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Like all natural ecosystems, Florida's native habitats have developed a complex system of checks and balances that prevents the overpopulation of plant and animal species and maintains a healthy natural environment. Every native plant in Florida has evolved with specific population regulating factors that include environmental restraints such as water levels, as well as natural enemies including herbivores and pathogens. These natural enemies play an important role in keeping the the native plant population in check.
When a non-native plant is introduced into a Florida habitat, it may have a competitive advantage over native plant populations because the natural factors that regulated the introduced plant in its native range may not exist in Florida. As a result, the non-native plant often flourishes and out-competes Florida's naturally controlled native plants. The introduced plant may replace native species, clog waterways, degrade water quality, and impede recreation and navigation.
One way to manage invading non-native plants in Florida's aquatic systems is to use biological control agents such as insects, fish and pathogens.
Biological control agents are used to decrease the invasive plants' competitive advantages over native plants, and to weaken the invading population by increasing leaf mortality, decreasing plant size, reducing flower and seed production, and/or limiting population expansion.
For more than fifty years, non-native biological control agents have been deliberately introduced or have arrived from elsewhere on their own to combat non-native invading plant populations in Florida. A total of eighteen biological control agents have been studied since the 1960s.
Developing Biological Control Agents
The development of a successful biological control agent for a problem non-native plant begins with rigorous procedures for identifying and testing potential organisms:
- Discovery and identification: Scientists travel to the target plant's native range in search of its natural enemies. Such field exploration, observation and preliminary testing takes years to complete.
- Approval for importation and study: When a candidate organism is identified in its native range, it is reviewed by the United States Department of Agriculture Technical Advisory Group (USDA TAG) before it is imported to the U.S. for further study.
- Quarantine studies: Once approval is obtained, the potential biological control agent is imported to the U.S. and placed within a secured quarantine laboratory. There, the organism is raised and extensively studied for host-specificity using appropriate plant species, including related species in other states. Feeding trials and life-history experiments are meant to ensure that the organism will affect only the targeted invading plant species and will not impact native or crop species. This testing often takes several years. Two state-of-the-art quarantine and rearing facilities exist in Florida – one in Ft. Pierce and one in Ft. Lauderdale. To learn more about these facilities, view this article.
- Initial field release: If the biological control agent is proven to be host-specific in quarantine and is capable of damaging the pest plant's growth or reproduction, a petition is prepared and submitted to obtain a permit for field release (see here). Once the permit is issued, the organism is released in specific areas to promote establishment. In the field, the biological control agent attacks the target plants. Subsequent generations of the introduced organism suppress the target plants over a long period of time.
- Monitoring: From the moment they are imported for quarantine study, all introduced biological control agents are tracked by state and federal scientists. Based on field establishment success and impact data, more releases of the organism may or may not occur. Releases and results are annually recorded with the USDA.
Biological Control Approaches
There are several approaches for using biological control. An approach is chosen after considering the target plant, its habitat, and the management objectives:
- Classical Biological Control: A biological control agent is imported into the U.S. after extensive study. The organism, usually an arthropod or pathogen, is released into its new habitat to attack the target weed. Classical biological control relies on subsequent generations of the biological control agent to suppress the invading species over a long period of time. The classical approach is the most common method of biological control.
- Non-classical Biological Control: This approach involves mass rearing and periodic release of resident biological control agents (native or introduced) to increase their effectiveness. The large number of agents is intended to immediately suppress the target plant. Although this type of biological control is generally used with mass-produced plant pathogens, repeated releases of some insects have occasionally been used to provide season long control of a target weed in areas where it is too cold for the insect to survive the winter.
- Adventive (or Fortuitous) Biological Control: Regulation of a pest population by a natural enemy that has arrived from elsewhere without deliberate introduction. Several examples are presented below.
The development of an effective biological control agent requires a significant amount of time and money, involves international cooperation, and may produce unpredictable results. For instance, the biological control agent may fail to reproduce and/or provide the desired control on the target weed.
However, the long-term benefits of an effective biological control agent can far exceed the development costs. The results from a successful biological control agent last longer than most management techniques and it reduces the need for, or amount of, chemical, mechanical, and physical controls. It is believed that successful biological controls save much time and money in aquatic and wetland plant management. During the past 50 years, eighteen biological controls have been evaluated overseas, studied in quarantine, and released in Florida and throughout the southeastern U.S. to control five invasive aquatic plant species.
Alligator weed (Alternanthera philoxeroides)
Three South American insects were released in the 1960s to control alligator weed, a prolific invasive aquatic plants infesting >80% of Florida's public waters. Because each of these insects stresses alligator weed in different ways, this suite of biological control agents has collectively had excellent results on this formerly problematic plant. Alligator weed is still present in more than 80% of Florida public waters, but at such low levels that it is rarely necessary to control it with other means.
