Bagworms are caterpillars that form silk bags covered in plant parts to camouflage themselves from predators. There are voracious pests of many tree and shrub species especially Leyland cypress and juniper, but in the case of the bagworms in this picture cherry laurel.
The bagworm caterpillars emerge in mid-summer to feed on foliage of several weeks. They then pupate, and adults become active in the fall. Female moths stay in the bags until males find them and mate. After the female lays eggs in the bag, she leaves and dies.
These creeps can defoliate whole trees if not found soon enough. That being said, bagworms are easily controlled with foliar applied insecticides.