SEE ALSO THE FOLLOWING ARTICLES Usually, this is achieved by the introduction of additional Agriculture / Biological Control, of Plants / Integrated Pest natural enemies (either new species or more of the same Management / Ladybugs / Wasps species) or by encouraging an increase in the abundance of local natural enemies by habitat modifi cation. FURTHER READING Bigler, F., D. Babendreier, and U. Kuhlmann, eds. 2006. Environmental BIOLOGICAL CONTROL STRATEGIES Impact of Invertebrates for Biological Control of Arthropods. Wallingford, Weed biological control efforts can be grouped into UK: CABI Publishing. Clausen, C. P. 1978. Introduced Parasitoids and Predators of Arthropod three different strategies: introduction, augmentation, Pests and Weeds: A World Review. Agricultural Handbook No. 480. and conservation. Introduction, or classical biological Washington, DC: USDA. control, is the movement of selected natural enemies of Greathead, D. J., and A. H. Greathead. 1992. Biological control of insect pests by parasitoids and predators: The BIOCAT database. Biocontrol a targeted plant species from its native range into the News and Information 13(4): 61N–68N. new area invaded by the weed. It is common for exotic Gurr, G., and S. Wratten, eds. 2000. Biological Control: Measures of Suc- weeds to lack natural enemies in their new area of inva- cess. Dordrecht: Kluwer. sion. When a plant is imported as an ornamental or crop Gurr, G. M., S. D. Wratten, and M. A. Altieri. 2004. Ecological Engineer- ing for Pest Management: Advances in Habitat Manipulation for Arthro- plant, effort is taken to ensure it is free of insects, mites, pods. Ithaca, NY: Cornell University Press. and disease. Similarly, plants accidentally introduced Heinz, K. M., R. G. Van Driesche, and M. P. Parrella, eds. 2004. Biocontrol are usually transported as seeds or pieces of stem or rhi- in Protected Culture. Batavia, IL: Ball Publishing. Neuenschwander, P., C. Borgemeister, and J. Langewald, eds. 2003. zome, plant parts too small to include or support natural Biological Control in IPM Systems in Africa. Wallingford, UK: CABI enemies that require leaves or larger pieces of the plant Publishing. to survive. If the plant escapes cultivation or is acciden- Van Driesche, R., M. Hoddle, and T. Center, eds. 2008. Control of Pests and Weeds by Natural Enemies: An Introduction to Biological Control. tally introduced and begins to spread, the reason for its Oxford: Blackwell. success as an invader may be due to the difference in the Waage, J. K., and D. J. Greathead. 1988. Biological control: Challenges level of herbivory it receives compared to plants native to and opportunities. Philosophical Transactions of the Royal Society of the area, which are damaged and infected by their own London B 318: 111–128. Wajnberg, E., J. K. Scott, and P. C. Quimby, eds. 2001. Evaluating Indi- group of natural enemies. This difference in the level rect Ecological Effects of Biological Control. Wallingford, UK: CABI of herbivory or disease has been proposed as one of the Publishing. reasons that exotic plants become invasive and is called the “enemy release hypothesis.” Classical biological con- trol involves the discovery of specifi c natural enemies in a plant’s native range, an evaluation of their safety (through host-specifi city testing) and effi cacy, and the BIOLOGICAL CONTROL, study of their release and establishment in the invaded range. The objective is for the exotic natural enemies to OF PLANTS permanently reduce the weed population. It is generally accepted that the weed will not to be eradicated and MICHAEL J. PITCAIRN that both the weed and biological control agents will California Department of Food and Agriculture, Sacramento permanently persist, but at densities below economic or ecological threshold levels where the weed is no lon- Plant populations are limited by many factors, including ger problematic. Classical biological control is the most abiotic conditions, resource limitations, germination safe common biological control method used against plants sites, plant-to-plant competition, predation (herbivory), and should generally be part of an integrated pest man- pollination, and plant disease. The large and various agement program. groups of herbivores and diseases that consume or infect Augmentative biological control is the addition of a particular plant are called its natural enemies, and the natural enemies, either native or exotic, to provide a damage they impart due to their feeding or infection temporary boost to the background level of herbivory. works together with the other limiting forces to main- Natural enemies released in an augmentative program are tain a plant’s population density around some reduced usually not expected to survive past their life spans or the level. Biological control of invasive plants is a pest control growing season and often do not become permanently method where the natural enemies of an organism are established. The released organisms are mass reared in lab- intentionally manipulated to further reduce its abundance. oratory cultures so that thousands are released at a time. BIOLOGICAL CONTROL, OF PLANTS 63 From Daniel Simberloff and Marcel Rejmánek, editors, Encyclopedia of Biological Invasions, Berkeley and Los Angeles: University of California Press, 2011. 