Summary: Scale are a common pest on houseplants and fruit trees. The female scale are immobile and cover themselves in a waxy shell so they look more like a seed case or malignant growth on the plant rather than an insect.
Scale are a group of insects that are small in size and usually parasites of plants, although they also occasionally feed on fungi. Like with many parasitic insects the most damaging stage of a scale's life cycle is during its larval or nymph stage. This occurs after the small insect has insect has emerged from its egg and needs energy to grow.
The scale gets its energy from tree sap which it collects from a host plant using a straw-like mouthpart that pierces leaves, roots, or other soft plant tissue and sucks out nutrients. Scale secrete honeydew, a very sweet kind of excrement that is a favorite food of ants and yellowjackets, which are often attracted to plants where the scale have inhabited. This can become a secondary pest control problem.
A third problem caused by scale is the presence of excess honeydew which can drip down off of trees onto sidewalks or cars below. The sticky substance is a nuisance if it covers your shiny new Cadillac. However, if you treat a scale problem successfully, you will eliminate the honeydew problem and the ant or yellowjacket problem as well.
When scale infest a plant they can do some serious damage. The plant often will drop many of its leaves as a defense mechanism, eventually causing the plant to die. In 1868 when orange groves in California were infested by the Cottony Cushion Scale brought over from Australia, orange farmers almost lost their entire crop that year. They saved the following year's orange crop by bringing in the species of lady bug from Australia that fed on the scale. This was one of the first examples of biological pest control.
Scale species are divided into two groups being the soft bodied scale and the armored scale. The soft bodied scale are usually larger. The soft bodied scale uses a waxy coating over its body as protection and the armored scale uses secretions to form a hard shell over its body. The female scale lays her eggs under her body or armor covers and is almost always immobile.
The scale's appearance is very different depending on the species. Scale can have brown and puffy shells, orange and flattened shells, black and fuzzy shells, or the shells can be white and cotton-like. When a plant is infested with scale it can appear as if the plant is covered in fish like scales, thus the origin of the name œscale.
Male scale have wings and resemble tiny wasps. They do not eat anything, so are not harmful to plants. Their only job is to mate with female scale, and then they die.
Killing scale insects can be accomplished with pesticides, but the scale's waxy coating can protect it from harm. Pesticides are best applied while scale are in their nymph stage when they are crawling around on the outside of the plant. They are often referred to as œcrawlers at this early stage. It is this stage that causes the most damage to the plant, but it is also the stage when they are most vulnerable. The nymphs can be sprayed off of the plant using a mixture of soap and water. This will be time consuming, but any pesticide that is applied externally to the plant must reach all of the scale and their eggs.
Oil based pesticides can be applied by a pest control professional that to suffocate the female scale. This might be a good option if you don't want to take the time to wait for the nymphs to appear and then spray all the infested plants yourself. Yet another option is to use a œsystematic pesticide that is applied to the soil around the base of the plant. These systematic pesticides are absorbed into the tree and combat insect parasites from the inside out. These will have the added benefit of not harming beneficial insects that feed on pesky insects, like the scale!
Scale have some interesting commercial uses that somewhat redeem them from their pest status. Cochineal scale, which feed on cacti, have been used since ancient times to create a crimson colored dye. Synthetic dyes are more commonly used today, but dye from Cochineal scale is still sometimes used to dye fabrics or in food coloring. I bet you didn't know that the average person eats about two or three drops of dye made from scale each year. If you are eating something with œnatural red food dye, it might come from Cochineal scale. Lac scale have also contributed to human products. Shellac is a type of resin used to polish wood that is made from the secretions of Lac scale.