Which organic pest controls are actually effective and which are a waste of time?

Effectiveness varies enormously, and being organic is not the same as being useful. It is worth being blunt about which is which.

Things that reliably work: physical barriers, above all. Fine mesh over brassicas stops cabbage white butterflies completely; fleece over carrots stops carrot fly; copper tape or wool pellets deter slugs with reasonable consistency. Handpicking, done in the evening for slugs and snails and in the morning for caterpillars, is unglamorous and genuinely effective on a garden scale. Encouraging predators works over a season — a pond, a beetle bank, flowering umbellifers and leaving some overwintering habitat measurably reduce aphid pressure. Biological controls such as nematodes for vine weevil and slugs, and Encarsia for glasshouse whitefly, are effective when applied at the right temperature and life stage.

Contact treatments — insecticidal soap, horticultural oil, neem — work but only where they land, with no residual action, so they need thorough coverage and repeat applications on a schedule matched to the pest's life cycle. They also kill beneficial insects they touch, so spray at dusk when pollinators are not active.

Things that mostly do not work: coffee grounds, eggshells and most household remedies against slugs, which have been tested repeatedly with unimpressive results. Companion planting as pest control specifically, as opposed to as habitat diversity, is much weaker than commonly claimed. Garlic and chilli sprays give short-lived deterrence at best. Ultrasonic devices have no credible evidence behind them.

The most effective single intervention is usually not a treatment at all: a healthy, appropriately sited, unstressed plant resists pests far better than a struggling one.