Scale Insects and Mealybugs, Superfamily Coccoidea

Scale insects and mealybugs are significant pests of many ornamental plants and orchard trees, and cost these industries millions of dollars each year. Many other insects and larger predators eat these tiny insects, so they do serve a purpose. Some scale insects cause the formation of galls. Learn the habits and traits of these interesting true bugs, which belong to the superfamily Coccoidea.

What Do Scale Insects Look Like?

Scale insects often go unnoticed, although they live on many common landscape and garden plants. They're small insects, usually just a few millimeters long. They tend to position themselves on the undersides of leaves or other plant parts, where they aren't exposed to the elements.

Scale insects are sexually dimorphic, meaning males and females look entirely different from one another.  Adult females are usually somewhat round in shape, lack wings, and often lack legs as well. Males are winged, and look somewhat like winged aphids or small gnats. To identify scale insects, it's often necessary to identify the host plant.

Although largely considered pests, scale insects have been used in some surprisingly beneficial ways throughout history. The red pigment found in cactus-feeding cochineal scales is used to make a natural red dye for food, cosmetics, and textiles. Shellac is made from the secretions from coccids called lac scales. Scale insects and their waxy secretions have also been used in various cultures for making candles, for jewelry, and even for chewing gum.

How Are Scale Insects Classified?

Kingdom - Animalia
Phylum - Arthropoda
Class - Insecta
Order - Hemiptera
Superfamily – Coccoidea

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There is still some disagreement on how scale insects should be classified and how the group should be organized.  Some authors rank the scale insects as a suborder rather than a superfamily. Family level classification is still very much in flux. Some taxonomists subdivide the scale insects into just 22 families, while others use as many as 45.

Scale Insect Families of Interest:

Margarodidae - giant coccids, ground pearls
Ortheziidae - ensign coccids
Pseudococcidae - mealybugs
Eriococcidae - felt scales
Dactylopiidae - cochineal insects
Kermesidae - gall-like coccids
Aclerdidae - grass scales
Asterolecaniidae - pit scales
Lecanodiaspididae - false pit scales
Coccidae - soft scales, wax scales, and tortoise scales
Kerriidae - lac scales
Diaspididae - armored scales

What Do Scale Insects Eat?

Scale insects feed on plants, using piercing mouthparts to suck the juices from their host plant. Most scale insect species are specialist feeders, requiring a particular plant or group of plants to meet their nutritional needs.

The Life Cycle of Scale Insects

It's difficult to generalize a description of the scale insect life cycle. Development varies greatly between scale insect families and species, and is even different for males and females of the same species. Within the Coccoidea, there are species that reproduce sexually, species that are parthenogenetic, and even some that are hermaphroditic.

Most scale insects produce eggs, and the female often guards them while they develop. Scale insect nymphs, particular in the first instar, are typically mobile and are referred to as crawlers. The nymphs disperse, and eventually settle on the host plant to begin feeding. Adult females are usually immobile and remain in one location for their entire lifespan.

How Scale Insects Defend Themselves

Scale insects produce a waxy secretion that forms a cover (called a test) over their bodies. This coating can vary greatly from species to species. In some scale insects, the test looks like a powdery substance, while others produce long strands of wax. The test is often cryptic, helping the scale insect blend in with the host plant.

This waxy coat performs several functions for the scale insect. It helps insulate it from temperature fluctuations, and also maintains the proper humidity around the insect's body. The test also camouflages the scale insect from potential predators and parasitoids.

Scale insects and mealybugs also excrete honeydew, a sugary liquid waste that is a by-product of eating plant sap. This sweet substance attracts ants. Honeydew-loving ants will sometimes protect the scale insects from predators to ensure their supply of sugar remains intact.

Where Do Scale Insects Live?

The superfamily Coccoidea is quite large, with more than 7,500 species known throughout the world. Roughly 1,100 species inhabit the U.S. and Canada.

Sources:

• _{Borror and DeLong's Introduction to the Study of Insects, 7^th edition, by Charles A. Triplehorn and Norman F. Johnson.}
• _{Encyclopedia of Entomology, 2^nd edition, edited by John L. Capinera.}
• _{"Superfamily Coccoidea – Scales and Mealybugs," Bugguide.net. Accessed online February 9, 2016.}
• _{"Systematic Studies of Scale Insects (Hemiptera: Coccoidea)," by Nathaniel B. Hardy, University of California Davis, 2008.}
• _{"Scale Management Guidelines – UC IPM," University of California Statewide Integrated Pest Management Program. Accessed online February 9, 2016.}
• _{ScaleNet: Scale Insects (Coccoidea) Database, USDA Agricultural Research Service. Accessed online February 9, 2016.}
• _{"Coccoidea," Tree of Life Web. Accessed online February 9, 2016.}