Biological control

Biological control of non-native, invasive weeds involves the relocation of natural enemies, typically insects, from the weeds' native range for release onto invasive populations of the weeds. Invasive weeds are targeted for biological control because they threaten and degrade natural resources, such as habitat and water quality, and displace native plant and animal species. Typically, biological control is considered for populations of invasive species that have exceeded the management capacity of other control methods, such as herbicide application and mechanical control. However, biological control can be integrated with those methods as part of an adaptive management plan.

The long-term goal of biological control is to reduce the ability of the weed to grow, spread, and compete with other plants, thereby reducing its impact as an invasive plant in natural ecosystems. Insect biocontrol agents, for example, can achieve this by boring into roots, shoots and stems, chewing on leaves, opening and feeding on seeds, or extracting plant fluids. Effective biological control agents are able to synchronize their life cycles to that of the target plant, adapt to new climates and habitat, find their host weed, reproduce rapidly, and cause significant damage to the targeted weed.

The objective of biological control is to reduce the weed population to a level below the ‘threshold’ at which other control methods and their associated economic costs become necessary (see Figure 1, below). The weed population is still present and may fluctuate in size over time. In terms of the Delta Region Areawide Aquatic Weed Project (DRAAWP), weeds targeted for biocontrol - water hyacinth, Brazilian water weed and arundo - cannot be eradicated in the Sacramento-San Joaquin Delta by biological control or any control method. Rather, the objective is environmentally and economically sustainable, integrated management of these weeds and others to protect water resources, allow navigation and water flow, and preserve Delta ecosystems.   

Biological control agents (usually insects, but occasionally plant pathogens) targeting a specific weed species must, prior to their release, be tested to make sure that they can feed, complete their development, and reproduce only on the targeted weed. This requires several years of laboratory research, during which time the life cycle and impact of the agent on the targeted weed are also determined. If a biocontrol agent is found to be effective and highly host-specific, a petition to release the agent is submitted to a technical peer-review panel consisting of experts from the U.S., Canada, and Mexico. If the panel recommends release, the USDA-Animal and Plant Health Inspection Service completes several additional reviews and, if no concerns are found, issues a permit to release the agent into the field. Specific public agencies or water resource managers may require an additional permit from them prior to release on their properties or managed areas.

Biological control agents are expected to disperse throughout the region where the targeted weed occurs. As a result, this control method is not specific to an invaded site or weed infestation. The vast majority of weed biological control agents have targeted invasive weeds of semi-natural and natural habitats such as rangelands, pastures, forests, riparian habitats along lakes and streams, and aquatic ecosystems. Among 46 terrestrial and aquatic invasive weeds targeted with biological control in the U.S. since 1945, one-third of the released agents have had impacts sufficient to reduce or eliminate the need for other control methods. For agents released since 1991, over 70% have established populations, although in some cases, a sufficient amount of time (at least five years) has not yet elapsed to evaluate impacts. Benefit:cost ratios for successful (widely established and impactful) agents range from 8:1 to 300:1 worldwide. Most of the costs are incurred up front, for the discovery, testing and permitting of new agents over a 3 to 10 year period, while the benefits continue to accrue indefinitely after the agent is released.

The history of biological control of aquatic weeds in the U.S. includes several major success stories. These projects, however, were initiated in the southeastern U.S., and so the biological control agents have, for the most part, not established populations here in California. Alligatorweed (Alternanthera philoxeroides) was the first aquatic plant to be targeted with host-specific biological control agents, including a leaf-feeding beetle (Agasicles hygrophila). The beetle has reduced alligatorweed in Florida and along the Gulf Coast, but has not established populations in southern California, where alligatorweed is present. Giant salvinia (Salvinia molesta) has been reduced in the southern U.S by a weevil (Cytrobagous salviniae) which, when re-released each year, can control giant salvinia on the Colorado River on the CA-AZ border. Hydrilla (Hydrilla verticillata) has been controlled in Imperial Valley canals by a different type of weed biocontrol agent, the grass carp (Ctenopharyngodon idella). This agent, unlike insects, is not host-specific, and can feed on both other invasive weeds and native aquatic plants. Also unlike insects, the grass carp are sterilized prior to release and so they cannot reproduce; new fish from tank colonies must be added each year. Their use is therefore most appropriate in human-created and managed aquatic systems such as canals.

Biological control of water hyacinth (Eichhornia crassipes), one of the weeds targeted for integrated, adaptive management under the DRAAWP, was initiated in the 1960s by the USDA-ARS. Two leaf- and stem-feeding weevil species (Neochetina bruchi and Neochetina eichhorniae and a leaf- and stem-boring moth (Niphograpta albiguttalis) from the South American native range were released in Florida and the Gulf Coast in the 1970s. In this region, they have reduced water hyacinth biomass by up to 70%, reducing the need for other control methods. The weevils were released in over 20 other countries, resulting in control of water hyacinth to low levels in tropical regions. All three agents were released in the Delta in the early 1980s by the U.S. Army Corps of Engineers, but only one of the weevil species, N. bruchi, established populations and is still present in the Delta. In 2011, the California Department of Food and Agriculture and then USDA-ARS began releasing a new agent, the planthopper Megamelus scutellaris. Studies to re-examine water hyacinth weevil populations in the Delta and to find a population of weevils better-adapted to Delta conditions began in 2015. Biological control of arundo (giant cane), another weed targeted by DRAAWP, was initiated by USDA-ARS about 15 years ago. This project led to the U.S. release of two agents, a shoot tip-galling wasp (Tetramesa romana) and a shoot- and root-feeding armored scale insect (Rhizaspidiotus donacis). Since 2014, both agents have been released in the Delta. Field surveys for new biological control agents of Brazilian waterweed (Egeria densa) are ongoing in the South American native range. Other aquatic weeds in the Delta, such as South American spongeplant (Limnobium laevigatum) and water yellow-primrose (Ludwigia hexapetala), are also being considered for biological control.

More information about DRAAWP biological control projects can be found under the Project Description tab of the DRAAWP website.