Alligator weed flea beetles (Agasicles hygrophila) were imported from Argentina and first released in Florida in 1964; an example of classical biological control. A member of the Chrysomelidae family, the insect consumes the leaves and parts of the stems of the aquatic form of alligator weed. This insect has been the most effective of the three biological control insects imported to control alligator weed. The U.S. Army Corps of Engineers cancelled all herbicide spraying against alligator weed three years after its introduction. Still, the beetle is less effective in southern Florida because of its sensitivity to climatic extremes.
Alligator weed thrips (Amynothrips andersoni) is native to Argentina and was first released in 1967. It is the least known of the alligator weed biological control insects. Leaf damage by the thrips affects the plant by stunting its growth. This insect is the only one of the three that successfully controls the terrestrial form of alligator weed.
Alligator weed stem borer (Arcola (=Vogtia) malloi) is a small brown moth from Argentina that was released in 1971. The larvae mine inside the stem and cause the plant to wilt and die. This insect is capable of migrating great distances and is the most cold tolerant of the alligator weed insects. Control is most effective when used in conjunction with the alligator weed flea beetle.
Brazilian peppertree (Schinus terebinthifolius)
Adventive Biological Control. The only insect currently causing some damage to Brazilian peppertree in Florida is the adventive torymid wasp, the Brazilian peppertree seed wasp (Megastigmus transvaalensis), which attacks the drupes or seeds. In recent years, this insect has been expanding its range throughout the Brazilian peppertree infested area. Megastigmus transvaalensis was probably introduced accidentally into the USA from Reunion or Mauritius via France in Brazilian peppertree seeds sold as spices in some food shops. In 2001, a detailed two-year study on the distribution and effect of M. transvaalensis on Brazilian peppertree in Florida observed that up to 31% of the drupes were damaged by the wasp during the major winter fruiting period, and up to 76% during the minor spring fruiting phase.
For more information on Brazilian peppertree and biological control, see this Extension publication.
Hydrilla (Hydrilla verticillata)
Worldwide surveys began in 1981 to search for an effective biological control agent for the submersed plant hydrilla. Some of the earliest research studied snails and pathogens, which produced unsatisfactory results. Currently, four insects and one fish have been released to control hydrilla, but only two of these insects are established, and only one is commonly associated with hydrilla in the southeastern U.S. None of the insects have been able to adequately control or stress rapidly increasing hydrilla populations, but the fish has proven to be very effective. During the past 40 years, the the FWC Invasive Plant Management Section (formerly DEP Bureau of Invasive Plant Management) has spent nearly $7.5 million – more than half of its research budget – to evaluate potential biological control candidates and release promising candidates that have passed quarantine regulations. This research has included collaborations with the University of Florida, US Army Corps of Engineers, and the USDA.
The hydrilla tuber weevil (Bagous affinis) was discovered in India and Pakistan and released in the U.S. in 1987. The adult lays eggs on rotting wood and organic matter. After hatching, the larvae burrow into the ground until they find hydrilla tubers. The tuber is destroyed as the insects feed on it. Hydrilla tuber weevils are specific to hydrilla and therefore do not pose a threat to other aquatic plants. The weevils failed to establish because they are only effective during drawdowns and Florida lakes are rarely dry.
The Asian hydrilla leaf-mining fly (Hydrellia pakistanae) was found in India and first released in the U.S. in 1987. The larvae of the Asian hydrilla leaf mining fly, together with the species described below, burrow inside the plant’s leaves. Each insect destroys up to 12 leaves throughout its developmental period. However, hydrilla has not been effectively controlled by these insects. Research efforts are underway to mass-rear them to use in an augmentative biological control strategy.
The Australian hydrilla leaf-mining fly (Hydrellia balciunasi) was found in Australia and first released in the U.S. in 1988. Although it has failed to establish on hydrilla in Florida, a small population of this insect has persisted in East Texas following its release.
The hydrilla stem borer (Bagous hydrillae) was imported from Australia and released in 1991. The larvae burrow into the submerged stems of hydrilla, causing them to fragment. This insect also failed to establish as the stem fragments require a dewatered sandy shoreline for larvae to develop within stem fragments – a rare situation in Florida waters.
The hydrilla miner (Cricotopus lebetis) is a midge that has been associated with hydrilla declines in several Florida locations since 1992. Developing larvae mine the growing shoot tips of hydrilla, which severely injures or kills them. The feeding damage alters the plant’s architecture by preventing new hydrilla stems from reaching the water surface. The life cycle of the hydrilla miner is completed in 1-2 weeks. It is not clear whether the midge is an adventive species or native insect that adapted to hydrilla.
