If you need a safe place to live, why not get a home built to your own specifications?
That’s what the larvae of Dasineura sisymbrii, a kind of gall midge do. The presence of the larvae amongst the developing flower buds of species of Rocket (Sisymbrium) or Yellow-cresses (Rorippa) cause the host plant to develop a mass of pale yellow growth in which the larvae can develop and feed. The mass of plant tissue also keeps the larvae safe from predators. I don’t think it helps the plant to set seed though!
These galls were found in a damp piece of grassland near Stenton Hall.