Microbes as biocontrol agents

What is biocontrol?

In plain words: instead of relying on toxic sprays, a farmer can let natural enemies help: predators that eat pests, and microbes that kill pests or check plant pathogens. NCERT’s microbial agents are narrow in their targets: Bt spares insects other than caterpillars, and baculoviruses spare non-target insects (NCERT page 157).

Biological control of pests and diseases

• In agriculture, one method of controlling pests relies on natural predation rather than introduced chemicals (page 156).
• A key belief of the organic farmer is that biodiversity furthers health: the more variety a landscape has, the more sustainable it is (page 156).
• The organic farmer therefore does not eradicate the insects that are sometimes called pests, but keeps them at manageable levels through a complex system of checks and balances within a living ecosystem (page 156).
• Conventional farming often uses chemicals that kill useful and harmful life forms indiscriminately. The organic approach is holistic: it seeks to understand the webs of interaction among the many organisms of the field (pages 156–157).
• Without the pests, the beneficial predatory and parasitic insects that depend on them as food or hosts could not survive; so the organic farmer keeps pests at manageable levels rather than eradicating them (pages 156–157). Biocontrol greatly reduces our dependence on toxic chemicals and pesticides (page 157).
• An important part of biological farming is knowing the life forms of the field, predators as well as pests, with their life cycles, feeding patterns and preferred habitats; this helps develop the right means of biocontrol (page 157).

NCERT slip (page 157): NCERT prints that the organic farmer sees eradicating pests as “not only possible, but also undesirable”. A ‘not only … but also’ pair should give two reasons against eradication, so the intended word is most likely “impossible” (our reading of the sentence). In an exam, write that the organic farmer keeps pests at manageable levels instead of eradicating them, because the beneficial predatory and parasitic insects depend on them as food or hosts.

Ladybirds and dragonflies

• The very familiar beetle with red and black markings, the ladybird, is useful to get rid of aphids (page 157).
• Dragonflies are useful to get rid of mosquitoes (page 157).

Both are insects, members of the phylum Arthropoda (NCERT Class 11 Biology, Chapter 4), used here as predators.

Bacillus thuringiensis (Bt)

• Bacillus thuringiensis, often written Bt, is a bacterium that can be introduced to control butterfly caterpillars (page 157).
• NCERT: Bt is available in sachets as dried spores, which are mixed with water and sprayed onto vulnerable plants such as brassicas and fruit trees, where the insect larvae eat them (page 157).
• In the gut of the larvae the toxin is released, and the larvae are killed (page 157).
• NCERT Chapter 10 adds how this works: the Bt toxin protein exists as an inactive protoxin in protein crystals; once an insect ingests it, the alkaline pH of the gut solubilises the crystals and converts it into the active toxin. The active toxin binds to the surface of midgut epithelial cells and creates pores, so the cells swell and lyse and the insect dies. Most Bt toxins are insect-group specific (NCERT Class 12 Biology, Chapter 10, Section 10.1, pages 179–180). Beyond the NCERT text: once the alkaline gut fluid has dissolved the crystals, gut proteases cut the protoxin into the active toxin, and only insects whose gut cells carry matching receptors are affected (Wikipedia, “Bacillus thuringiensis”, facts in our words).
• With genetic engineering, scientists have introduced the B. thuringiensis toxin genes into plants; such plants are resistant to attack by insect pests. Bt-cotton is one example, grown in some states of India; NCERT returns to it in Chapter 10 (page 157). The protection of such plants covers the insect groups that their toxin targets (reasoned from NCERT Chapter 10, page 180: most Bt toxins are insect-group specific).

Trichoderma

• The fungus Trichoderma is a biological control being developed to treat plant disease (page 157).
• Trichoderma species are free-living fungi, very common in root ecosystems, and effective biocontrol agents of several plant pathogens (page 157). NCERT Class 11 lists Trichoderma among the deuteromycetes, the imperfect fungi (Chapter 2, page 18). Do not confuse the two roles of the genus in this chapter: NCERT also names Trichoderma polysporum as the source of the immunosuppressant cyclosporin A (NCERT Section 8.2, page 153).

