Regulation of pest abundance below the level of economic injury is the target of biological control, which is usually done by study, importation, augmentation and conservation of beneficial organisms for the regulation of harmful animal’s population. Most of the agricultural pests are insects and these have natural enemies, which are also mostly insects. Therefore most of the examples of biological control come from insects.
Definition: Biological control is the action of natural enemies (parasites, predators and pathogens) in maintaining another organism’s population density at a lower level than would occur in their absence.
The importance of biological control has lately been enhanced due to the fact that overwhelming use of insecticides has led to the resurgence of the pests and resistance to insecticides by the pests like mosquitoes, houseflies and stored grain pests. Biological control is based on the utilization of ecological principles; hence it is frequently called Applied Ecology. Maintenance of the balance of nature is an important aspect of biological control.
Natural Biological Control: includes role of natural enemies to contain pest populations in an undisturbed environment.
Applied Biological Control: includes manipulation of biotic factors (natural enemies) by man to reduce the population of a pest species.
Pests of foreign origin usually do not cause serious damage in their native country because there they are kept under check by natural enemies. But when accidentally introduced into a new country they multiply unchecked and become serious pests. Role of biological control is to find out natural enemies of such pests and introduce them in the areas of pest outbreak. Against pests of domestic origin also exotic natural enemies of species closely related to the indigenous pest are imported and released.
Examples: Control of cottony cushion scale (Icerya purchasi) by using vedalia beetle (Rodolia cardinalis) in California in 1988 is an outstanding success story. In 1887, citrus industry in California suffered massive destruction by the cottony cushion scale. Chemical control had failed. A German scholar, Albert Koebele, was assigned the job to find out natural enemies of this pest in its native home, Australia and New Zealand. In Australia, Koebele found a ladybird beetle, Rodolia cardinalis and a dipteran fly, Chryptochaetum feeding on the pest stages. He dispatched many consignments of the two species to USA for release in the orchards. Chryptochaetum failed to establish but Rodolia multiplied so fast that by July, 1889 the scale was virtually wiped out from the valley.
Another outstanding example is the control of cactus, Opuntia stricta by the Argentine moth, Cactoblastis cactorum in Australian grasslands in 1927-30.
Bacillus thuringiensis is a pathogen which is widely used to control caterpillars of many pest species commercially.
Many outstanding works on biological control have been done in Australia, Canada, Chile, Fiji, Hawaii, Japan, New Zealand, and India. Successes have been reported in over 60 countries. Commonwealth Institute of Biological Control, with headquarters in London and Indian station at Bangalore, is noteworthy for its contribution.
Some terms and definitions
Parasite: An organism that derives its nutritional requirements from another organism, killing it slowly or not killing it at all.
Predator: When one organism kills another instantly for food.
Ectoparasite: Parasites which feed on the host from outside their bodies. Examples are larvae of parasitic Hymenoptera and Diptera.
Endoparasite: Parasites which live inside the body of the host and derive their nutrition from it.
Parasitoid: Larvae of Diptera and Hymenoptera which live as parasites in early stages but behave like predators when nearing maturity and kill the host before emergence.
Protelian parasites: Insects in which only the immature stage is parasitic and not the adult. Examples are parasitic Hymenoptera and Diptera.
Superparasitism: When a parasite oviposits in a host, which is already parasitized by the parasite of the same species.
Hyperparasitism: When a parasite develops on another parasite, which is on the host. The parasite that attacks the host is called primary parasite and the one that attacks the primary parasite is called secondary parasite.
Multiple parasitism: When two or more species of parasites oviposit in the same host. Normally only one parasite manages to develop to maturity.
Methodology
Applied biological control is practiced in the following three ways:
1) Importation and colonization of exotic natural enemies: When the target pest is of foreign origin, it is always advantageous to search for its natural enemies in the country of its origin. It is taken as a general rule that the predominant natural enemy occurring at relatively low host densities in the native home offers greatest promise for introduction to new environment. Usually the dormant stage of the parasite (eggs or pupae), or the dormant stages of the parasitized host are shipped. Releases should be timed with the availability of the host stages to be parasitized. To evaluate the effectiveness of the natural enemy introduced, samples are collected at regular intervals and analyzed as life-table data.
2) Conservation and inundative releases of indigenous natural enemies: Conservation of natural enemies demands judicious and minimal use of insecticides on crops, so that parasites and predators are not unnecessarily killed. Selective insecticides which are not harmful to the natural enemies are used, such as organophosphates and methyl esters. Use of favorable application technique, e.g. soil application of systemic insecticides, seed treatment and use of baits, helps to conserve the natural enemies. Sometimes natural enemies are collected from the field, mass-bred in labs and then released in the field, much like biological insecticides, e.g. use of Trichogramma and Bacillus thuringiensis.
3) Manipulation of natural enemies: When a parasite or predator fails to become effective, ecological, biological and physiological studies are conducted to find out reasons for failure. There are various possible ways of enhancing the effectiveness of natural enemies as follows:
· Development of resistant strains of parasites by artificial selection under controlled conditions.
· Provision for supplementary food for adults.
· Use of behavior modifying chemicals (semiochemicals) is sometimes helpful. Extracts of tomato sprayed on corn increases parasitization of Heiothis zea. The predator Chrysopa is strongly attracted to honey dew of aphids. Synomones are chemicals produced by plants which attract natural enemies and Kairomones are chemicals released by host insects that attract natural enemies.
· Genetic improvement of natural enemies by hybridization and artificial selection of different strains, which increases vigor and effectiveness of parasites and sometimes even resistance to insecticides. Intercropping is known to augment parasitic activity.
Advantages and disadvantages of biological control
Advantages: It is a long-time self-perpetuating control of the target pest. Unlike insecticides, there is no fear of pest developing resistance. There is no fear of environmental pollution. In this method balance of nature in the ecosystem is not disturbed. This is a long-term control method and cost of controlling the pest is economical. There is no fear of pest resurgence, as normally happens by the application of insecticides.
Disadvantages: Biological control is a long-term process and takes years before natural enemies could be established and during this period the pest can cause immense damage. Often natural enemies fail to establish, leading to failure of the entire programme. In case of pest outbreak, biocontrol fails to provide immediate relief. In some cases a natural enemy also damages some useful animals or plants. Biocontrol doesn’t provide surety. The projects usually have equal chances of failure or success.
Some Indian examples of biological control
1. Biological control of cottony cushion scale in southern India by the introduction and release of Rodolia cardinalis from USA in 1928.
2. Control of woolly apple aphid, Eriosoma lanigerum, by Aphelinus mali introduced from USA in 1941.
3. Partial control of San Jose scale by the introduction and release of Prospaltella perniciosi and Aphytis diaspiditis from USA in 1961.
4. Control of sugarcane stemborers in some states of India by inundative releases of Trichogramma minutum, T. japonicum and T. australicum.
5. Control of Lantana weed by the anthocorid bug, Teleonemia scrupulosa along the foothills of Himalaya.
6. Biological control of mosquito larvae by the fish Gambusia and Nothobranchius guntheri.
7. Partial control of Opuntia spp. by the cochineal insect, Dactylopius tomentosus introduced from South America in 1900.