This article was published originally on 6/29/2011
It's a funny question, all right, but one that turns out to be more important than you might suppose. Scale insects are sap-feeding insects named for the scale or shell-like waxy covering that conceals their bodies.
Scale insects are generally divided into two categories: soft scale and armored (hard) scales. The names are moderately descriptive of the group, but not entirely, nor does hardness versus softness describe the whole difference.
- Soft scales produce a soft, thin, cottony, powdery or waxy layer over themselves that cannot be separated from the insect body. These scale insects often produce copious amounts of honeydew.
- Armored scales have a hard, shield-like cover composed of shed skins and wax that conceals the body but is not attached to the body of the insect.
In Iowa, most species of shade trees, fruit trees, and ornamental shrubs are subject to scale insect attacks. Depending on the species, scale insects may be found on plant stems, twigs, trunks, foliage, or fruit. Most scale insects are small and inconspicuous. The size of scale insects ranges from 1/8 to ½ inch. Color, shape, texture and other features vary with the species. Scale insects can weaken and even kill trees, shrubs and houseplants, but in general, complete loss of the plant is rare.
Scale insects feed by sucking sap from trees and shrubs through piercing-sucking mouth parts. Sap feeding by scale insects may cause yellowing or wilting of leaves, stunting or unthrifty appearance of the plants, and eventually death of all or part of the plant when infestations are heavy. Weakened plants may lose vigor and become more susceptible to injury caused by drought, severe winters, attack by other insects (such as borers), or infection by diseases.
While feeding, soft scale insects excrete a sweet, sticky substance called honeydew. Honeydew is a mixture of undigested sugar and water passed through the insect's digestive system and deposited onto leaves and stems. Honeydew may make the plant appear shiny and wet and also attracts flies, ants, bees, and other insect scavengers. The honeydew may encourage a fungus called sooty moldthat gives the plants a characteristic black, “sooty” appearance. Honeydew can foul sidewalk, cars, and houses beneath scale-infested trees.
Scale insects have a simple life. Eggs are laid underneath the scale covering of the adult female. When the eggs hatch, tiny immatures, known as nymphs, emerge. Nymphs have legs and antennae and are called “crawlers” because they walk away from the maternal scale to settle at new feeding sites. For most common scales, this is the only stage that crawls about on the plant. When the crawlers arrive at a suitable location, they insert their mouthparts into the plant, and begin to feed on the plant’s sap. The shell or scale characteristic of the species develops soon after feeding begins. The legs and antennae of most species are lost as the nymphs grow.
Nymphs and adult females for most species remain at the same location for the rest of their lives. Adult males are tiny, flying, gnat-like insects that fly to new females for mating. Scales may go through one or more generations each year.
For more on scale insects found in Iowa, see ISU Extension publication IC-415, Scale Insects on Ornamental Plants.
Here is a brief list of the common soft and armored scales of ornamenal plants in Iowa.
Pine Tortoise Scale
Spruce Bud Scale
Pine Needle Scale
Cottony Maple Scale
San Jose Scale