reviewed and regulated
by the U.S. Environmental Protection Agency (USEPA) under the Federal Insecticide,
Fungicide, and Rodenticide Act (FIFRA 1974; 7 J.S.C. 135 et seq., Public Laws 92-516,
94-140, and 95-356) and recent amendments.
The printed information and instructional material that is sold with a registered
herbicide is known as the "label," and constitutes a legal document. These instructions
are considered a part of compliance with FIFRA and other Federal regulations, and
failure to use a herbicide in accord with label restrictions can lead to severe
penalties. The label provides information on the chemical compound(s) comprising
the active ingredient(s) of the herbicide, directions for correct use on target
plant species, warnings and restrictions, and safety and antidote information.
Labels change frequently, and the herbicide’s current label should be reviewed for
the most recent conditions or restrictions before it is used. Read all labels carefully
and comply with their site-use directions. For the very latest label information
on a given herbicide, contact the manufacturer, or the company or distributor that
sells the product.
Current pesticide labels can be accessed on-line through the Pesticide Product Label
System (PPLS) provided by the EPA. This information can be found at
http://www.epa.gov/pesticides/pestlabels/index.htm.
It is important to remember that aquatic herbicide products are safe when used according
to the label. The U.S. Environmental Protection Agency (EPA) approved label provides
guidelines protecting the health of the environment, the humans using that environment,
and the applicators of the herbicide. Currently, no product can be labeled for aquatic
use if it poses more than a one in a million chance of causing significant damage
to human health, the environment, or wildlife resources. In addition, it may not
show evidence of biomagnification, bioavailability, or persistence in the environment
(Joyce 1991). In most states, there are additional permitting or regulatory restrictions
on the use of these herbicides. Some states prohibit the use of certain herbicides,
or the application of herbicides to sensitive areas such as irrigation canals. In
these states, application without a specific permit is illegal. The individual label
lists those jurisdictions or situations where use is not allowed. Since State and
local regulations regarding herbicide use may be more restrictive than Federal regulations,
always check and comply with all state and local regulations. Check annual updates
from state regulatory and environmental agencies for changes in label restrictions
and application policies or permit requirements, before developing or implementing
any plans for applying herbicides.
An important part of any aquatic herbicide label is the restrictions on use of treated
water, and the time periods that must be imposed before treated water can be used
by humans or livestock, or for fishing, irrigation, swimming, use of fish and shellfish,
etc.
There are certain herbicides that are registered as "Restricted Use" herbicides.
They can be legally applied only by trained and certified applicators or by people
under their direct supervision. These are compounds or formulations that have a
high potential to harm humans (if not used according to label guidance), or to damage
nontarget species.
Classification of Herbicides
Herbicides can be grouped on the basis of their chemical structure and physiological action, and on the timing and method of their application. Chemicals with similar structure usually produce the same type of physiological reaction in plants, and control similar species. Herbicides labeled for aquatic use can be classified as either "contact" or "systemic" (Table 1). Contact herbicides act immediately on the tissues contacted, typically causing extensive cellular damage at the point of uptake but not affecting areas untouched by the herbicide. Typically, these herbicides are faster acting, but they do not have a sustained effect, in many cases not killing root crowns, roots, or rhizomes. In contrast, systemic herbicides are translocated throughout the plant. They are slower acting but often result in mortality of the entire plant, including crowns and rhizomes.
Table 1. Characteristics of U.S.
Environmental Protection Agency – approved aquatic herbicides
|
Compound
|
Trade Name
|
Company
|
Formulation
Contact vs. Systemic
|
Mode of Action
|
|
Complexed Copper
|
Cutrine-Plus
Clearigate
Nautique
Captain
Komeen
K-tea
|
Applied
Biochemists
SePRO
|
Various complexing
agents with copper, superior to CuSO4
|
Plant cell toxicant
|
|
2,4-D
|
Navigate
Aqua-Kleen
DMA 4 IVM
Weedar 64
|
Applied
Biochemists
United Phosphorus
Dow AgroSciences
Nufarm
|
BEE salt
DMA liquid
Systemic
|
Selective plant-growth
regulator
|
|
Carfentrazone-ethyl
|
Stingray
|
FMC Corporation
|
Liquid
Contact
|
Disrupts chlorophyll synthesis, cell membrane
disruption
|
|
Diquat
|
Reward
Weedtrine-D
Littora
|
Syngenta
Applied
Biochemists
SePRO
|
Liquid
Contact
|
Disrupts plant cell
membrane integrity
|
|
Endothall
|
Aquathol K
Aquathol SuperK
Hydrothol 191
|
United Phosphorus
|
Liquid or granular
Contact
|
Inactivates plant
protein synthesis
|
|
Fluridone
|
Sonar AS
Sonar SRP
Sonar PR
Sonar Q
Avast!
