The physical form of juvenile and adult are different. Young nymphs are round and mostly red, looking somewhat like small Lady Beetles. As they develop, bands of white lines form and they grow in size (see photo gallery). Adults are neither red, nor black, but a drab brown. This coloring is camouflage and allows them to get close to their insect prey undetected.
Adults hibernate over winter and emerge in the spring to seek mates. Females lay fertilized eggs under leaves or other objects where they hatch within a week in the right conditions. After several instars, P. placidus reaches maturity in a month.
References: Entomology Collection at University of Alberta, The Pentatomidea of North America by McPherson (SIU)