This is the archetypal shield bug as it is shapes just like a shield and large enough to be noticeable. It occurs in a variety of habitats, wherever there is shelter and food, but particularly around deciduous trees and shrubs such as hazel and oak. They are more or less all green when first adult (except for the membranous wingtips) but become reddish before hibernation, re-emerging bright green in the spring.  They feed on a wide variety of plant material.