BIOLOGICAL CONTROL-MEANING, TYPES & EFFECTS
Introduction
- Biological control is a means of controlling pests including weeds and plant diseases using other living organisms or natural enemies.
- Natural biocontrol is the reduction of a species’ populations by natural enemies with no manipulation of the natural enemies by man. In contrast, applied biocontrol is the reduction of a species’ populations by natural enemies when man manipulates the natural enemy populations for the purpose of population control.
- It is an important component of integrated pest management(IPM) programs.
- Natural enemies of insect pests are also called as biological control agents. These are predators, parasitoids, pathogens, and competitors.
- Biological control agents of plant diseases are most often referred to as antagonists.
- The term “biological control” was first used by Harry Scott Smith.
- The first reported case of a classical biological control attempt in Canadawas parasitoidal wasp Trichogramma minutum.
Types of Biological control
Three forms of applied biocontrol are generally recognized based on how the natural enemies are manipulated.
Importation
- Importation or classical biological control involves the introduction of a pest’s natural enemies to a new locale where they do not occur naturally.
- In classical biocontrol, exotic natural enemy species are imported and released in the area where the pest occurs. If the introduced natural enemy survives and adapts to its new habitat, it may increase in numbers, disperse throughout the pest region over the time, and suppress the pest population.
- This technique is often practiced against exotic pest species because these pests usually invade their new habitats without the natural enemies that suppress their populations in their native range. However, classical biocontrol is also practiced against native pests when it is thought that an exotic natural enemy species may be able to suppress the pest better than native natural enemies.
- Classical biocontrol can result in complete, sustained, and broad-scale control of the target pest.
- In order to effectively control a pest, a biological control agent requires a colonizing ability which allows it to keep pace with changes to the habitat in space and time.
- One of the earliest successes was in controlling Icerya purchasi (cottony cushion scale) in Australia, using a predatory insect Rodolia cardinalis (the vedalia beetle).
Augmentation
- It refers to periodic releases of natural enemies into the habitat where the pest occurs, boosting the naturally occurring populations there.
- It may be necessary when existing natural enemies fail to colonize the pest habitat each season or too late or otherwise fail to attain numbers sufficient to control the pest.
- In inoculative release, small numbers of the control agents are released at intervals to allow them to reproduce, in the hope of setting up longer-term control and thus keeping the pest down to a low level.
- In inundative release, large numbers are released in the hope of rapidly reducing a damaging pest population, correcting a problem that has already arisen.
- An example of inoculative release occurs in the horticultural production of several crops in greenhouses. Periodic releases of the parasitoidal wasp, Encarsia formosa, are used to control greenhouse whitefly.
- The egg parasite Trichogramma is frequently released inundatively to control harmful moths.
Conservation
- Classical and augmentation biocontrol directly manipulate natural enemies through natural enemy releases.
- Conservation biocontrol, in contrast, fosters existing natural enemy populations indirectly by making the environment more favorable for natural enemies. This can involve the removal of factors that negatively influence natural enemies, or the addition of factors that positively influence natural enemies.
- Conservation biocontrol practices seek to minimize disruptions to natural biocontrol; however, it is also an important part of any classical or augmentation biocontrol effort.
- Cropping systems can be modified to favor natural enemies, a practice known as habitat manipulation.
Biological control Agents
Predators
- These are mainly free-living species that directly consume a large number of prey during their life. Lady beetles, and in particular their larvae which are active between May and July in the northern hemisphere, are voracious predators of aphids, and also consume mites, scale insects and small caterpillars.
- Several species of nematode are important predators of insect and other invertebrate pests. Entomopathogenic nematodes form a stress–resistant stage known as the infective juvenile. These spread in the soil and infect suitable insect hosts. Upon entering the insect they move to the hemolymph where they recover from their stagnated state of development and release their bacterial symbionts. The bacterial symbionts reproduce and release toxins, which then kill the host insect.
- Species used to control spider mites include the predatory mites Phytoseiulus persimilis, Neoseilus californicus, and Amblyseius cucumeris.
- For rodent pests, catsare effective biological control.
Parasitoids
- Parasitoids lay their eggs on or in the body of an insect host, which is then used as a food for developing larvae. The host is ultimately killed. Most insect parasitoids are wasps or flies.
- Parasitoids are most effective at reducing pest populations when their host organisms have limited refuges to hide from them.
- Parasitoids are among the most widely used biological control agents. Encarsia formosa is a small parasitoid wasp attacking whiteflies, sap-feeding insects which can cause wilting and black sooty moulds in glasshouse vegetable and ornamental crops.
- Birds are a natural form of biological control.
Bacteria
- Bacteria used as control agent infect insects via their digestive tracts, so they are used for controlling insects with sucking mouth parts such as aphids and scale insects.
- Bacillus thuringiensis, is the most widely applied species of bacteria used for biological control, used against Lepidopteran (moth, butterfly), Coleopteran(beetle) and Dipteran (true fly) insect pests.
- Paenibacillus popilliae has been found useful in the control of Japanese beetle, killing the larvae.
- Bacillus, fluorescent Pseudomonads, and Streptomycetes are controls of various fungal pathogens.
Fungi
- Beauveria bassiana is used to manage whiteflies, thrips, aphids and weevils.
- Lecanicillium is used against white flies, thrips and aphids.
- Metarhizium is used against pests including beetles, locusts and other grasshoppers, Hemiptera, and spider mites.
- Paecilomyces fumosoroseus is effective against white flies, thrips and aphids; Purpureocillium lilacinus is used against root-knot nematodes, and 89 Trichoderma species against certain plant pathogens.
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Viruses
Baculoviruses have been shown to be useful in biological pest control. RNA mycoviruses are controls of various fungal pathogens.
Side Effects
An introduced control does not always target only the intended pest species; it can also target native species. Hence, biological control can affect biodiversity through predation, parasitism, pathogenicity, competition, or other attacks on non-target species.