Biological Control Agents and Examples – What are the Different Types?

Environmental biology is one of the critical Biology Topics that involves understanding how humans impact the environment and how to address environmental issues.

Types, Merits, and Demerits of Biocontrol Agents – Biological Pest Control

Chemical pesticides appear to be very useful for protecting crops from pests, but they pose serious problems in various aspects.

• The production of chemical pesticides causes environmental pollution and is much more expensive.
• Pests continue to develop resistance against pesticides.
• Chemical pesticides are toxic and biocides.
• They kill even useful organisms and harm human beings and animals.
• Insecticides kill insects thereby hampering pollination and leading to low production of crops.
• They result in biomagnification.

Therefore, present-day scientists are of the opinion to use biological agents for controlling pests. The idea is not to eradicate pests and pathogens but to keep them at manageable levels through a system of checks and balances operating in the ecosystem. The natural method of pest and pathogen control involving the use of natural predators and pests (viruses, bacteria, and other insects) is called biological control or biocontrol. In natural methods, the natural enemies of any organism, e.g., predators, parasites, and parasitoids may be used as biocontrol agents.

For instance, predators like Lady Bird Beetle and Dragonfly prey on aphids and mosquitoes respectively. Free-living fungi like Trichoderma and Arthrobotrys can attack various plant pathogens, especially nematodes. Baculovirus is used against controlling insects and arthropods. These are species-specific narrow-spectrum bioinsecticides without any side effects on the plants or any other consuming organisms. These techniques are taken under the Integrated Pest Management (IPM) programme.

Biological Control of Pests

Characteristics of a Natural Enemy (Agents)

• Must be host-specific.
• Absence of hyperparasite.
• The reproductive rate should be high.
• Ability to adapt to new environments.

Agents are
A. Vertebrates:

• Fish, tadpoles, frogs, and toads – are used to eat mosquito larvae.
• Toads, salamanders, and lizards – used to eat insects on land.

B. Invertebrates:

• Protozoa: More than 80 protozoa cause diseases in insects. European corn borers are controlled by Perezia pyruvate.
• Nematodes: More than 200 species of pest insects, are susceptible to Nematodes. Popillia japonica Japanese beetle.
• Arthropoda: Spider – through spider net or directly attack insects, Mites – As parasites on many plants damaging pests, Insects (as parasites and predators)
• Largest and most significant group. It is believed that 25-33% of insects are beneficial to farmers
  • Tiger beetle Lophyra sp. used to eat Gandhi rice bug.
  • Adult and grubs of Ladybird beetles destroy aphids.
• Microbes: Fungi, bacteria, and viruses play a role in biological control.
  • The Entomophthora genus is used against flies and grasshoppers.
  • Popillia japonica (Japanese beetle) is controlled by Bacillus papillae in milky disease.
  • Many larvae of Lepidoptera are controlled by spores of Bacillus thuringiensis.

Significance of Biological Control

• Insect pests never become resistant to biological agents.
• No adverse effect on beneficial insects.
• It has no side effects and degrades into nontoxic substances where soil acts as a sink.
• Effects are quick and permanent.

Biopesticides

Biopesticides are biological agents that are used for the control of weeds, insects, and pathogens. Such agents are viruses, bacteria, protozoa, fungi, and mites. The most important name of a bacterial biopesticide is Bacillus thuringiensis. It is an endospore-forming bacteria pathogenic to insect larvae. Endospores produce an insecticidal Cry protein. The protein crystal, called the parasporal body, is produced with the endospore cell. When it is ingested by any insect (especially bol worms, and gypsy moths), it gets dissolved and causes destruction of the surface tissues of the larval gut, causing death. However, the protein does not harm animals or humans. Its commercial preparation contains a mixture of spores, Cry protein, and an inert carrier. This bacterium was the first biopesticide to be used on a commercial scale in the world and in India.

Biopesticides are typically created by growing and concentrating naturally occurring organisms and their metabolites including bacteria and other microbes, fungi, nematodes, proteins, etc. They are often considered to be important components of integrated pest management (IPM) programmes and have received much practical attention as substitutes to synthetic chemical plant protection products (PPPs).

Biopesticides fall into three major categories:

• Microbial pesticides consist of bacteria, entomopathogenic fungi, or viruses.
• Entomopathogenic nematodes are also often classed as microbial pesticides, even though they are multicellular.
• Biochemical pesticides are naturally occurring substances that control pests.
• Plant-incorporated protectants (PIPs), have genetic material from other species incorporated into their genetic material (e.g., GM crops).

Biopesticides have usually no known function in photosynthesis, growth, or other basic aspects of plant physiology; however, their biological activity against insects, pests, nematodes, fungi, and other organisms is well documented. Every plant species has developed a built-in unique chemical complex structure that protects it from pests. These biodegradable, economical, and renewable alternatives are used especially under organic farming systems. Biopesticides are of two types: bioherbicides and bioinsecticides.

