Biological control of insect pests
Predators, parasitoids and pathogens against pests: classical, augmentative and conservation biological control, famous successes, risks and practice.
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Most potential pests never become pests, because something eats them, parasitises them or makes them sick. Biological control is the deliberate use of these natural enemies to keep pest populations below damaging levels. It is the oldest form of crop protection and, when it works, the cheapest and most lasting.
Biological control sits at the heart of integrated pest management. Exams ask for its three approaches, the kinds of natural enemies, famous examples, and its limits and risks.
The natural enemies
Predators
Predators kill and eat many prey during their lives. They are usually larger than their prey and often feed on several species.
- Lady beetles (Coccinellidae), adults and larvae, mainly on aphids, scales and mites.
- Lacewings (Chrysopidae), whose larvae are voracious "aphid lions".
- Hoverfly larvae (Syrphidae) on aphids.
- Predatory bugs (anthocorids, mirids, reduviids) on thrips, whiteflies, eggs and small larvae.
- Predatory mites (Phytoseiidae) on spider mites, thrips and whiteflies.
- Spiders, ground beetles and ants.
- Birds, bats and other vertebrates.
Parasitoids
A parasitoid is an insect, usually a tiny wasp or a fly, whose larva develops on or inside a single host and kills it. Parasitoids are often highly specific, which makes them valuable and safe.
- Endoparasitoids develop inside the host; ectoparasitoids outside it.
- Egg parasitoids, such as Trichogramma wasps, kill the pest before it hatches.
- Larval and nymphal parasitoids, such as Cotesia wasps in caterpillars, Aphidius in aphids and Encarsia in whitefly nymphs.
- A parasitised aphid often turns into a swollen, papery "mummy", a useful sign in the field.
Pathogens
Microorganisms that cause disease in insects, used as biopesticides:
- Bacteria, chiefly Bacillus thuringiensis (Bt), toxic to specific insect larvae.
- Fungi, such as Beauveria and Metarhizium, which infect through the cuticle and need humidity.
- Viruses, especially baculoviruses, highly specific to particular caterpillars.
- Entomopathogenic nematodes (Steinernema, Heterorhabditis), which carry symbiotic bacteria and kill soil-dwelling larvae.
The three approaches
Classical biological control
Many serious pests are invasive: they arrived without the natural enemies that control them at home. Classical biological control finds those enemies in the pest's region of origin, tests that they will not attack non-target species, and introduces them, once, so that they establish and spread permanently.
- Cottony cushion scale and the vedalia beetle (1888–1889). An Australian scale insect devastating Californian citrus was brought under control within a few years by an introduced Australian lady beetle, Rodolia cardinalis, and a parasitic fly. It founded the discipline.
- Cassava mealybug in Africa (1980s). A mealybug from South America, accidentally introduced to Africa in the 1970s, spread across the cassava belt and threatened a staple of hundreds of millions of people. A small parasitic wasp from South America, Anagyrus lopezi, was released from 1981 across more than twenty countries and brought it under control. Economic studies have estimated returns of hundreds of units for every unit spent, and the programme's leader, Hans Herren, received the World Food Prize in 1995.
- Prickly pear in Australia (1920s–1930s). Invasive Opuntia cacti covering millions of hectares were cleared by the introduced moth Cactoblastis cactorum, a classic case of biological control of a weed.
Augmentative biological control
Natural enemies are reared in large numbers and released, because those present are too few or arrive too late.
- Inoculative releases: a small number released early in the season to multiply and give control over a longer period, common in greenhouses.
- Inundative releases: large numbers released for quick control, like a living pesticide, with little expectation that they will persist.
Augmentation is standard in protected cropping in many countries, where parasitoids and predatory mites control whiteflies, aphids, thrips and spider mites on tomatoes, cucumbers, peppers and ornamentals, often replacing most insecticides. In field crops, egg parasitoids against moth pests of sugarcane, maize, rice and cotton are widely used in parts of Asia, Latin America and Europe.
Planning an egg parasitoid release
A recommendation calls for releasing Trichogramma egg parasitoids at 50,000 per hectare, three times at weekly intervals, beginning when moth egg-laying starts (detected with pheromone traps). Parasitoids are supplied as "cards" of parasitised host eggs, each card yielding about 20,000 wasps.
