If you have this white froth on your plants it is cuckoo spit and inside each one there is a froghopper nymph. Doesn't do any real harm to the plant but you can always use the hose pipe to remove.

In the USA we call them spittle bugs. They are the immature form of a bug called the frog hopper. They are a member of the insect group called Homoptera. Cicadas, scale insects and aphids are members of this order too. The nymph sucks plant juices an uses some to make a protective and defensive froth. It's harmless.