The Royal Horticultural Society (RHS) has issued a plea for help to stop bugs transmitting a deadly disease to the UK's plants.
The Xylella fastidiosa bacteria has already taken hold in France, Spain and Italy, where it has killed millions of olive trees.
The bacteria is not yet present in the UK, but the RHS has described it as its "number one concern".
The disease prevents water travelling from roots to leaves, making plants look as if they have been damaged by drought or frost.
A report by the EU's European Food Safety Authority earlier this month concluded there is no known way to eliminate the bacteria in the wild.
If Xylella is discovered in the UK, all host plants within 100 metres would need to be destroyed and there would be an immediate restriction of movement for some plants within a five kilometre radius, the RHS said.
The society has asked gardeners and nature lovers to report any sightings of spittle on their plants, which is left by a small insect known to transmit the bacteria.
Spittlebugs, also called froghoppers, are small sap-sucking insects that move from plant to plant to feed.
The most common species is the meadow spittlebug, which is around five millimetres long and can vary in colour and pattern from black to brown.
The bugs are named after the small white blobs of spittle left by their nymphs on leaves and branches.
Volunteers are asked to report sightings of spittle on the Biological Records Centre's iRecord website.
The RHS is working with Forest Research and the University of Sussex, and will use data about the bugs to track the threat of Xylella in the UK.
More than 500 plants are at risk, including lavender, oleander, rosemary and flowering cherry.
Gerard Clover, head of plant health at the RHS, said: "Xylella remains our number one concern but is not an issue bound by the garden fence.
"Understanding how and where the disease's primary vectors move is fundamental to understanding how we can stop the devastation of our gardens and environment should it arrive."
Alan Stewart, of the University of Sussex, said: "Records of spittle submitted by the public will help us to build up a picture of where spittlebugs are found, what plants they feed on and how much they move around.
"This information will be essential for deciding how best to respond should Xylella arrive in the UK.