|
SePRO
|
Liquid or granular
Systemic
|
Disrupts carotenoid
sysnthesis
|
|
Glyphosate
|
Rodeo
AquaPro
Aquamaster
AquaNeat
Touchdown
|
Dow AgroSciences
SePRO
Monsanto
United Phosphorus
Syngenta
|
Liquid
Systemic
|
Disrupts synthesis of
amino acids
|
|
Imazamox
|
Clearcast
|
BASF
|
Liquid
Systemic
|
Disrupts synthesis of
amino acids
|
|
Imazapyr
|
Habitat
AquaPier
|
BASF
SePRO
|
Liquid
Systemic
|
Interupts DNA synthesis
& cell growth
|
|
Penoxsulam
|
Galleon SC
|
SePRO
|
Liquid
Systemic
|
Disrupts synthesis of
amino acids
|
|
Triclopyr
|
Garlon 3A
Renovate 3
Renovate OTF
|
Dow AgroSciences
SePRO
|
Liquid
Systemic
|
Selective plant growth
regulator
|
Contact herbicides act immediately on the tissues contacted, typically
causing extensive cellular damage at the point of uptake but not affecting areas
untouched by the herbicide. Typically, these herbicides are faster acting, but they
do not have a sustained effect, in many cases not killing root crowns, roots, or
rhizomes. In contrast, systemic herbicides are translocated throughout the plant.
They are slower acting but often result in mortality of the entire plant, including
crowns and rhizomes.
a) Complexed copper compounds include a variety of formulations from different companies,
under different names and labels, in which copper is chelated in an organic complexing
agent that keeps it in solution. Formerly, copper sulfate was used in applications
predominantly for the control of phytoplankton. However, the copper rapidly precipitated,
especially in harder water, and was no longer available. This problem was solved
by the development of complexed copper agents. Complexed copper is very effective
for algal control, somewhat effective for several vascular plants (particularly
hydrilla), and is also used in tank mixes with diquat to increase its effectiveness.
b) 2,4-D is a widely-used aquatic herbicide for many broadleaf species, such as
Eurasian watermilfoil. A selective systemic herbicide, it effectively controls broadleaf
plants with a relatively short contact time, but does not generally harm the pondweeds
or water celery (vallisneria). However, it is also not effective against hydrilla,
elodea or northern naiad.
c) Carfentrazone-ethyl is a fast acting contact herbicide. It causes desiccation within a few hours; necrosis and death occur in subsequent weeks. It is not effective on roots, rhizomes or tubers. Carfentrazone can be used to treat both submersed and emergent/floating plants.
d) Diquat is a contact herbicide which will act on a very short contact time. It
causes a rapid die-off of the shoot portions of the plant it contacts, but is not
effective on roots, rhizomes or tubers, requiring subsequent applications. Diquat
will bind to particulate and dissolved organic matter, which restricts its use in
some water bodies. It is also effective in a tank mix with copper compounds.
e) Endothall is another contact herbicide. Unlike Diquat, it is not affected by
particulates or dissolved organic material. It should not be used in tank mixtures
with copper, as it can have an antagonistic reaction with chelated copper compounds.
The dimethylamine salt formulations need to be used strictly according to the label
in order to avoid damage to fish populations.
f) Fluridone is a nonselective systemic aquatic herbicide. It requires very long
exposure times but may be effective at very low concentrations (parts per billion
rather than parts per million). Fluridone is widely used for both hydrilla and Eurasian
watermilfoil management. It appears to work best where the entire lake or flowage
system can be managed, but not in spot treatments or high water exchange areas.
g) Glyphosate is not used on submersed plants but is a very useful broad-spectrum
systemic herbicide for emergent and shoreline aquatic and wetland plants such as
spatterdock and cattail.
h) Imazamox is a systemic herbicide that can be used to control submersed, emergent and floating vegetation. It is active on both monocot and broadleaf aquatic plants.
i) Imazapyr is not used on submersed plants but is a broad-spectrum systemic herbicide
for emergent and floating leaf plants such as cattail and water hyacinth.
j) Penoxsulam is a nonselective systemic herbicide that requires a very long exposure period. It should be use in quiescent bodies of water or on exposed sediments of quiescent bodies of water. Penoxsulam can be used to control submersed, floating and emergent plants.
k) Triclopyr is a selective systemic herbicide, similar in activity to 2,4-D, and
can effectively control broadleaf plants, including Eurasian watermilfoil, without
harming desirable species such as pondweeds or vallisneria. It is not effective
against hydrilla.
