Integrated Management Across Two Environments
Alligator weed management requires a coordinated approach that addresses both the aquatic mat form and the terrestrial/riparian form simultaneously. Treating only the aquatic population without managing adjacent land populations ensures rapid recolonization — terrestrial alligator weed at the water margin continuously contributes fragments and new stems to the aquatic environment. Effective programs integrate herbicide treatment, biological control with the flea beetle, and prevention of new introduction through equipment decontamination. All aquatic herbicide applications require state permits.
Permits Required: Aquatic herbicide application requires permits from your state's department of natural resources or environmental protection agency. Agricultural herbicide applications near water require careful attention to label requirements and state restrictions. Contact your state agency before beginning any management program.
Biological Control: The Alligator Weed Flea Beetle
One of the most significant and historically important biological control programs in aquatic plant management was the introduction of the alligator weed flea beetle (Agasicles hygrophila) from Argentina to control alligator weed in the southeastern United States. Beginning in the 1960s, USDA researchers imported this small beetle, studied its host specificity extensively to ensure it would not attack non-target plants, and released it in Florida in 1964. The beetle spread northward through natural dispersal and deliberate redistribution, eventually establishing throughout much of the alligator weed range in the Southeast.
Agasicles hygrophila adults and larvae feed exclusively on alligator weed — adults create "window pane" holes in leaves; larvae mine between leaf surfaces. Heavy beetle feeding can defoliate entire aquatic mats, causing dramatic suppression. In warm, humid climates with mild winters (Florida, coastal Georgia, Louisiana), the beetle provides outstanding sustained control, reducing alligator weed aquatic mats to low density in many infested water bodies without ongoing chemical input. In cooler-winter areas (North Carolina, Virginia, Tennessee), beetle populations are reduced by winter cold and provide less consistent control, requiring supplementation with other management approaches.
Two additional biological control agents have been approved: a thrips (Amynothrips andersoni) that feeds on alligator weed flowers and leaves, and a moth (Vogtia malloi) whose larvae feed on alligator weed stems. The thrips is established in Florida. Together with the flea beetle, these agents provide a self-sustaining biological suppression system that has made alligator weed management in Florida dramatically less chemical-intensive than in the 1950s before beetle introduction.
Herbicide Treatment: Aquatic Form
When herbicide treatment of the aquatic form is warranted (large mats, areas where biological control is insufficient, new infestations requiring rapid suppression), the following herbicides are effective:
2,4-D (various formulations)
2,4-D is effective against alligator weed and has been used since the earliest management programs. Aquatic formulations applied to the floating mat by spray or injection kill above-ground growth within 1–3 weeks. 2,4-D is the most commonly used aquatic herbicide for alligator weed where biological control is supplemented rather than primary.
Imazamox (Clearcast)
Imazamox is registered for alligator weed control in aquatic environments and has favorable water use restriction profiles. It is often used where 2,4-D is less appropriate due to adjacent crop or water use concerns.
Triclopyr (Renovate)
Triclopyr is effective against alligator weed in some applications, particularly the terrestrial form.
Herbicide Treatment: Terrestrial Form
The terrestrial growth form of alligator weed requires different herbicide considerations than the aquatic form. Standard terrestrial herbicides (glyphosate formulations) can be used on terrestrial alligator weed outside the water, but buffer zones and label requirements must be carefully observed near waterways. Agricultural herbicides labeled for use near drainage systems must be selected based on applicable state and federal registrations. Work with your state department of agriculture and DNR to select appropriate herbicides for the specific site conditions and water proximity.
Mechanical Control and Prevention
Mechanical removal of alligator weed mats by hand, rake, or aquatic harvester is feasible in small infestations but not sustainable as the primary management approach for large or well-established populations — regrowth from terrestrial edge populations and from surviving stem fragments is rapid. The most important use of mechanical removal is elimination of newly introduced small populations before they establish, combined with complete removal of all plant material from the water body. All removed material must be completely taken to a composting or disposal site away from any waterway — alligator weed fragments left at the water's edge will re-establish. Equipment decontamination (cleaning all plant fragments from boats, trailers, and farm equipment) is critical for preventing spread to new water bodies.
References
- Spencer, N.R. & Coulson, J.R. (1976). Biological control of alligatorweed. Aquatics 9:4–17.
- Buckingham, G.R. (1996). Biological control of alligatorweed with flea beetles. Castanea 61:232–260.
- USDA APHIS PPQ. Alligatorweed biological control. aphis.usda.gov