On this page
How to identify it
- Adults of both species are somewhat rounded weevils with long snouts and two dark markings on the wing covers. The mottled waterhyacinth weevil (N. eichhorniae) is usually a darker mottled gray and brown, with the two markings longer and set nearer the head; the chevroned waterhyacinth weevil (N. bruchi) is tan or brown, often with a lighter V-shaped band low on the wing covers, and its two markings are shorter and sit midway down.[1]
- Larvae are C-shaped, white with a yellow-orange head, up to 9 mm long, found by splitting the leaf stalks. Pupae are white, in cocoons attached to the roots below the water. Eggs are white and oval, embedded in leaf and stalk tissue.[1]
- Feeding scars 2 to 3 mm wide on the leaves and stalks are the field sign; no other water hyacinth natural enemy established in North America leaves this external damage. Scars on the newest center leaves mean adults are usually sitting at the base of the stalk inside the leaf sheath.[1]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Water hyacinth moth (Niphograpta albiguttalis) | Another South American agent whose larvae mine the leaf stalks, but it is a moth: the larvae are cream-colored with dark spots, up to 2 cm, and the mining browns and curls the leaf rather than leaving the weevils' external scars.[1] |
| Salvinia weevil (Cyrtobagous salviniae) | A much smaller weevil, up to 2 mm and shiny black when mature, that lives on common and giant salvinia rather than water hyacinth.[4] |
Where it lives
Native to South America. Weevils from Argentina were released in Florida in 1972 (N. eichhorniae) and 1974 (N. bruchi) and later moved to Texas, Louisiana and California; both are well established in five states and in Mexico. The mottled weevil usually dominates on the Gulf Coast, while in California the chevroned weevil dominates and the mottled weevil is scarce. They thrive wherever water hyacinth persists through the year. Texas A&M lists the Neochetina beetles among the biological management options for water hyacinth in Texas ponds.[1][2]
Biology and life cycle
Both species breed continuously, so generations overlap. In warm areas where temperatures stay above freezing every stage overwinters, adults may live more than a year, and they can be found year-round.[1]
Adults feed on the leaves and stalks, leaving scars 2 to 3 mm wide. A female lays 300 to 400 eggs in her lifetime, embedded in leaf and stalk tissue; the chevroned weevil may put several eggs in one spot, the mottled weevil lays them singly.[1]
The larvae feed through three stages and mine the leaf stalks toward the root crown; chevroned weevil larvae develop somewhat faster. They pupate in cocoons fastened to the roots under water, and the new adults climb above the surface to feed and mate.[1]
Because water hyacinth is their main food, the weevils depend on the plant: in a 54-site Florida survey, sites kept nearly clear by repeated herbicide treatment held far fewer weevils simply because there were few plants, while at untreated sites the dense weevil populations had stressed the plants so much that fewer females were reproductive.[2][3]
Damage and signs
- Adult scarring and larval tunnels in the stalks stunt the plants and reduce both flowering and vegetative spread. Under heavy feeding the stalks become thin and brittle, the plants waterlog and gradually sink.[1]
- On the Gulf Coast the two species' damage is hard to tell apart and probably complements each other. In some parts of those states the weevils have reduced water hyacinth to less than a third of its original abundance, and managed waters need significantly less herbicide, applied much less often, though the weed remains a problem elsewhere in the region.[1]
- In California, where the chevroned weevil dominates, the impact has been only minor, probably because cold limits the populations. In Mexico results vary from excellent control in some water bodies to little in others.[1]
- The Florida survey found that herbicide maintenance control actually improved the nutritional quality of surviving plants and so stimulated weevil reproduction, but also guaranteed regrowth; the USDA-ARS authors recommended integrating the two, using herbicides to hold water hyacinth below management thresholds in ways that conserve the weevil populations.[3]
- Early spillover feeding on native pickerelweed, canna lily and other plants growing mixed with water hyacinth was insignificant and temporary, and no nontarget attack has been seen more recently.[1]
Health risk
Aquatic plant-feeding weevils; none of these sources describes any risk to people.[1][3]
Control
Prevention
No control of the weevils is needed; they are approved biological control agents of water hyacinth and are commonly used for that purpose.[1][2]
Biological control
Where water hyacinth is also sprayed, the USDA-ARS advice is to integrate the two: keep infestations below management thresholds with herbicide while leaving enough plants to carry the weevil population, rather than clearing every mat.[3]
Common questions
- What are the small scars all over the water hyacinth leaves?
- Adult feeding scars of the waterhyacinth weevils, 2 to 3 mm wide. No other established water hyacinth natural enemy makes them. If the scars are on the newest center leaves, look for the adults at the base of the stalk inside the leaf sheath, and split a stalk to find the white larvae.[1]
- Which weevil do I have?
- The mottled weevil is darker, mottled gray and brown, with longer dark marks near the head; the chevroned weevil is tan or brown with shorter marks midway down the wing covers and often a lighter V-shaped band near the tips. On the Gulf Coast the mottled weevil is usually the common one; in California it is the chevroned weevil.[1]
- Will spraying the hyacinth wipe out the weevils?
- Spraying removes their only food, so weevil numbers fall with the plants. The USDA-ARS recommendation is to use herbicides to keep the weed below thresholds while conserving enough plants for the weevils, as the survey's authors concluded the two should be integrated.[3]
Sources
- North American Invasive Species Management Association (PDF hosted on Bugwood), “Smith (USDA-ARS Invasive Plant Research Laboratory), Goode (University of Florida) and Harms (US Army Engineer Research and Development Center). Waterhyacinth Biocontrol Agents: History and Ecology in North America. In: Winston, ed., Biological Control of Weeds in North America, NAISMA-BCW-2022-16-WATERHYACINTH-A (2022).” bugwoodcloud.org/resource/files/25358.pdf (accessed 2026-11-10)
- Texas A&M AgriLife Extension, “How to Control Water Hyacinth (AquaPlant: Management of Pond Plants and Algae).” aquaplant.tamu.edu/management-options/water-hyacinth/ (accessed 2026-11-10)
- Environmental Management (Springer), “Center, Dray, Jubinsky and Grodowitz (USDA-ARS Aquatic Plant Control Research, Fort Lauderdale). Biological control of water hyacinth under conditions of maintenance management: can herbicides and insects be integrated? Environmental Management 23: 241-256 (1999), doi:10.1007/s002679900183.” doi.org/10.1007/s002679900183 (accessed 2026-11-10)
- North American Invasive Species Management Association (PDF hosted on Bugwood), “Pham and Diaz (Louisiana State University) and Nachtrieb (US Army Engineer Research and Development Center). Salvinia Biocontrol Agents: History and Ecology in North America. In: Winston, ed., Biological Control of Weeds in North America, NAISMA-BCW-2022-20-SALVINIAS-A (2022).” bugwoodcloud.org/resource/files/26407.pdf (accessed 2026-11-10)
- iNaturalist, “Neochetina eichhorniae taxon page (order and family).” www.inaturalist.org/taxa/269433 (accessed 2026-11-10)