Popular Christmas plants include holly, mistletoe, ivy and Christmas trees. Other popular holiday plants include holly, mistletoe, red amaryllis, and Christmas cactus. Its preferred fruits include those of the mistletoe, holly and yew. The Christmas Tree' refers to couples wanting to stop beneath the mistletoe. Plants usually considered hemiparasites include "Castilleja", mistletoe, Western Australian Christmas tree, and yellow rattle. This tree is host to fir mistletoe ("Phoradendron pauciflorum"), a parasitic plant. These include the roundheaded pine beetle, European mistletoe and caterpillars. A parasitic plant sometimes utilizing "P. menziesii" is Douglas-fir dwarf mistletoe ("Arceuthobium douglasii"). The breeding season appears to correspond with the flowering of key eucalyptus and mistletoe species. The bark of areas infected with dwarf mistletoe also appears to be preferred. The mistletoe berry juice resembles semen. Cedar elms are very susceptible to mistletoe. It is a Christmas custom for a couple who meet under a mistletoe to kiss. It was featured on Bieber's Christmas album "Under the Mistletoe". Hanging mistletoe was part of the Saturnalia festival.