Two species of water hyacinth weevils (Neochetina bruchi and N. eichhorniae) have been introduced into the United States as biological control agents to suppress the growth and reproduction of water hyacinth[1, 3]. Both weevils were imported from Argentina to help manage water hyacinth. These weevils are the natural enemies of water hyacinth in its native range . The adult weevils feed on water hyacinth leaves, while the larvae damage the plant by burrowing into the stem .
Water hyacinth mites and water hyacinth moths can also help control these plants. The water hyacinth mite (Orthogalumna terebrantis) is a small, black arachnid believed to be native to the United States . The mite damages the plant in a similar manner to the weevils, with the adults feeding on the leaves and the larvae burrowing into the plant. However, the mite alone usually cannot effectively control water hyacinth .
The water hyacinth moth (Niphograpta albiguttalis) was introduced to the United States in 1977 to control water hyacinth . The larvae of these moths burrow into and feed on the plant, damaging the leaves and stems. They slow the growth of the plants by destroying new leaves .
The water hyacinth weevils (Neochetina bruchi and N. eichhorniae) are native to South America, where they feed on water hyacinth. Feeding damage caused by both species of weevils is complimentary and contributes to the suppression of water hyacinth . The weevils were first released in Florida in the 1970’s and are now found across the entire distribution range of water hyacinth in United States.
N. eichhorniae and N. bruchi
Eggs: White and ovoid, eggs are less than 3/64 inches (1 mm) in length. Adults can lay 300-400 eggs over a life span .
Larvae: Larvae are white with orange heads and legs are absent. Third instar larvae are approximately 5/16 inch (8-9 mm) in length. Length of larval period is less than 30-45 days .
Pupae: Pupae are white and enclosed in a cocoon attached to the rootstock under the water’s surface. Pupal stage is approximately seven days, but young (teneral) adults sometimes remain in cocoons for extended periods of time .
Figure 1. Lateral view of N. eichhorniae adult.
Figure 2. Dorsal view of N. eichhorniae adult.
Adults: Males and females, excluding heads, are approximately between 1/8 and 9/62 inch (3.2 and 3.7 mm) long, respectively (Figures 1 and 2). Adults are covered in grey scales and mottled with brown. Dense, yellow water shedding scales are located at the base of the head and antennae and lower leg segments are reddish-brown. Male snouts are thick and slightly curved and females are more slender and strongly curved. Compared to the male, female snouts are also noticeably shiny near the tip. This characteristic is common to both species .
Figure 3. Dorsal view of N. bruchi adult.
Adults: Males and females, excluding heads, are approximately 9/64 inch and 11/64 inch (3.5 and 4.5 mm) long, respectively  (Figure 3). Adults are densely covered in brown and tan scales, with tan scales forming a V-shaped chevron on its back. This feature distinguishes it from N. eichhorniae . Yellow, water-shedding scales are located on the underside and at the leg joints. Lower leg segments and antennae are reddish-brown. Weevil snouts are thick and curved. Compared to males, female snouts are more slender, longer and more curved .
Larvae of water hyacinth weevil. Image courtesy of USDA-ARS.
Adult weevils burrow into the water hyacinth stem and deposit their eggs directly into the plant . The eggs will hatch in 7-10 days and the larvae will remain in the plant for over month . Once the larvae are fully developed, they crawl out of these holes and move to the bottom of the plants, where they create a cocoon using material from the roots . Here in the cocoons, they transition from larvae to adults. In about 30 days, the adults emerge from the cocoons and will be ready to reproduce within a few days .
The larvae eat their way through the stems of the plant, creating tunnels called galleries . They will often feed on and destroy the apical bud, which is where new leaves grow from. The adult weevils feed on the leaves, leaving a distinctive circular scar . The images above show water hyacinth leaves with adult weevil feeding damage.
Neochetina weevils are specialist herbivores and feed solely on water hyacinth . Studies indicate that both weevils have a narrow host range; moreover, they have developed a method of pupation underwater that relies on specific aspects of the water hyacinth’s root system .