02_Simberloff10_B_p43-91.indd 63 9/10/10 4:09:50 PM Augmentative biological control was originally developed An example is Collego, a commercial product consist- against insect pests in greenhouses and fi eld crops, where ing of spores of the fungus Colletoctricum gloeosporioides it is economically feasible to produce thousands of para- f. spp. aeschynomene, for control of northern jointvetch, a sitic or predatory insects on high value crops, especially native leguminous weed in rice and soybean crops in the if the pests are resistant to insecticides. There are very southeastern United States. Most of these products have few examples of augmentative biological control being not been economically viable because they are effective used on plants, probably because it is not cost-effective against a single weed species and must compete in a mar- compared to herbicides, which are able to control broad ketplace with broad-spectrum herbicides effective against classes of weeds. For invasive plants that infest large areas, many weed species. The use of plant pathogens as an aug- augmentative strategies are not likely to be cost-effective. mentative biological control tool has great potential and Some have called the use of sheep or goats an augmenta- needs to be explored further. tive control activity, but the use of grazing is tradition- Recently, a stem-boring wasp and a scale insect have ally considered a cultural control method because the been proposed for use in augmentative control releases animals must be herded. Some plant diseases have been against Arundo donax, a giant reed that has invaded the developed for use as bioherbicides and can be classifi ed riparian community along the Rio Grande, the river that as augmentative control. Unlike insect agents, impacts serves as a border between Mexico and the United States. from a bioherbicide may occur for several generations Both insects are exotic and were obtained from Spain, of the pathogen, but they usually do not extend beyond where A. donax is native. The proposed objective is to rear a single fi eld season. Eight pathogens worldwide have hundreds of thousands of these species in a mass-rearing been registered for use as bioherbicides against weeds. facility and then release them early in the growing season STEPS IN A CLASSICAL WEED BIOLOGICAL must be summarized and submitted for consideration to the regu- CONTROL PROGRAM latory authority before a permit will be issued. The review process can take months to years. 1. Target selection. Identify weed species using morphological and molecular techniques and identify area of origin. Resolve 5. Implementation. Upon approval for introduction, initial release confl icts regarding the commercial or environmental value of the and establishment of the biological control agent will occur in target weed. Perform cost–benefi t analysis. fi eld nursery sites, areas with high densities of the target weed located in climatic areas deemed optimal for the control agent. 2. Foreign exploration in weed’s area of origin. Examine litera- Usually, only a few organisms (usually fewer than 1,000) are ture and explore target weed’s native range to discover and col- available for initial releases. Once they are established and their lect potential biological control agents. When extensive, native numbers increase, collections of surplus agents will be used areas with the most similar climate to the invaded range should to redistribute them throughout the invaded range. Regional be identifi ed as priority. Correct identifi cation of all collected redistribution can be facilitated through outreach events, such material is critical for purposes of safety and project success. as “fi eld days” where local land managers and property owners Plants closely related to the target weed should be examined in are invited to visit the nursery site, learn the biology of the tar- the native range to see if they are damaged by candidate control get weed and control agent, and receive a small quantity of the agents. agent for release on their property. 3. Host specifi city studies. All potential biological control agents 6. Post-release monitoring. Following their initial release, nurs- should be subjected to a series of choice