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Gardening glossary

Biological control

Biological control (often shortened to "biocontrol") uses one organism to suppress another. It is one of the oldest pest-management approaches and one of the most effective when done well — properly deployed biocontrols can eliminate the need for chemical pesticides on many crops.

The three main types:

**1. Predators.** Free-ranging insects or mites that hunt and eat pests. - **Ladybirds (Coccinellidae)** eat aphids — up to 50 per day per adult. - **Lacewing larvae** ("aphid lions") eat aphids, mealybugs, scale, thrips, and whitefly nymphs. - **Hoverfly larvae** — voracious aphid predators. - **Predatory mites (Phytoseiulus persimilis, Amblyseius spp.)** target spider mites and thrips in greenhouses. - **Ground beetles, rove beetles, spiders** — generalist soil and surface predators that keep slug and grub populations in check.

**2. Parasitoids.** Insects that lay eggs in or on pests; the larvae develop inside the host, killing it. - **Encarsia formosa** — a tiny parasitic wasp that lays eggs in whitefly scales. The whitefly nymph turns black as the wasp larva develops inside; a new wasp emerges within 2–3 weeks. Used in tomato greenhouses worldwide. - **Aphidius colemani / ervi** — parasitic wasps targeting aphids. Aphid "mummies" (brown spherical husks) on stems mark a successful attack. - **Trichogramma** — minute wasps that parasitise lepidopteran (caterpillar) eggs.

**3. Pathogens.** Bacteria, fungi, viruses, or nematodes that infect and kill pests. - **Bacillus thuringiensis (Bt)** — a soil bacterium whose crystal proteins kill caterpillars when ingested. See the [Bt safety profile](/glossary/systemic-insecticide) for context — Bt is safe for bees, mammals, fish, and beneficial insects, but it can affect butterfly caterpillars too, so apply selectively. - **Steinernema and Heterorhabditis nematodes** — microscopic worms that hunt soil-dwelling pest larvae (vine weevil grubs, chafer grubs, leatherjackets, sciarid fly larvae). Sold as packets of dormant nematodes activated with water and watered into the soil. - **Beauveria bassiana** — a fungal pathogen of thrips, whitefly, and aphids.

How to use biocontrols at home:

1. **Identify the pest correctly first.** Biocontrols are pest-specific. 2. **Order from a reputable supplier** (UK: Defenders, Dragonfli, Green Gardener; US: Arbico Organics, Beneficial Insectary). They ship live organisms with care instructions. 3. **Time release for low pest pressure** — biocontrols work best when populations are small. Late release into a massive infestation rarely catches up. 4. **Stop or reduce chemical sprays** for at least 7 days before and after release. Even "organic" insecticidal soap and neem oil can kill released predators. 5. **Provide habitat for resident beneficials.** Calendula, alyssum, fennel flowers, dill, and yarrow feed adult parasitic wasps and hoverflies.

The biggest mistake home gardeners make: treating biocontrol as a one-off rescue. It is most effective as ongoing preventative practice — a garden with diverse plantings, undisturbed corners, and a no-spray policy supports a permanent population of beneficial insects.

Where this comes up in our guides

Related terms

Integrated pest management (IPM)

Integrated pest management is the EPA-recognised, environmentally-sensitive framework for pest control. It combines pest monitoring, action thresholds, prevention, biological controls, and targeted chemical sprays — used only as a last resort — to manage pests with the lowest possible risk to people, beneficial wildlife, and the environment.

Systemic insecticide

A systemic insecticide is absorbed by the plant and moves through its tissues, killing sap-sucking insects that feed on the treated plant. Neonicotinoids are the best-known class — but they are heavily restricted in the UK and EU because of harm to pollinators. Use is now banned outdoors in the UK as of 2025.

Plant quarantine

Plant quarantine is the practice of isolating new, sick, or recently-imported plants from the rest of your collection for 2 to 4 weeks to make sure they are not carrying pests or disease. The single highest-leverage habit for protecting an indoor plant collection.

Neem oil

Neem oil is a yellow-brown vegetable oil pressed from the seeds of Azadirachta indica (the neem tree). Diluted as a foliar spray, it acts as a contact suffocant, feeding deterrent, and partial systemic against aphids, whiteflies, thrips, mites, scale, and some fungal diseases. Generally low-toxicity to mammals but harmful to bees if sprayed directly.

Aphid

Aphids are tiny pear-shaped sap-sucking insects, usually green, black, or pink, that cluster on new growth and undersides of leaves. They reproduce rapidly and excrete sticky honeydew that fuels sooty mold.