Appropriate Application Techniques
In treating submersed species, the applicator is actually treating the water with
a herbicide, and allowing the plant to take up herbicide from the water. The applicator
needs to know the exchange rate of the water in order to know how long the plant
will be exposed to the herbicide at the required treatment concentration (Getsinger
et al. 1991). The response of different plant species to different herbicides is
a function of the properties of both the plant and the herbicide. The applicator
also needs to match a herbicide with an appropriate concentration and exposure-time
relationship for the target species (Netherland 1991)
The concentration and exposure-time relationship for a given compound have been
determined from laboratory experiments. For instance, if it is known from water
exchange studies that the exposure time will ensure only 24 hours of contact with
1 mg/liter of 2,4-D if applied at full label rate, then a 75% control rate for Eurasian
watermilfoil can be expected. If longer exposure times are expected, then lower
concentrations can be applied. Where lower application rates are possible, they
allow applicators to save money on herbicides and to introduce a lower total amount
of herbicide into the aquatic environment. For faster exchange rates, the applicator
will have to use higher concentrations of the contact herbicides such as diquat
or endothall; slower exchange rates may allow the use of systemic herbicides. However,
some systems are limited in selecting herbicides for use, because it is never permissible
to use concentrations of herbicides higher than the allowed EPA maximum label rate.
When treating emergent plants, any label directions that prohibit application of
the compound to water must be carefully followed.
Selectivity
Some herbicides (e.g., 2,4-D and triclopyr) are intrinsically selective, being very
effective for controlling broadleaf plants such as Eurasian watermilfoil but not
narrow-leaved plants or grasses such as hydrilla
Table 2. Use suggestions for US
Environmental Protection Agency – approved aquatic herbicides
|
Compound
|
Exposure Time
|
Advantages
|
Disadvantages
|
Systems where used
effectively
|
Plant species response
|
|
Complexed Copper
|
Intermediate (18-72
hours)
|
Inexpensive, rapid
action, approved for drinking water
|
Does not biodegrade,
but biologically inactive in sediments
|
Lakes, higher exchange
rate areas
|
Broad spectrum, acts in
7-10 days or up to 4-6 weeks
|
|
2,4-D
|
Intermediate (18-72
hours)1
|
Inexpensive, systemic
|
Non-target plants may
be affected
|
Lakes and slow flow
areas
|
Selective to
broadleaves, acts in 5-7 days up to 2 weeks
|
|
Carfentrazone-ethyl
|
Short (hours)
|
Rapid action, degrades
quickly
|
Does not effect
underground portions
|
Shoreline, localized
treatments, higher exchange rate areas
|
Broad spectrum,
symptoms occur in hours, plant death in subsequent weeks
|
|
Diquat
|
Short (12-36 hours)2
|
Rapid action, limited
drift
|
|
Shoreline, localized
treatments, higher exchange rate areas
|
Broad spectrum, acts in
7 days
|
|
Endothall
|
Short (12-36 hours)3
|
Rapid action, limited
drift
|
Does not affect
underground portions
|
Shoreline, localized
treatments, higher exchange rate areas
|
Broad spectrum, acts in
7-14 days
|
|
Fluridone
|
Very long (30-60 days)4
|
Very low dosage
required, few label restrictions, systemic
|
Very long contact
period
|
Small lakes, slow
flowing systems
|
Broad spectrum, acts in
30-90 days
|
|
Glyphosate
|
Not applicable
|
Widely used, few label
restrictions, systemic
|
Very slow action, no
submersed control
|
Emergent &
floating-leaved plants only
|
Broad spectrum, acts in
7-10 days, up to 4 weeks
|
|
Imazamox
|
7-14 day ½ life
|
Systemic, few label
restrictions
|
Growth regulation of
submersed plants not plant death
|
Quiescent bodies of
water
|
Growth regulation of
submersed plants, acts in 1-2 weeks or more for foliar applications
|
|
Imazapyr
|
Not applicable
|
Systemic
|
No submersed control
|
Emergent &
floating-leaved plants only
|
Nonselective, acts in
2-4 weeks, up to 6-8 weeks
|
|
Penoxsulam
|
Very long (60 days)
|
Selective, few label
restrictions, systemic
|
Very long contact
period
|
Quiescent bodies of
water
|
Broad spectrum, acts in
60 to 120 days
|
|
Triclopyr
|
Intermediate (12-60
hours)5
|
Selective, systemic
|
Can injure other nearby
broadleaved species
|
Lakes & slow flow areas
|
Selective to
broadleaves, acts in 5-7 days, up to 2 weeks
|
1 Green and Westerdahl 1990
2 Westerdahl 1987
3 Netherland et al. 1991
4 Netherland 1992
5 Netherland and Getsinger 1992
Other herbicides may be used selectively only through application
based on the target and non-target plant's biology.
