Tom Turpin
Professor of
Purdue University


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Insect scales weigh on plants

There are many types of scales. Some are small plate-like structures that form the external covering of fish or reptiles. There are flakey scales, such as the rust that forms on metal. A system of ordered marks, such as a ruler with inches and centimeters, used for measurements is also called a scale. Another type of scale is a series of tones in music. Other scales, such as the one in your bathroom, are designed for weighing things.

There are also scales in the world of insects - a fact that almost anyone who raises plants for either fun or profit can attest. That's because scales of the insect kind are major plant pests.

Scale insects get their name because the immatures and females of many species resemble fish scales. Throughout most of their life, female scale insects are wingless, and often legless. As you can probably guess, without legs and wings these insects don't go scampering around. They just sit and use their piercing mouths to suck sap from plants.

Male scale insects are more insect-like than the females. The males do have wings and resemble small gnats. Unlike the sap-sucking females, these little male scale insects don't feed. In fact, they don't even have mouthparts.

Scale insect biology generally works this way. Newly hatched scale insects have legs and are active. In this stage, they are known as crawlers and move away from their hatching site. Then, they molt, lose their legs, become stationary and begin to form the waxy substance that constitutes their scale-like covering.

When the female scale insects mature into adults, they remain under the scale. The males develop wings and leave the protective scale. The males flutter about and mate with the females that are still concealed under their protective scales. The females produce their eggs under the scale. The eggs generally spend the winter under the scale of the female.

Some scales are known as armored scales, and this group includes some very important pest species. One is the San Jose scale. This pest was introduced from the Orient. It attacks trees and shrubs, and sometimes kills the plants on which it feeds. Another armored scale is called the oystershell scale because - and I bet you could have guessed this - it looks like the shell of an oyster. It, too, does major damage to fruit and ornamental trees. Another scale in this group is called the pine needle scale. This scale is found throughout the U.S., as anyone who owns any pine trees can tell you.

Scale on plant
Cottony maple scale insect

The so-called soft scales include a number of injurious species. The black scale, which is a major pest of citrus, is one of the soft scales. Another is called the cottony maple scale because its primary host is maple trees. Several other species of this group attack plants in homes and greenhouses.

The insects known as mealybugs are also scale insects. They get their name because of the mealy wax that covers their bodies. Like the other scale insects, the mealybugs feed on plant species that are ornamentals or fruits and, consequently, have not endeared themselves to people who raise such species.

But all scale insects are not pests. Some are considered beneficial insects. The mostly tropical lac insects are beneficial. One of the lac insects is of commercial value. It grows on fig and banyan (a tropical tree with aerial roots) in the Far East. The wax, or lac, produced by the insect sometimes coats the twigs with a layer half an inch deep. The twigs are cut, and the lac is melted. The melted lac is used to make shellac and varnishes.

Red scale
Cochineal insect

Another scale insect for which humans have found a use is known as the cochineal insect. This insect resembles the mealybugs, except the females are red in color. It feeds on prickly pear cacti. The females are removed from the cacti and dried. The red pigments are extracted from the dried bodies and used to produce cochineal dye.

manna scale on branch
Tamarisk manna scale insect

Another beneficial scale insect is the tamarisk manna scale. The females excrete large quantities of honey dew, which accumulates on the plants, to form a sweet material called manna. This is the same manna from heaven, which is mentioned in the Bible, that the children of Israel used.

So, even if one or two species of this group of plant-feeding insects are beneficial, the weight of the evidence implicates most scale insects as pests. Case closed! 


Writer: Tom Turpin
Editor: Olivia Maddox