Biological controls: when and how to deploy them

Biological controls are living organisms that parasitise or predate specific pests. They are expensive, perishable and fussy about temperature, but in the right circumstances they work with surgical precision. You are not spraying a chemical; you are releasing an ally that hunts on your behalf. Once the pest is gone, the control dies out or moves on. There is no residue, no resistance, no collateral damage.

Nematodes are microscopic worms applied as a drench. Steinernema kraussei targets vine weevil grubs; apply to moist, warm soil (above 5°C) in late August or early September and again in March. Phasmarhabditis hermaphrodita targets slugs; apply every six weeks from March to October when soil is above 5°C. Nematodes arrive by post, live in the fridge for a few weeks, and die in dry or cold soil. They are not a one-time fix; you reapply each season. They cost £15–25 per application for a small garden.

Parasitic wasps come in species-specific varieties. Encarsia formosa parasitises whitefly in greenhouses; release at first sign of infestation and hang the cards among the plants. Aphidius colemani targets aphids, again under cover. These controls need warmth (18°C minimum) and a steady pest population to sustain themselves. If the greenhouse drops below 10°C at night, they fail. They are not suitable for outdoor use in the UK climate.

Bacillus thuringiensis (Bt) is a bacterium sold as a spray. The caterpillar-specific strain kills only lepidopteran larvae (caterpillars) and is harmless to everything else, including bees, hoverflies and ladybirds. Spray onto brassica leaves at dusk when caterpillars are active; they ingest it, stop feeding within hours, and die in two to three days. Repeat every seven to ten days through the cabbage white season. It breaks down in sunlight, so evening application is critical.