Vine weevil
How to recognise it
Adults notch ragged edges into leaves at night; the grub stage causes sudden collapse of a container plant with no obvious cause, with white C-shaped grubs visible in the compost when checked.
Life cycle and conditions
Adults active spring to autumn, laying eggs in compost through summer; grubs feed on roots through autumn and winter, causing the most damage in the cold months.
Prevention
Check compost of at-risk containers regularly; gritty, free-draining compost is less attractive for egg-laying.
How to treat it
A biological nematode drench applied to affected compost while soil temperature is above roughly 5°C (spring or early autumn) is highly effective and leaves no residue harmful to pollinators, pets or soil life.
Chemical compost drenches exist for severe cases, but the nematode option above is usually just as effective without the wider impact on soil organisms — reach for this only if nematodes have already failed.