Neochetina eichhorniae and N. bruchi were introduced to control the exotic pest, waterhyacinth. Florida's sub-tropical climate and warm, shallow, fertile waters are ideal for waterhyacinth proliferation. This floating macrophyte was introduced into the United States at a New Orleans exposition in 1884. Samples are said to have been given to fair-goers, and within 4 years, coastal fresh waters were infested from Texas to Alabama. By 1972, the infestation in Florida was estimated to be 200,000 acres. Large, floating mats of waterhyacinth obstruct navigation, clog irrigation works, disrupt the natural ecology of wetlands in many ways, exacerbate mosquito problems, and are costly to the tourism and recreation industries.
Three insects were introduced into Florida in an attempt to control waterhyacinth. The weevils N. eichhorniae and N. bruchi, natives of Argentina and surrounding areas in South America, were released in 1972 and1974, respectively. The pyralid moth, Sameodes albiguttalis, was released in 1977. N. eichhorniae has proven to be the most important control agent.
N. bruchi: The body is broad, robust, and densely covered with fused brown and tan scales. The tan scales form a V-shaped chevron on the back that distinguishes this species from N. eichorniae. Antennae and lower-leg segments are reddish-brown. There are yellowish water-shedding scales on the leg joints and parts of the underside. The snout is thick and weakly curved to straight in males; in females it is longer, more slender, and more curved. Male length is about 3.5 mm long (excluding head), and female length is about 4.5 mm long (excluding head). The female snout is noticeably shiny near the tip where the scales have been rubbed away. This easily distinguishes females from males in both species.
N. eichhorniae: The body is covered mostly with gray scales mottled with brown, and the antennae and lower-leg segments are reddish-brown. The snout is thick and weakly curved in males, and moderately slender and rather strongly curved in females. There are dense, yellow, water-shedding scales at the base of the head. The male is about 3.2 mm long (excluding head) and the female is about 3.7 mm long (excluding head).
The biologies of N. bruchi and N. eichhorniae are very similar. Studies in the United States show that adults are reproductive all year, without producing distinct generations. In Florida, females undergo a reproductive quiescence during the winter. Reproduction begins again after December. Oviposition is in the leaf tissue, N. eichhorniae placing eggs singly into holes chewed by the female in the leaf epidermis, and N. bruchi inserting eggs either singly or in groups of up to 25 into the petiole. About 30°C is optimal for feeding and oviposition in both species.
Eggs hatch in 7-17 days, depending on temperature. There are three instars, the larval stages taking from 36 to 90 days-- reports vary widely. Larvae feed and develop in the petioles. They burrow downwards, usually arriving at the petiole base by the third instar. They may gouge pockets into the stem (rhizome) and feed on developing axillary buds. Fully grown larvae move into the water to the upper root zone, create a ball around themselves formed from lateral roots and pupate attached to the main root axis, possibly using the root as an oxygen source. Pupation lasts approximately 7-9 days at 27° C. Feeding begins within 24 hours of leaving the pupal chamber, and eggs are usually laid about 6 days later. Adults are nocturnal.
Total generation time is from 70 to 140 days for N. eichhorniae and 56 to 94 days for N. bruchi, depending on time of year. Females lay about 200 eggs during their lifetime and sometimes as many as 400 or more. Fecundity is heavily dependent on abundant, healthy plants.
The biological control of waterhyacinth is considered a qualified success. Waterhyacinth infestation in Florida was estimated to be only 2600 acres in 1989. Although this figure was obtained after herbicide treatment of 25,000 acres, earlier herbicide treatment of more acreage had not reduced the infested area. Also, many of the early study sites used to investigate the performance of waterhyacinth biological control agents were situated in undisturbed backwater locations, and most of those sites no longer harbor waterhyacinth. Although declines of these waterhyacinth populations were well documented and readily attributable to biological control, the documentation was never published.
Although immature weevils have no way to escape dying and sinking plants, experiments have shown that Neochetina adults are able to tolerate applications of 2,4-D, Kover II, diquat, and polymer "573" (see Haag, below, for concentrations) and can move freely between sprayed and unsprayed plants. It is therefore desirable to keep an area of waterhyacinth unsprayed in order to provide habitat for the weevil population needed to continue biological control.
Not available commercially.
Thanks to Ted Center and Gary Buckingham for reading previous drafts of this page and to Gary Buckingham for supplying photographs and additional information.
Center, T.D. (1994) Biological control of weeds: waterhyacinth and waterlettuce. In Pest Management in the Subtropics: Biological Control--a Florida Perspective. Rosen, D., Bennet, F.D., and Capinera, J.L. (Eds.). Intercept, Ltd., Andover, UK. 737 pp.
Haag, K.H. (1985) Does herbicide application affect water hyacinth weevils? Aquatics, 7: 13-15.
O'Brien, C.W. (1976) A taxonomic revision of the new world subaquatic genus Neochetina (Coleoptera: Curculionidae: Bagoini), Ann. Entomol. Soc. Am., 69: 165-174.