Every year in May/June I'm observing on different plants, such as strawberries, lavender (pictured below) and different weeds, a foamy liquid containing a green insect or a larvae.
At first I thought that this is the work of slugs, but now, that I was able to find a "bigger" exemplar and I saw it was moving.
I'm really sorry for the quality, I only had my cellphone at hand at that time:
What is it and is it a pest or canI just leave them alone? On affected plants I never noticed any damage or negative side-effect, but that doesn't mean there aren't.