Felt scales are very diverse and comprise a number of apparently unrelated groups. The most commonly intercepted eriococcids at U. S. ports-of-entry are those of the Acanthococcus type. They produce a white, gray, or yellowish ovisac that encloses the pyriform body of the adult female. Body color varies from pink or red to purple, green or brown. The posterior end of the sac has a small opening that allows the first instars to emerge. Other eriococcids occur under the bark of the host, produce little or no ovisac secretion and often are pink or red. Many species produce galls including one of the most interesting genera, Apiomorpha which induces very ornate structures on various species of Eucalyptus.
Recent research (Cook, Gullan, and Trueman 2002 & Hardy et al. 2008) has shown that the family Eriococcidae is not a single monophyletic group but is an aggregation of different groups. In several instances, species that look very similar morphologically are apparently only distantly related and species that look different belong together. Considerably more research is required before the relationships of the organisms currently labeled as eriococcids can be sorted into logical family groups. Eriococcidae Targioni Tozzetti was first used as a family by Brues and Melander (1932).
Felt scales occur in all zoogeographical regions of the world. They are most speciose in the Australasian region, and are nearly absent from the Afrotropical region.
Eriococcids occur on a wide diversity of hosts including trees, shrubs, and even grasses. They are found on all parts of the host with the possible exception of small diameter rootlets. The greatest diversity in the southern hemisphere is on older families such as the Myrtaceae whereas in the northern hemisphere they are most diverse on more advanced plant groups such as the Asteraceae.
Felt scales have 3 instars in the female and 5 in the male. Most eriococcus-type species have 1 or 2 generations each year. The overwintering stage usually is the adult female or egg in the ovisac. First instars appear in early spring and settling often occurs within hours of emergence from the ovisac. Second-instar males feed for a short period then produce a narrow, felt sac that encloses the body. Development of the prepupa, pupa, and adult male occurs within this sac. Soon after molting, the adult female mates and produces the ovisac several days later. Usually 50 to 100 eggs are laid. Some eriococcids have very unusual life histories. One example is the gall-forming genus Apiomorpha which can have females that live for a year or more and produce separate male galls that often are induced on the gall of the female.