- Scale insects belong to a large group of sucking insects that attack a wide variety of trees and shrubs. Generally, they are divided into two categories, armored (hard) and soft scale. Heavy infestations may cause leaf yellowing, stunting, and dieback.
- Eggs are laid under female adult coverings.
- Typically, eggs hatch in spring or summer. The stage after the eggs hatch is the immature stage or commonly called the 'crawler' stage because they walk about on plants to find new feeding sites. This is how a scale infestation spreads.
Appearance of soft scales
- Appear as raised bumps on twigs and branches and can be scraped off.
- They may be over 1/3 inch when mature and can vary in color.
- Produce honeydew (a sweet liquid, secreted by aphids and some (soft not armored) scale insects as they feed on plant sap) and sooty mold (a fungus that grows on honeydew), which may be present on leaves below infested branches.
- Honeydew can drip on to cars parked under trees or patios and is often the first noticeable sign of a scale infestation.
- Common examples include azalea bark scale, Magnolia scale, Indian wax, cottony camellia scale, and European fruit lecanium.
Examples of Soft Scales
Azalea bark scale, Eriococcus azalea
A scale infestation is indicated by sooty mold (black coating) on leaves, yellowing of leaves, and twig dieback. This scale is most obvious from May through June when white egg sacs may be found in twig forks. Heavy infestations over several seasons may kill plants. Overwintering immature scales (nymphs) are about 2 mm long, gray, and are usually found in twig forks. This scale primarily attacks azalea and rhododendron but has also been found on Andromeda (Pieris), maple, arborvitae, willow, poplar, and hackberry. There are 2 generations a year in Maryland.
Cottony camellia scale is also known as Cottony taxus scale
Cottony maple scale, Neopulvinaria innumerabilis
A native soft scale that feeds on different species of shade trees, including maple, hawthorn, dogwood, sycamore, and linden.
European fruit lecanium scale, Parthenolecanium corni
Contrary to its name this is a native scale. It has a varied host range but prefers maples, oaks, and ornamental fruit trees.
Elm scale, Gossyparia spuria
The preferred host is American elm but will infest Zelkova.
Indian wax scale, Ceroplastes ceriferus
Has a large host range which includes Chinese and Japanese hollies, euonymus, spirea, and boxwood. A difficult scale to control. Pruning out infested stems
and branches and gently scraping the scale covers off bark are recommended.
Calico scale, Eulecanium cerasorum
A native soft scale with a wide range of host plants. They include dogwood, magnolia, maple, oak, redbud, and many others.
Magnolia scale, Neolecanium cornuparvum
This native scale is one of the largest in size. It feeds only on magnolias.
Fletcher scale, Parthenolecanium fletcheri
A native soft scale that can be found on evergreens like arborvitae, yew, and possibly on juniper and cypress.
Pine tortoise scale, Toumeyella parvicornis
Pine tortoise scale differs from other scale insects that occur on pine. Mature female covers are 1/4 inch long, hemispherical and dark brown to black with light brown to cream colored mottling. Males are small and inconspicuous. This soft scale attacks Scotch, jack, Virginia, Austrian, Swiss mountain, red, white loblolly, shortleaf, slash and Chinese pines. It has one generation and overwinters as immatures on twigs. The damage symptoms first appear as honeydew, followed by sooty mold build up on branches. Needles turn yellow and branches eventually die. Small pines on exposed sites may are most susceptible and may be killed. To monitor for this pest, look for ants (seeking honeydew), honeydew, and sooty mold near branch terminals. In the spring the large females are found at the base of needles on twigs. Crawlers usually hatch the last two weeks of June.
Spruce bud scale, Physokermes piceae
The main host is Norway spruce but will infest other types of spruce. Similar looking to the buds of spruce trees so they often go undetected. They are often clustered in groups at the base of new growth.
Scale insects are difficult to control. To manage a scale infestation, prune out branches with severe symptoms. Parasites generally provide control of many scale infestations. To protect parasites, avoid spraying with insecticides during the summer. If treatment is necessary, spray the tree with a dormant rate of horticultural oil during the dormant season to control overwintering scales. Keep trees and shrubs watered during droughty periods and do not over-fertilize.