Biological pest control is the use of living organisms — predators, parasites, or pathogens — to suppress pest populations. It is a cornerstone of organic gardening and integrated pest management (IPM), offering an effective alternative to chemical insecticides that avoids the harmful side effects of pesticides on beneficial insects, soil health, and the wider environment.
How Does Biological Control Work?
Biological control exploits the natural relationships between pests and their enemies. In nature, every pest species has predators, parasitoids, or diseases that keep its population in check. Biological control involves either encouraging these natural enemies in the garden (conservation biological control) or introducing them deliberately (augmentative biological control).
| Approach | Description | Example |
|---|---|---|
| Conservation | Encouraging existing natural enemies by providing habitat and reducing pesticide use | Planting wildflowers to support hoverflies and ladybirds |
| Augmentative (inundative) | Releasing large numbers of commercially reared natural enemies | Introducing parasitic wasps (Encarsia formosa) to control whitefly in greenhouses |
| Classical | Introducing a natural enemy from the pest's country of origin to provide long-term control | Introduction of the Harlequin ladybird (though this had unintended consequences) |
Key Biological Control Agents for UK Gardens
Ladybirds (Coccinellidae)
Ladybirds are among the most effective natural predators of aphids. A single seven-spot ladybird (Coccinella septempunctata) can consume over 5,000 aphids during its lifetime. Both adults and larvae are voracious aphid predators, with the alligator-like larvae often consuming more aphids per day than the adults.
Lacewings (Chrysopidae)
Green lacewing larvae are nicknamed "aphid lions" for their ferocious appetite. A single larva can consume 200–300 aphids before pupating. They also eat mealybugs, thrips, whitefly, and small caterpillars. Lacewings can be encouraged by providing overwintering habitat (bug hotels with corrugated cardboard) and by avoiding pesticides.
Parasitic Wasps
Tiny parasitic wasps (parasitoids) lay their eggs inside or on pest insects, and the developing wasp larvae consume the host from within. Key species include:
- Encarsia formosa: Controls glasshouse whitefly; commercially available for greenhouse use
- Aphidius species: Parasitise aphids, turning them into brown, papery "mummies"
- Ichneumon wasps: Parasitise caterpillars, beetle larvae, and other pests
Commercially Available Biological Controls
- Nematodes (Steinernema feltiae): Watered into soil to control vine weevil larvae, chafer grubs, and leatherjackets
- Nematodes (Phasmarhabditis hermaphrodita): Applied to soil to control slugs
- Encarsia formosa: Parasitic wasp for glasshouse whitefly
- Phytoseiulus persimilis: Predatory mite for red spider mite in greenhouses
- Cryptolaemus montrouzieri: Mealybug destroyer (a type of ladybird)
- Atheta coriaria: Rove beetle for fungus gnats and shore flies
Did you know? The use of biological control is not new. In 1888, the Australian vedalia beetle (Rodolia cardinalis) was introduced to California to control the cottony cushion scale that was devastating the citrus industry. The introduction was spectacularly successful and is considered the first major triumph of biological pest control.
Using Nematodes in the Garden
Nematodes are microscopic roundworms that are among the most effective biological controls available to home gardeners. They are watered into the soil using a watering can and seek out their target pests in the soil:
- Choose the right nematode: Different nematode species target different pests. Steinernema feltiae controls vine weevil larvae; Steinernema carpocapsae targets leatherjackets; Phasmarhabditis hermaphrodita controls slugs.
- Check soil temperature: Nematodes require a minimum soil temperature of 5–12°C (depending on species) to be active. Most are applied in spring or autumn.
- Water the soil first: Nematodes need moist soil to move through. Water the area thoroughly before and after application.
- Apply in the evening: Nematodes are sensitive to UV light. Applying them in the evening gives them time to move into the soil before dawn.
- Keep the soil moist: Maintain soil moisture for at least two weeks after application to ensure nematode survival and activity.
Advantages and Limitations
| Advantages | Limitations |
|---|---|
| No chemical residues on food crops | Slower-acting than chemical pesticides |
| Safe for humans, pets, and wildlife | May require specific environmental conditions (temperature, humidity) |
| Targets specific pests without harming beneficials | Cannot eliminate pests entirely — aims for management, not eradication |
| No pest resistance develops | Commercially reared agents can be more expensive than chemicals |
| Sustainable and self-perpetuating (conservation approach) | Timing of application is critical for success |
Creating a Garden That Supports Natural Enemies
- Plant diverse flowers: Many beneficial insects (hoverflies, lacewings, parasitic wasps) feed on nectar and pollen as adults. Umbellifers (fennel, dill, cow parsley) and compositae (yarrow, marigolds) are particularly valuable.
- Provide shelter: Bug hotels, log piles, and dense hedges offer overwintering sites for ladybirds, lacewings, and ground beetles.
- Avoid broad-spectrum pesticides: These kill natural enemies as well as pests, often causing pest populations to rebound worse than before.
- Tolerate low pest levels: A small aphid population is necessary to sustain the ladybirds and hoverflies that prevent outbreaks.
Key Takeaway
Biological pest control uses natural enemies — predators, parasitoids, and pathogens — to manage pest populations without chemicals. From ladybirds eating aphids to nematodes controlling vine weevil larvae, biological control is effective, sustainable, and safe for the environment. The most successful approach combines conservation of existing natural enemies with targeted introductions of commercially reared biological agents, all within a garden managed to support biodiversity.