The water hyacinth weevils, Neochetina bruchi Hustache and N. eichhorniae Warner (Coleoptera: Curculionidae), are native to South America. They were introduced in India in 1983 for the biological suppression of water hyacinth (Eichhornia crassipes (Mart) Solms.), which is one of the most serious aquatic weeds in India.

The weevils have been introduced in at least 15 Indian states and are established in Assam, Andhra Pradesh, Gujarat, Karnataka, Kerala, Maharashtra, Punjab and Tamil Nadu.

Distinguishing features between Neochetina bruchi and N. eichhorniae

Neochetina bruchi Hustache (Chevroned water hyacinth weevil)	Neochetina eichhorniae Warner (Mottled water hyacinth weevil)
Elytra grey to brown with a distinct lighter brown to tan chevron / crescent-shaped marking in the posterior half (occasionally absent)	Elytra darker, more or less uniformly covered with gray scales mottled with brown or dark brown scales
Elytra with a pair of short, dark, glabrous sutural ridges or raised lines located behind middle	Elytra with a pair of dark, glabrous sutural ridges or raised lines located a little forward of middle, longer than those in N. bruchi
Striae on elytra fine	Striae on elytra relatively

O'Brien (1976) revised the genus Neochetina and provided descriptions and a key to species. An interactive key to these two species is also available at http://www.famu.org/weeviltool/. A brief account of the two species is given below.

Chevroned water hyacinth weevil

• Body broad, robust, 3.5-4.5 mm long and densely covered with fused gray and brown scales.
• Elytra with a V-shaped chevron or crescent shaped marking (this marking is distinct when present, occasionally absent) in the posterior half; with a pair of short, dark ridges or raised lines located behind the middle of elytra.
• Antennae and lower-leg segments reddish-brown, yellowish water-shedding scales present on the leg joints and parts of the underside.
• Snout / rostrum thick and weakly curved to straight in males; longer, more slender, and more strongly curved in females.
• Female snout noticeably shiny near the tip where the scales have been rubbed away which easily distinguishes females from males.
• Striae on elytra fine.

       Adult weevils feeding on water hyacinth

Mottled water hyacinth weevil

       Adults feeding on water hyacinth

The general biology of N. bruchi and N. eichhorniae is more or less similar and the immature stages also closely resemble each other. Eggs are deposited directly within the tissues of water hyacinth. Adult females make a hole in the lamina or petiole of the leaf and deposit a single egg. Eggs may also be deposited around the edges of adult feeding scars. The weevils prefer to lay eggs in the tender central leaves or ligules surrounding the leaf bases.

    Eggs

Larvae of both species are similar in external appearance. They are white or cream coloured with a yellow-orange head. They are apodous (legless) The posterior end of the body is blunt with a pair of dorsally projecting spiracles that the insect is thought to insert into the plant tissues to extract oxygen. There are three larval instars - the first instar larvae are very small (2 mm long); the full grown (third instar) larvae are grub-like, C-shaped, and 8-9 mm long. The larvae feed within the bases of leaves and petioles, occasionally entering the apex of the stem, where they destroy the apical bud.

     
Larva of Neochetina sp. feeding on water hyacinth

Pupae of both species are similar in external appearance and difficult to distinguish. They are creamy white and enclosed within a cocoon that is formed among the lateral rootlets below the water surface. Pupae within their case have the appearance of small "balls" about 5 mm in diameter typically on roots near the stem.

Singh (1994a, b) detailed the methods for the multiplication of the weevils in portable or permanent water tanks and pools, their field release and impact assessment.

 
Plastic-lined, portable, aluminium tanks used for the mass production of Neochetina spp.

• O'Brien, C.W. 1976. A taxonomic revision of the new world subaquatic genus Neochetina (Coleoptera: Curculionidae: Bagoini). Annals of the Entomological Society of America, 69: 165-174.
• Singh, S.P. 1994a. Technology for production of natural enemies. Technical Bulletin No. 4, Project Directorate of Biological Control, Bangalore, India. 221 p.
• Singh, S.P. 1994b. Fifteen years of AICRP on biological control. Technical Bulletin No. 8, Project Directorate of Biological Control, Bangalore, India. 320 p.
• Identification tool for weevil biological control agents of aquatic and terrestrial weeds in the United States and Canada.