Body of adult female usually large, often red or brown, with conspicuous legs (legs absent in Xylococcus filiferus) and antennae. Cysts of Xylococculus and Xylococcus located under bark, with a thin wax tube protruding from bark that is attached to anal opening. Adult females and cysts of Stigmacoccus species occur in tests on the branches of the host.
The composition of this family has not changed dramatically since the definitive classification presented by Morrison (1928). The most important difference is the removal of the Matsucoccidae into a separate family group. This hypothesis is supported by the phylogeny presented by Gullan and Sjaarda (2001). Xylococcidae Pergande was first treated as a family by Zahradník (1959a).
Xylococcids are found on trees in the following genera: Alnus, Betula, Calocedrus, Cupressus, Fagus, Inga, Juniperus, Quercus, and Salix.
A definitive life history study was undertaken on Xylococculus macrocarpae (Coleman) in California (Tait et al. 1990). Females have 4 instars and males have 5. There is a single generation each year. Adult females lay eggs on the foliage in a sac that is formed by the abdomen; no waxy ovisac is produced. Crawlers move to the branches and bole of the tree where they molt twice into legless cysts. In early spring, adults emerge from the cyst and migrate back to the foliage. Second-instar males molt to a legged prepupa that moves to the lower bole and produces a cocoon. Prepupae develop in the cocoon and adult males emerge soon after. This life history differs slightly from those reported for other species. Oguma (1919) suggested that there were 4 female instars and 6 male instars in Xylococcus japonicus Oguma. Florence (1917) indicated that there were 5 male and 5 female instars in X. macrocarpae and Hubbard and Pergande (1898) found 5 female instars and 6 male instars in X. betulae. It has been suggested that the dramatic difference between mobile and settled crawlers might explain some of the discrepancy. Birds are frequently attracted to the large quantities of honedew produced by species of Stigmacoccus (Greenberg et al. 1993).