The adventive hydrilla moth (Parapoynx diminutalis) from Asia probably entered the US via the aquarium trade. It was discovered feeding on hydrilla in Florida in 1976. The life cycle of Parapoynx is completed in 4-5 weeks. The moth was never approved for release, but large populations of hydrilla are occasionally completely defoliated by the larvae. It was later found that the moth is not a hydrilla specialist.
Chinese grass carp (Ctenopharyngodon idella), a fish from China, is one of the most effective biological control agents for hydrilla and a number of other aquatic plants. The voracious herbivore prefers hydrilla and 2-25 fish can completely control one acre of the plant. Unfortunately, the fish does not eat only hydrilla and also will consume most submersed and emersed aquatic plants once hydrilla is depleted. Florida's interconnected surface waterways make it nearly impossible to restrict the range of grass carp. Because of the potential environmental damage caused by a breeding population of grass carp, a sterile "triploid" grass carp can be produced by treating fertilized eggs with cold, heat or pressure. It is legal in Florida to use grass carp for biological control with a permit from the Florida Fish and Wildlife Conservation Commission (FWC). An efficient means of recapturing grass carp has not yet been developed and this limits the feasibility of employing the fish as a biological control agent. Triploid grass carp are stocked at very low rates (1-2 fish/acre) to control hydrilla in about 80 small Florida public waters (less than 500 acres in size and relatively self-contained).
For more detailed information about the use of grass carp, go to this page of the website.
Water hyacinth (Eichhornia crassipes)
Three biological control insects have been imported, studied, and released to control invasive water hyacinth, a floating macrophyte that was introduced to the U.S. during in the late 1800s. Together, these insects reduce the size and vigor of water hyacinth, and reduce flower and seed production. Individually, however, they are not able to control water hyacinth.
The mottled water hyacinth weevil (Neochetina eichhorniae) was first released in 1972. The adults feed on the leaves and petioles of water hyacinth, where they produce characteristic feeding scars. The larvae tunnel in the petioles and crown of the plant. The mottled water hyacinth weevil has been the most effective biological control insect for water hyacinth. It is able to stress plants, reduce flowers and seeds, and reduce plant vigor.
The chevroned water hyacinth weevil (Neochetina bruchi) is very similar to N. eichhorniae. It was first released in 1974. Both weevils reduce plant vigor and seed production and are damaging to young water hyacinth stands. Studies have shown a substantial decrease in plant growth when the insect is used in conjunction with herbicides. The weevils are unable to effectively control plants growing in water bodies with high nutrient loads (e.g., at wastewater treatment facilities); the plants simply outgrow the effects of herbivory.
The water hyacinth moth (Niphograpta (=Sameodes) albiguttalis) was first released in 1977. The larvae feed by tunneling into the petioles of the younger, bulbous form of water hyacinth. The moth has been less successful as a biological control agent because it disperses rapidly, has patchy distribution, and may be completely excluded by the weevils on the older, non-bulbous plants.
The water hyacinth planthopper (Megamalus scutellaris) was released in Florida in 2010. Both the nymphs and adults feed on the sap of water hyacinth, and the females deposit eggs into the leaf tissue. The insect's population increases rapidly, which will enable it to quickly impact water hyacinth. Nymphs are active and readily hop off the plant if disturbed. Because of its mobility, this insect may integrate better with existing maintenance control programs utilizing herbicides.
Water hyacinth mite (Orthogalumna terebrantis) is an arachnid native to the U.S. In high numbers, these mites can desiccate water hyacinth foliage and cause leaves to turn brown. Severe damage may occur in small areas, but rarely does this mite attain high enough populations to provide area wide control of water hyacinth.
Water lettuce (Pistia stratiotes)
Two South American insects have been released in Florida to combat water lettuce. Only one of these insects is established, but it has not adequately controlled or stressed the plant populations in most situations
The water lettuce leaf weevil (Neohydronomus affinis) was imported from South America after showing promising results as a biological control agent in Australia and South Africa. It was imported to the U.S. in 1986 and 1988. Two years after its release, the weevil population increased and effectively suppressed water lettuce at several sites. It is now established and distributed widely throughout the state, but rarely suppresses water lettuce growth. Adults and larvae feed on the leaves, crown and newly emerging shoots, and the characteristic holes in leaves indicates high weevil densities. Feeding by multiple larvae destroys the spongy leaf bases, which causes plants to lose buoyancy. The life cycle of the weevil is completed in 3 to 4 weeks. The weevil has not contributed to long-term suppression of the plant in the US, but has provided successful biological control of water lettuce in other countries. It is thought that the weevil is heavily preyed upon by imported fire ants in Florida. If true, this provides an interesting example of an exotic insect controlling a valuable potential biological control agent.
