The common pests and what they actually do

Aphids are small, soft-bodied and legion. Greenfly, blackfly, woolly aphid, mealy cabbage aphid — they pierce plant tissue and suck sap, weakening growth and spreading viruses. They reproduce without mating through spring and summer, which is why numbers explode so fast. A single aphid can produce fifty offspring in a week. They cluster on soft new growth because it is easiest to pierce. They excrete honeydew, which attracts ants (who farm them for it) and sooty mould (a black fungus that blocks light).

Slugs and snails are molluscs, not insects, and they chew. Spanish slug — large, orange-brown, mucus-thick — is the worst: resistant to desiccation, active in dry weather, impossible to drown. Native species do less damage and are important food for hedgehogs, thrushes and ground beetles. Slugs feed at night and hide by day under pots, boards and mulch. They devastate seedlings — one slug can take out an entire tray of lettuce in a night — but tend to leave tougher, established plants alone.

Vine weevil is a night-feeding beetle that notches the edges of leaves (rhododendron, heuchera, bergenia) in a distinctive scalloped pattern. The real damage is underground: fat, white, C-shaped grubs eat roots from autumn to spring. Container plants are especially vulnerable. Lily beetle is pillar-box red, unmistakable, and its larvae cover themselves in their own black excrement as camouflage. Both adults and larvae strip the leaves of lilies and fritillaries to bare stalks in days.

Cabbage white caterpillars are the larvae of those cheerful white butterflies. Large white lays clusters of yellow eggs on brassica leaves; small white lays singly. The caterpillars are green, well-camouflaged and phenomenally hungry. They bore into the heart of cabbages and leave trails of dark green frass. Codling moth caterpillars tunnel into apples and pears in summer, leaving the telltale hole plugged with brown frass. Carrot root fly larvae are pale maggots that tunnel into carrot, parsnip and celery roots, making them inedible.