Exposure Time
The length of time that a herbicide can be maintained in the water column touching the plant is usually the primary consideration when choosing a chemical for aquatic weed control. The physiological action of each herbicide, and whether it needs to be taken up systemically by the plant or can be effective through contact alone, requires different periods of exposure.
Thus, the hydrology and type of aquatic system must be considered when selecting a herbicide for aquatic plants.
For example, lotic (flowing water) systems may require consideration of a different herbicide than lentic (quiescent water) systems. Factors such as flow rate or retention time may prevent sufficient plant/herbicide contact time to achieve the desired level of control.
In those cases, temporary decrease in flow, or the use of impoundments, may be considered.
Applications may be repeated to maintain the required concentration, as long as the total applied does not exceed labeled rates. Some aquatic herbicide label restrictions may specifically prohibit use of a herbicide in flowing waters. For those aquatic systems that encompass both flowing and static waters, the specific water management units (i.e., aquatic environment containing the nuisance plants) must be considered, and appropriate herbicides selected.
Below is a table that illustrates contact time needed to achieve control as related to herbicide, action, and concentration.
Fast-acting contact herbicides such as copper, diquat and endothall can eliminate plants after only 4 to 24 hours of exposure time. 2,4-D and triclopyr can be effective if plants are exposed for a few days. Fluridone, in contrast, requires exposure times greater than 60 days to eliminate plants, because it does not kill tissue directly.
Location of Application
Some states prohibit the use of certain herbicidal chemicals or formulations, or restrict use to limited areas. Sometimes local conditions are such that problems will result from chemicals that can be used safely elsewhere. Note that the restrictions given on the herbicide label are legally binding.
The following herbicides are not labeled for use in New York:
| Diquat |
REWARD |
| Diquat |
LITTORA |
The following herbicides have recently received Section 3 national labels for use in aquatic sites by the USEPA and are currently undergoing state reviews (Jan. 08):
| Penoxsulam |
GALLEON SC |
| Imazamox |
CLEARCAST |
Herbicide Guides
For the very latest label information on a given herbicide, contact the manufacturer,
or the company or distributor that sells the product.
Numerous books are available on herbicides and their use in vegetation control.
The following books focus on compounds and commercial formulations and on specific
aquatic weeds:
Herbicide Handbook, 1994 (7th edition)
available from: Weed Science Society of America
1508 West University Avenue
Champaign, Illinois 61821-3133
Telephone: (217) 352-4212
Weed Control Manual, published biennially
available
from: Meister Publishing Co.
37733 Euclid Avenue
Willoughby, Ohio 44094-5992
Telephone: (216) 942-2000
Westerdahl, H.W., and Getsinger, K.D., eds. 1988. Aquatic Plant Indentification
and Herbicide Use Guide; Vol. I: Aquatic Herbicides and Application Equipment; Vol.
II: Aquatic Plants and Susceptibility to Herbicides. Technical Report A-88-9, U.S.
Army Engineer Waterways Experiment Station, Vicksburg, MS.
available from:
US Army Engineer Research and Development
Center
ATTN: Aquatic Plant Control Research Program, EP-L
3909 Halls Ferry Road
Vicksburg, MS 39180-6199
Telephone: (601) 634-3654