The water lettuce leaf moth (Spodoptera pectinicornis) is native to Southeast Asia and was imported from Thailand. The caterpillar was first released in Florida in 1990, but failed to establish. Fire ant predation also may have prevented establishment of the moth. In its native range, augmentive releases of the moth have been successfully used to control water lettuce in rice paddies.
Melaleuca (Melaleuca quinquenervia)
Four insects have been released in Florida to combat melaleuca, an invasive weedy tree intentionally imported from Australia in 1906. Two of these biological control insects are well-established and significantly impacting melaleuca. The third insect failed to establish but the fourth is now well-established.
The melaleuca leaf weevil (Oxyops vitiosa) was imported from Australia in 1992, and released in 1997 to slow the spread of melaleuca. The insect feeds on the young leaves, stems, and buds of the trees and interferes with normal plant processes such as seed production and plant growth. A new weevil generation is produced in approximately 3 months. This biological control agent is now widely established except in flooded areas, where the insect is prevented from completing its development in the soil.
The melaleuca psyllid (Boreioglycaspis melaleucae) was released in 2002. Itspread very quickly due to wind dispersal and a short generation time of 1.5 months. Damage to melaleuca is caused primarily by the nymphs, which attack older leaves and woody stems as well as new leaves and seedlings. Unlike the weevil, the psyllid is able to complete its development entirely in the tree canopy under flooded conditions that effectively exclude the weevil.
The melaleuca bud-gall fly (Fergusonina turneri) was first released in Florida in 2005. This insect is associated with a parasitic nematode. The female fly deposits her eggs, along with juvenile nematodes, into developing melaleuca buds. The nematodes induce gall formation and the fly larvae feed on the gall tissue. A new generation is produced in about 2 months. To date, this insect has failed to establish, even after multiple releases.
The melaleuca stem-gall midge (Lophodiplosis trifida) was released in 2008, and establishment of this insect has been confirmed. Adults are tiny, fragile flies with long legs and antennae. They do not feed and are short-lived. Females are recognized by the red-orange eggs visible through the abdomen. Eggs are deposited singly or in groups on young stems, buds, and leaves of melaleuca. Larval feeding induces gall formation, primarily on the stems. Gall formation impedes stem growth, and small plants that are attacked often are killed.
Common and giant salvinia (Salvinia spp.)
A tiny black weevil, Cyrtobagous salviniae, is the only insect that has been released as a biological control agent of giant salvinia (Salvinia molesta). Adventive weevils of one biotype that were discovered in Florida in 1960 are used to control common salvinia (Salvinia minima), whereas weevils of another biotype released in 2001 from a Brazilian population are used as biological control agents for giant salvinia.
Adult salvinia weevils (Cyrtobagous salviniae) feed on leaf buds and leaves. Larvae tunnel inside the plant, killing leaves and rhizomes. The entire life cycle of the Cyrtobagous weevil takes approximately 46 days. Attacked plants turn brown and eventually lose buoyancy. Cyrtobagous weevils from Australia are currently of great interest to researchers and have been introduced as a biological control agent for giant salvinia in the U.S. The effectiveness of these weevils for controlling salvinia in the US was recently confirmed in Texas and Louisiana.
Other biological controls studied in the past include: Some snails feed on several species of aquatic plants but have not proven effective as biological control agents.
Research, implementation, and results of biological controls are slow. Therefore, it is important to explore other control methods such as chemical, mechanical and physical, while establishing new biological control agents. Protocols for integrating biological control agents with other control practices must also be developed.
Preventing the introduction and spread of non-native plants in Florida's waterways is the most effective and least expensive means of restoring Florida's natural freshwater habitats.
Like all plant management techniques, biological controls are costly and time-consuming tools to use to combat the non-native aquatic plant populations that are infesting Florida's lakes, rivers and wetlands.
Time and money spent on managing invasive species can be saved in the first place by preventing the introduction and spread of invasive species in the state's waterways. Public cooperation is an essential part of restoring Florida's natural habitats.
For More Information
For more information about biological control in Florida, visit the UF-IFAS Integrated Pest Management website.
Biological Control of Invasive Plants in the Eastern United States – An online book that provides a reference guide for field workers and land managers concerning the historical and current status of the biological control of invasive plant species in the eastern United States.
Biology and Control of Aquatic Plants – A Best Management Practices Handbook. Haller, W.T., Gettys, L.A., Bellaud, M., (eds.), Aquatic Ecosystem Restoration Foundation, Marietta, GA, Chapters 8, 9 & 10 by Dr. James Cuda.