Mealybugs (or pseudococcids) in the field are often characterized by a white, mealy or powdery secretion that covers the body. Species that occur in concealed habitats such as grass sheaths either lack this secretion or have only small amounts of it. Frequently marginal areas of the body have a series of protruding lateral wax filaments. These filaments may be absent, confined to the posterior 1 or 2 abdominal segments, or occur around the entire body perimeter. A filamentous secretion often is produced that encloses the eggs and at least part of the body.
Mealybugs are serious pests particularly as invasive species. In the United States there are 56 introduced species of mealybugs and 45 of them are pests (Miller et al. 2005). Species such as the citrus mealybug, longtailed mealybug, Madeira mealybug, Comstock mealybug, and grape mealybug are serious pests not only outdoors, but also in greenhouse and internal landscape environments.
The species that were chosen for the key were selected because they were intercepted at U. S. ports-of-entry during the last 5 years. The list was augmented by species listed in Williams (2004a) as being picked up in U. S. quarantine in southern Asia.
The following combination of characters will assist in determining if an unknown scale insect is a mealybug. With ostioles; cerarii at least on anal lobe; 1 or more circuli; swirled-type trilocular pores; translucent pores on hind legs; 2 pores on each surface of trochanter; without basal denticle on claw. Other characters are: trochanter pores parallel to front edge of femur, not oriented transversely; 3 labial segments; usually 3 pairs of anal-ring setae; more than 4 setae on tibia; tubular ducts without invagination. No single character can be used to determine a specimen as a pseudococcid. Mealybugs are a large and diverse group and exceptions occur for every character. There are species without ostioles, cerarii, circuli, trilocular pores, and translucent pores. Although the family is quite distinct, it is best to use a combination of characters for its recognition.
Pseudococcids occur in all zoogeographical regions of the world. Click here for a list of genera and species from the Australasian region, Afrotropical region, Nearctic region, Neotropical region, Oriental region, and Palearctic region. Of described species, they are most speciose in the Palearctic region, and least numerous in the Neotropical area. There are about 2,000 species of mealybugs world wide.
Based on an analysis of host information in the mealybug catalogue by Ben-Dov (1994), mealybugs occur on about 250 families of host plants. The most common host family is Poaceae with 585 species. The Asteraceae is a distant second with 250 species. The top ten most common host families are: Fabaceae 225; Rosaceae 116; Rubiaceae 101; Euphorbiaceae 97; Myrtaceae 94; Labiatae 85; Moraceae 82; Cyperaceae 75. It is interesting that grasses and composites are such important hosts of mealybugs, but are far less common as hosts of armored scales. This might be explained by the tendency for mealybugs to occur on more herbaceous plants and on fewer trees and woody shrubs. There are surprisingly few mealybugs on families such as Salicaceae, Pinaceae, Palmaceae, and Betulaceae.
Mealybugs have 4 female instars and 5 instars in the male. Mealybugs have a diverse array of life history strategies from occurring in grass blade sheaths, to feeding on rootlets, to occurring exposed on leaves. Thus, any generalized life history will have many exceptions. Many mealybugs overwinter as second instars, although adults, first instars, and eggs also can play this role. Eggs or first instars are laid by the adult female. Eggs are normally laid in an ovisac that can enclose all or part of the body of the female. Most species that lay first instars rather than eggs lack any substantial ovisac. Even though the majority of species have legs in all instars, most mealybugs remain relatively stationary throughout their life; a few species such as some members of the genus Phenacoccus, move to different areas of the host for overwintering, feeding, oviposition, and molting. Most species have 1 or 2 generations a year, although some are reported to have as many as 8 generations in the greenhouse. Both parthenogenetic and sexual species are common.
Scale insects are so diverse morphologically that in most instances there is no single character, or even combination of characters, that can be used to diagnose a particular family all of the time. Those who have looked at lots of taxa for a long period of time, have developed a kind of family gestalt for most of the groups, but this is difficult to characterize in a logical and definitive manner. In fact, some of the family groups that traditionally have been defined are not consistent with recent molecular data. To assist inexperienced users, we have taken two steps to help make family identifications easier.