- Wasps needed for 10 ha: 50,000 × 3 × 10 = 1,500,000.
- Cards: 1,500,000 ⁄ 20,000 = 75 cards, about 2.5 cards per hectare per release, spread evenly and placed in shade so the emerging wasps survive.
Timing matters more than numbers: released before or long after egg-laying, the wasps find nothing to parasitise and die. Release rates and card sizes vary by product and pest; follow local guidance.
Conservation biological control
The most widely used approach, though rarely noticed, is protecting the natural enemies that are already present:
- Avoid broad-spectrum insecticides; use thresholds, selective products and spot treatments.
- Provide food and shelter: flowering strips and hedgerows supply nectar and pollen for parasitoids and hoverflies; beetle banks and field margins shelter ground predators.
- Diversify fields with intercrops and rotations.
- Manage ants that protect aphids, scales and mealybugs from their enemies.
Risks and rules
Biological control is not risk-free, especially classical introductions:
- Non-target effects: generalist enemies can attack native species. The cane toad, introduced to Australia in the 1930s against sugarcane beetles, failed as a control agent and became a pest itself; the cactus moth that saved Australian rangelands later spread into North America, where it threatens native cacti; the harlequin lady beetle, released for aphid control, has become invasive in Europe and North America.
- Modern practice requires careful host-range testing, risk assessment and approval before release. The International Plant Protection Convention sets out guidelines for the export, shipment, import and release of biological control agents (ISPM 3), and national authorities regulate them.
Other practical limits: biological control acts more slowly than chemicals; it works best against pests with natural enemies that can be reared or conserved; product quality and storage matter; and one sprayed neighbour can undo a programme, which is why area-wide approaches help.
Key points
- Biological control uses predators, parasitoids and pathogens to keep pests below damaging levels.
- Predators eat many prey; parasitoids develop in a single host and kill it; pathogens cause disease and are used as biopesticides.
- Classical: introduce enemies from the invasive pest's home region (vedalia beetle and cottony cushion scale; Anagyrus lopezi and cassava mealybug).
- Augmentative: release reared enemies, inoculatively (few, early) or inundatively (many, quick).
- Conservation: protect existing enemies with selective spraying, habitat and diversity.
- Introductions need host-range testing and regulation (ISPM 3) because of non-target risks.
What the exams ask
- School-leaving and entrance papers: examples of predators, parasitoids and pathogens, and the meaning of biological control.
- Degree and research-fellowship entrances: the three approaches with classic examples, parasitoid biology, inoculative versus inundative releases, mass rearing, and risks of introductions.
- Plant protection examinations: designing release programmes, conservation measures, compatibility with pesticides, and regulation of biocontrol agents.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Is biological control always better than chemicals?
It is safer and often cheaper in the long run, and it does not cause resistance in the same way. But it can be slower, needs knowledge and planning, and does not work for every pest. In IPM it is combined with other methods.
Why do natural enemies sometimes fail to control a pest?
Because they arrive too late, are killed by sprays, lack food or shelter, are disrupted by ants, or are poorly matched to the pest or climate. Fixing those conditions is often more effective than releasing more enemies.
Can I buy natural enemies for my farm?
In many countries, yes: parasitoids, predatory mites, lady beetles and microbial products are sold, especially for greenhouses. Choose products approved locally and follow the supplier's release instructions.
Sources
- DeBach, P. and Rosen, D. Biological Control by Natural Enemies, 2nd edition. Cambridge University Press, 1991.
- Heimpel, G. E. and Mills, N. J. Biological Control: Ecology and Applications. Cambridge University Press, 2017.
- Neuenschwander, P. "Biological control of the cassava mealybug in Africa: a review." Biological Control 21, 214–229 (2001).
- van Lenteren, J. C. et al. "Biological control using invertebrates and microorganisms: plenty of new opportunities." BioControl 63, 39–59 (2018).
- IPPC. ISPM 3: Guidelines for the export, shipment, import and release of biological control agents and other beneficial organisms. FAO, 2005.
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