Baculoviruses and integrated pest management

• Baculoviruses are pathogens that attack insects and other arthropods. The majority of baculoviruses used as biological control agents are in the genus Nucleopolyhedrovirus (page 157). Beyond the NCERT text: the family is now divided into four genera, Alphabaculovirus (nucleopolyhedroviruses of moths and butterflies), Betabaculovirus (granuloviruses), Gammabaculovirus and Deltabaculovirus (Wikipedia, “Baculoviridae”, facts in our words). For NEET, give NCERT’s Nucleopolyhedrovirus.
• They are excellent candidates for species-specific, narrow-spectrum insecticidal applications, and have been shown to have no negative impacts on plants, mammals, birds, fish or even on non-target insects (page 157). Beyond the NCERT text: baculoviruses cannot replicate in the cells of mammals and plants (Wikipedia, “Baculoviridae”, facts in our words).
• This is especially desirable when beneficial insects are being conserved to aid an overall integrated pest management (IPM) programme, or when an ecologically sensitive area is being treated (page 157). Like all viruses, baculoviruses are non-cellular and replicate only after infecting a host cell (NCERT Class 11 Biology, Chapter 2, pages 19–20).

Solved examples

Common mistakes to avoid

Quick revision summary

• Biocontrol: the use of biological methods for controlling plant diseases and pests; it greatly reduces our dependence on toxic chemicals and pesticides.
• Organic farming: biodiversity furthers health; pests kept at manageable levels by checks and balances, not eradicated, so that their predators and parasites survive.
• Predators: the ladybird eats aphids; dragonflies eat mosquitoes.
• Bacillus thuringiensis (Bt): dried spores sprayed on plants; eaten by caterpillars; toxin released in the gut kills them; other insects unharmed. Bt toxin genes in crops: Bt-cotton.
• Trichoderma: free-living fungi of root ecosystems; control plant pathogens.
• Baculoviruses (mostly Nucleopolyhedrovirus): species-specific, narrow-spectrum; useful in integrated pest management (IPM) and ecologically sensitive areas.

Questions students ask

What is integrated pest management (IPM)?

Integrated pest management (IPM) is, beyond the NCERT text, an approach that combines several ways of keeping pests in check and saves chemical pesticides for when they are really needed; NCERT names IPM (page 157) but does not define it. The US Environmental Protection Agency (public domain) calls IPM “an effective and environmentally sensitive approach to pest management that relies on a combination of common-sense practices”, worked in four steps: set action thresholds, monitor and identify pests, prevent them, and only then control them, choosing the less risky methods first. In the EPA’s words, “broadcast spraying of non-specific pesticides is a last resort.”

Who discovered Bacillus thuringiensis, and how did it get its name?

Beyond the NCERT text, Bacillus thuringiensis (Bt) has two discoverers: the Japanese scientist Shigetane Ishiwata (also spelt Ishiwatari) first found it in 1901 as the cause of a disease of silkworms, and Ernst Berliner rediscovered it in Germany in 1911 as the cause of a disease of Mediterranean flour moth caterpillars; its species name comes from Thuringia, the German province where those infected moths were found (Kumar and colleagues, Egyptian Journal of Biological Pest Control, 2021, CC BY). The same review dates the first commercial Bt product to 1938, in France, and sums up that Bt “was initially considered a risk for silkworm rearing but later it became an insecticidal agent”.

What is the difference between Trichoderma and Trichogramma?

Trichoderma is a fungus and, beyond the NCERT text, Trichogramma is a tiny wasp, though both are used in biological control: NCERT describes Trichoderma species as free-living fungi, very common in root ecosystems, that are effective biocontrol agents of several plant pathogens (page 157). Trichogramma are “minute parasitoid wasps that develop within other insect eggs”, less than half a millimetre long, and are “released as biological control agents against major moth and butterfly pests” (Lindsey and colleagues, BMC Biology, 2018, CC BY). So Trichoderma fights plant diseases, while Trichogramma is an insect used against insect pests, whose eggs it parasitises (our comparison).

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. Which insect predators does NCERT name, and what does each control?

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   The ladybird, which gets rid of aphids, and dragonflies, which get rid of mosquitoes (NCERT page 157).

2. Q2. In what form is Bt sold, and on which plants is it sprayed?

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   As dried spores in sachets, mixed with water and sprayed on vulnerable plants such as brassicas and fruit trees (NCERT page 157).

3. Q3. To which genus do most baculoviruses used as biological control agents belong, according to NCERT?

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   Nucleopolyhedrovirus (NCERT page 157). Beyond the NCERT text: the family is now divided into four genera, and the nucleopolyhedroviruses of moths and butterflies are placed in Alphabaculovirus (Wikipedia, “Baculoviridae”, facts in our words). For NEET, give NCERT’s Nucleopolyhedrovirus.