1. Bioherbicides

Bioherbicides are those organisms or their extracts that destroy weeds without harming useful plants. Bioherbicides may be of various types:

• Predator herbivores: An insect Cactoblastis cactorum is used in India and America to reduce the growth of cacti.
• Smoother Crops: Some crops do not allow weeds to grow, e.g., sunflower, soybean, barley, rye, sorghum, etc. Rotation of these crops is helpful to protect from weeds.
• Mycoherbicides: A fungus Phytophthora palmivora is used to control milkweeds in Citrus gardens. The spore of the fungus is marketed in the name of “Devine” and “College” and they are used in weed control.
• Transgenic Crops: Transgenic tobacco and tomato have been developed for controlling weeds. Transgenic crops possess genes of smoother crops, and pest and herbicide resistance.
• Transgenic tomatoes and tobacco are active against hornworm larvae and against herbicides respectively.
• Vegetables: Certain weeds can be eliminated or made useful by using them as vegetables or fodder, e.g., Amaranthus, or Chenopodium.

2. Bioinsecticides

Organisms or their products which are used to kill or repel specific insects are called bioinsecticides. They are of different types.

(a) By using Predator Animals:

• Ladybird beetle Radiola cardinals are effective in controlling Citrus pests, Icerya purchase.
• The beetles are allowed to reproduce in a confined space and then they are set free in the garden of the citrus plant to remove Icerya.
• In controlling aphids hoover fly (Syrphid) larva is very effective.
• In south India, the stripped bug is controlled by ducks in the paddy field.
• Mosquito larvae in the ponds and drains may be controlled by gambusia or tilapia fish.
• Apart from several larvae-eating small birds, insect-eating Mexican frogs or different types of spiders like crabs or wolf spiders are all used for controlling pests.

(b) By using Parasites and Pathogens:

• The bacteria Bacillus thuringiensis usually infects the insects and kills them. They produce one type of ectotoxin by which the insect dies.
• If spores of these bacteria are applied to plants, the plants may be rescued from the attack of pests.
• The Japanese beetle has been controlled by using Bacillus papillae and B. lentivirus.
• The bacteria cause an infection called milky disease in the larvae, where the blood of the larvae becomes filled with bacteria and has a milky appearance.
• The infected larvae release more spores when they die, thus infecting more larvae of the insects and killing them as well.
• In many countries, the larval pest of many Lepidoptera, Coleoptera, could be controlled using such methods.
• Armyworm or Mythimna is a pest of maize plant, it is controlled by the parasite Apanteles.

In USSR the fungus Beauveria bassiana has been successfully employed in controlling potato beetle and codling moth.

(c) Natural Insecticides:
There are some natural organic compounds obtained from plants as well as animals that act like pesticides.

• Nicotine: Obtained from the Nicotiana species. The purified chemical is highly poisonous. Nicotine sulfate is one of the most toxic insecticides.
• Pyrethrum: Obtained from the inflorescence of Chrysanthemum sp. The active compounds are pyrethrin, and cinerin. Pyrethrin is also used in fly sprays, aerosols, mosquito coils, etc.
• Rotenoid: Some leguminous plants produce rotenoid. Rotenoid is insoluble in water but soluble in organic solvents. There are about six types of rotenoid and among them, rotenone is a powerful insecticide obtained from Derris elliptica and Lonchocarpus nicou. These are harmless to warm-blooded animals.
• Azadirachtin: Obtained from Azadirachta indica (neem). It occurs in margosa extract. Spray of the same keeps away the Japanese beetles and other leaf-eating pests because of the antifeedant property of azadirachtin.

Other Methods of Biological Control of Pests

(a) Sterile Male Technique:
In this technique, adult males are sterilized by radiation and then, they are usually released in nature in the pest-infected field. When the sterilized males mate with the females, the females may only lay unfruitful eggs. By this pest population may be decreased substantially. This method not only controls pest damage but also keeps the environment pollution-free as with the application of this technique application of pesticides could be cut. From 1962 to 1971, the population of screwworms could be destroyed totally in America by using this technique. The screwworm is a pest of cattle.

(b) Built-in Resistance to Pests:
In many cases, resistance to the pest could be developed by genetic engineering in plants. Toxin toxin-producing gene was isolated and introduced into the plants which could produce toxins against the insect pest (i.e., trioxide). The toxin-producing ‘Bt’ gene of Bacillus thuringiensis, could be introduced into the tomato plant and the transgenic tomato plant could be produced. Such tomato plants could be grown easily without a pest attack.

(c) Use of Hormones:
There are many insects in which adult females secrete pheromones before mating. Pheromone is volatile and males may be attracted to it. Therefore, by using pheromone the pest may be engaged. Besides this, if one piece of paper pheromone is kept at one site of the field the males will be confused and fail to perform mating with the females.