Water hyacinth weevil infestations can inhibit flowering, cause stunted growth, and ultimately cause the death of the water hyacinth plant . Scars from feeding damage can also make the plant more vulnerable to diseases .
Water hyacinth’s growth in waterways is only limited by temperature and availability of nutrients . The aesthetic value of water hyacinth in the aquarium trade is far outweighed by the potential damage the plant can cause. It can alter natural communities and prefers environments similar to desirable fish. Water hyacinth can impede navigation, displace native vegetation and reduce the dissolved oxygen in a water body if not properly managed . Water hyacinth weevils feed heavily on plant tissue. Larvae feed internally and adults feed externally. Feeding damage by both life stages suppress the growth of the plant often inhibiting flowering . In addition, feeding scars by weevils can promote plant pathogens to attack the plant and contribute to the overall suppression of the plant population .
Water hyacinth mite on top of leaf. Image courtesy of Willey Durden, USDA Agricultural Research Service, Bugwood.org
The water hyacinth mite (Orthogalumna terebrantis) is a small, black arachnid native to the United States . They feed on the leaves and stems of water hyacinth and the closely related native plant, pickerelweed (Pontederia spp.) The adults often congregate in the scars left by water hyacinth weevil feeding .
Water hyacinth mites on top of leaf. Images courtesy of National Bureau of Agricultural Insect Resources, India.
Adult water hyacinth mites are barely visible with the naked human eye, often measuring less than 0.5 mm long. They are glossy black with teardrop shaped bodies . Like other arachnids, they have 4 pairs of legs. The tiny larvae (0.24 mm long) are white and have 3 pairs of legs. The nymphs of this mite are between 0.32 mm and 0.5 mm and are an amber color .
The females deposit their eggs in the leaves of the water hyacinth. These eggs will hatch in about a week and the small, white larvae emerge. Over the course of the next 15 days, these larvae will go through several stages as nymphs before growing into fully mature mites .
Water hyacinth mites feeding on leaf. Image courtesy of National Bureau of Agricultural Insect Resources, India
The adults and larvae feed on the leaf surface, leaving small red spots. The nymphs create burrows in the leaves, called galleries. As nymphs emerge from these galleries, they chew exit holes through the leaf surface.
In heavy water hyacinth mite infestations, large numbers of galleries created by juvenile mites lead to the death of the leaves of water hyacinth . Severe damage such as this is usually only confined to a few plants but using the mite in conjunction with other biological control agents, such as the water hyacinth weevil, will often prove effective at controlling this invasive plant .
Water hyacinth moth on stem. Image courtesy of Willey Durdin, Forestry Images, forestryimages.org
The water hyacinth moth (Niphograpta albiguttalis), like the water hyacinth weevils, is also native to South America . They are natural enemies of water hyacinth. These insects were first released into the Florida to help control water hyacinth in the late 1970’s and is now found across the gulf coast .
Water hyacinth moth. Image courtesy of Monica Krancevic, inaturalist.com
Water hyacinth moth adults are approximately one third of an inch of an inch in length, with a wingspan of roughly three quarters of an inch . Coloration varies from yellow to grey with black, white, and brown patterns on the wings. Larvae are typically no longer than one fifth of an inch . They have black to brown heads and yellowish-white bodies with dark markings .
Newly hatched larvae feed on the surface of water hyacinth leaves for 1 to 2 days. They then bore into the stems and feed until they are ready to pupate. Once fully developed, the larvae bore into the plant’s main bud to begin pupation, which lasts about 6 days. After pupation, the newly emerged adult will live for about 7 days before dying. In this time period, the moth does not feed at all, but instead devotes most of its energy to mating. Female moths will lay around 370 eggs within their adult lifetime .
Feeding from the water hyacinth moth larvae can cause water hyacinth plants to sink by chewing holes into their buoyant, air-filled stems . They can also destroy the buds of the plant, which hinders plant growth . In high densities, the insects can cause severe damage to the plants, though these effects alone are usually not enough to fully control water hyacinth [1, 8]. However, when used in conjunction with other biological control agents like the weevils and the mite, these insects can completely control this invasive plant without the need for herbicide application .
Numbers in brackets are references and can be found on the resources page.