What is the white froth on my plant's stems?

This is one of the most frequently asked and least consequential things in a British garden. The foam is produced by the insect from plant sap and air to protect itself from drying out and from predators, which is why the nymph is always found inside it rather than on the surface.

Step by step

1. Confirm what it is.White, soapy-looking foam in a blob roughly the size of a thumbnail, on a stem, a leaf joint or a flower stalk, in late spring and early summer. Wipe it away with a finger and you will find a small, soft, pale green or yellowish nymph, a few millimetres long.
2. Understand what is inside it.The nymph of a froghopper, a sap-sucking insect. The adult is a small brown or green bug that jumps remarkably far when disturbed, and it does not make foam. There may be more than one nymph in a large blob.
3. Recognise the damage, which is minimal.A little wilting or distortion of the shoot the nymph is on. On a garden plant this is trivial and the plant grows on normally. Lavender, rosemary, roses, chrysanthemums and willowherb are common hosts.
4. Wash it off if you want to.A jet from a hose removes the foam and dislodges the nymphs. That is the whole of the treatment, and even that is optional. On a lavender hedge or a plant you handle, it is worth doing simply because the foam is unpleasant to touch.
5. Do not spray anything.There is no case for an insecticide here. The damage does not justify it, the foam protects the insect from contact sprays anyway, and you would be treating a plant that has nothing wrong with it.
6. Know why entomologists are interested in froghoppers.Froghoppers and related sap-feeding insects are the vectors of Xylella fastidiosa, a serious bacterial plant disease that has devastated olives in southern Europe. Xylella is not established in the UK, and cuckoo spit in your garden is not a sign of it. Recording sightings of froghoppers and cuckoo spit contributes to surveillance, which is why several research projects ask for them.
7. Report a suspected plant health problem properly if you ever need to.Suspected notifiable plant diseases in the UK are reported to the plant health authorities — in England and Wales through the Animal and Plant Health Agency, in Scotland through the relevant Scottish authority. That route exists for genuine suspicions, not for cuckoo spit.

Tips

• The name comes from the timing rather than the bird — it appears at the same time of year cuckoos are heard.
• It is at its most obvious in June and largely gone by midsummer, when the nymphs mature into adults and stop foaming.
• If you find foam and no insect inside, something has already eaten it. That is the garden working.
• Do not confuse cuckoo spit with the white waxy fluff of woolly aphid or mealybug, which is dry and fibrous rather than wet and bubbly.

Common mistakes

• Spraying an insecticide at cuckoo spit — It causes no meaningful damage, the foam shields the insect from contact sprays, and you are treating a healthy plant.
• Mistaking it for a disease or a fungus — It is entirely an insect product. Wipe it away and the nymph is visible in seconds.
• Assuming foam on a plant means Xylella — Xylella is not established in the UK, and the presence of froghoppers says nothing about it. Cuckoo spit is a normal part of a British summer.

Questions people ask