I particularly like them because of their relationship with Buckthorn. Brimstone butterflies' larvae feed exclusively off our native Alder Buckthorn and Purging Buckthorn, Rhamnus frangula and Rhamnus cathartica. If you grow them, as we're doing, chances are you'll find Brimstones moving in sooner rather than later. They stay for successive generations, year after year. Buckthorn is critical for Brimstone butterflies' survival. There are many other similar exclusive relationships between our native flora and fauna. Insects don't just need flowers to provide nectar and pollen; they often have other demands on them too. These mean it's easy to play God. If you want Brimstone butterflies, plant Buckthorn. Small Blues? Kidney Vetch. I love the sort of immediate cause and effect you can get in your garden. Buckthorn is a nice easy-to-grow native shrub anyway. We're growing Alder Buckthorn here, as it copes so well with our waterlogged clay. Purging Buckthorn is more of a chalky soil lover, so wherever you are, you can give Buckthorn a try!