Juvenile hormone is essential in the initial phase of differentiation in insects, in the latter phase this hormone is totally unnecessary. If this hormone is present, a larva cannot mature into an adult instead, the larva achieves a giant size. Therefore, during the metamorphosis of larva or nymph if the juvenile hormone is applied adult insect pests cannot be developed. By this, the population of the pest may be reduced. Moulting hormone causes premature moulting of larvae resulting in death.

Application of Biopesticides

Biopesticides are typically microbial biological pest control agents that are applied in a manner similar to chemical pesticides. In order to implement these environment-friendly pest control agents effectively, it can be important to pay attention to the way they are formulated and applied.

Biopesticides for use against crop diseases have already established themselves on a variety of crops. For example, biopesticides already play an important role in controlling downy mildew diseases. Their benefits include a 0-day Pre-Harvest Interval, the ability to use under moderate to severe disease pressure, and the ability to use a tank mix or in a rotational program with other registered fungicides. Because some market studies estimate that as much as 20% of global fungicide sales are directed at downy mildew diseases, the integration of bio fungicides into grape production has substantial benefits in terms of extending the useful life of other fungicides, especially those in the reduced-risk category.

A major growth area for biopesticides is in the area of seed treatments and soil amendments. Fungicidal and biofungicidal seed treatments are used to control soil-borne fungal pathogens that cause seed rots, damping-off, root rot, and seedling blights. They can also be used to control internal seed-borne fungal pathogens as well as fungal pathogens that are on the surface of the seed. Many bio fungicidal products also show capacities to stimulate plant host defenses and other physiological processes that can make treated crops more resistant to a variety of biotic and abiotic stresses.

Advantages of Biological Control of Pests

• Harmful residues were not detected.
• Can be cheaper than chemical pesticides when locally produced.
• Can be more effective than chemical pesticides in the long term (as demonstrated, by the LUBILOSA Programme)
• Biodegradable.

Disadvantages of Biological Control of Pests

• High specificity may require an exact identification of the pest/pathogen and the use of multiple products to be used; although this can also be an advantage in that the biopesticide is less likely to harm species other than the target.
• Often slow speed of action (thus making them unsuitable if a pest outbreak is an immediate threat to a crop.
• Often variable efficacy is due to the influences of various biotic and abiotic factors (since biopesticides are usually living organisms, which bring about pest/pathogen control by multiplying within the target insect pest/pathogen).
• Living organisms evolve and increase their resistance to biological, chemical, physical, or any other form of control. If the target population is not exterminated or rendered incapable of reproduction, the surviving population can acquire a tolerance of whatever pressures are brought to bear, resulting in an evolutionary arms race.

Pest

The organisms that damage cultivated plants, food, crop, domestic animals, health, and happiness of human, is known as a pest. The physical, biological or chemical substance which is used to control pests is known as a pesticide. A pest is any living organism that is invasive or prolific, detrimental, troublesome, noxious, destructive, a nuisance to either plants or animals, human or human concerns, livestock or human structures. Often animals are derided as pests as they cause damage to agriculture by feeding on crops or parasitizing livestock, such as codling moths on apples or boll weevils on cotton. An animal could also be a pest when it causes damage to a wild ecosystem or carries germs within human habitats. Examples of these include those organisms that vector human disease, such as rats or fleas that carry the plague disease, mosquitoes that vector malaria, and ticks that carry Lyme disease.

Types of Pesticides:

Pesticides	Examples
1. Fungicides (kill fungal pathogens)	Bordeaux mixture [CuSO4 + Ca(OH)2 + H2O], Burgandy mixture [CuSO4 + Na2CO3 + H2O], lime sulphur
2. Algicides (kill algal pathogen)	CuSO4
3. Weedicides (kill weeds)	Carbamates, Triazines.
4. Insecticide (kill insect)	Organophosphate, organochlorine. Some natural insecticides are Azadirachtin, Pyrithrum, and Rotenone.
5. Nematocides (kill nematode)	Ethyl dibromide, Methyl bromide.

Some Synthetic Pesticides:

Synthetic Pesticides	Examples
1. Organochlorine	DDT, BHC, Endrin, Aldrin.
2. Organophosphates	Malathion, Parathion, Tetra-acetyl pyrophosphate.
3. Carbamates	Carbofuran, Dithane
4. Triazines	Atrazine, Simazine

Integrated Pest Management (IPM)
It also known as integrated pest control (IPC), is a broad-based approach that integrates practices for economic control of pests. IPM aims to suppress pest populations below the economic injury level. IPM implies the improved use of chemical inputs, and cultivation practices, including traditional control, pest population control, use of nonpollutant pesticides, etc.