of Rhode Island GreenShare Factsheets
Scale insects are small, immobile insects with no visible
legs or antennae, pressed tightly against the plant on which
they are feeding. Many are common and serious pests of trees,
shrubs and indoor plants.
Scale insects feed on plant sap. They have long, threadlike
mouthparts (stylets) which are six to eight times longer than
the insect itself. Scale feeding slowly reduces plant vigor;
heavily infested plants grow poorly and may suffer dieback of
twigs and branches. An infested host is occasionally so weakened
that it dies. Scales often secrete a sticky honeydew which is
attractive to wasps and ants and which supports the growth of
black sooty molds.
Scale insects are generally controlled by natural enemies,
including tiny parasitic wasps and predators such as ladybugs.
It is very common for ladybugs to move onto a plant with a growing
scale infestation; before deciding upon treatment, look for adult
and immature ladybugs on your plants. Dormant oil treatments
can be used against almost all scale problems and are generally
applied in very early spring, before bud break. Summer oils can
also be very effective against most scales, but as with dormant
oils, some plants are sensitive to these treatments. Check labels
to make sure your plant is not harmed by the oil treatment you
are considering. Most other insecticides, including insecticidal
soaps, can be used only against the mobile crawler stage of scales
since adult scales are protected from insecticides by a waxy
covering. These treatments are very effective, but must be carefully
timed as crawlers are only active for a limited period.
Scale insects can be roughly divided into two groups: armored
scales and soft scales. Armored scales secrete a protective cover
over their bodies. Most species of armored scales overwinter
as eggs beneath the female cover. In spring, these eggs hatch
into tiny mobile crawlers which migrate to new feeding sites.
The crawlers settle after a few days, insert their mouthparts
in the plant, and begin to feed. Soon they secrete a protective
cover and lose their legs. Large populations can build up before
plants begin to show visible symptoms. Our most common armored
scale pests are described and illustrated below.
San Jose Scale (Quadraspidiotus perniciousus)
Mature scales are circular, slightly convex and smoky black.
They are about 2 mm (l/l6 inch) long. A conspicuous, dark gray,
concentric ring is visible under a magnifying glass. San Jose
scale is perhaps the most widely distributed and most destructive
scale insect pest of fruit trees, shade trees and ornamental
shrubs in the United States. Over 60 host plants are known, including
apple, pear, peach, cherry, ash, poplar, lilac, elm, willow,
pyracantha and cotoneaster. There are at least three generations
a year; broods often overlap, making treatment of crawlers difficult.
Both dormant oils and summer oils are effective.
Oystershell Scale (Lepidosaphes ulmi)
These scales are shaped like the shell of an oyster. They
are chestnut to dark brown, sometimes with lighter transverse
bands. Infested twigs are often completely encrusted with scales.
This is a common and destructive pest of over l20 different species
of fruit trees, shade trees, and woody ornamental shrubs. Hosts
include apple, lilac, dogwood, boxwood, birch, elm, sycamore,
viburnum and many others. Eggs hatch in late May. Use a dormant
spray in April or treat crawlers between late May and mid-June.
Pine Needle Scale (Chionaspsis pinifoliae)
Mature scales are pure white and shaped like oyster shells.
Pine needle scales are common and serious pests of ornamental
pines and various spruces. Less preferred hosts include hemlock
and fir. Ornamental plants, Christmas tree plantations and nursery
stock are more frequently infested than forest trees. In heavy
infestations, needles may be completely whitened by a continuous
layer of scales. Use dormant or summer oils against adults or
treat crawlers when lilacs are in bloom.
Euonymus Scale (Unaspis euonymi)
Females are pear-shaped and blackish-brown. Males are elongate
and white. This is a common and serious pest of evergreen euonymus,
often causing defoliation and dieback. Pachysandra and bittersweet
are also suitable hosts. There are two generations per year.
Crawlers are active in early June and mid-July. Avoid use of Euonymus
japonica, a highly susceptible species. Watch for a small
black ladybug with 2 red spots which often provides good control
of this scale.
Juniper Scale (Carulaspis juniperi)
Females are round and dirty-white with yellow centers. Under
a magnifying glass they resemble miniature fried eggs. Males
are also white, but smaller and narrower. Hosts include junipers,
arborvitae, incense cedar and cypress. Crawlers are active mid-June
to early July.
Hemlock Scales (Abgrallaspis spp.)
We have two species of hemlock scale--one is round and the
other oblong. Both are brown and found on the undersides of needles
of hemlock and occasionally on other conifers. These scales are
usually effectively controlled with biological controls, but
dormant or summer oils may be used when needed.
In general, soft scales are larger and more convex than
armored scales. Many resemble miniature tortoise shells. Soft
scales usually cover themselves with wax, but they lack the detachable
protective cover for which armored scales are named. Most soft
scales overwinter as immature, fertilized females. In spring
they resume feeding, mature and lay eggs. These hatch into tiny
crawlers. After locating suitable feeding sites, crawlers settle
and begin feeding. Some species lose their legs once they've
settled; others retain legs and are able to crawl short distances
to find suitable overwintering sites in the fall. Except for
those soft scales which infest indoor plants, most have only
a single generation per year at our latitude. Our most common
soft scale pests are described and illustrated below:
Magnolia Scale (Neolecanium cornuparvum)
Our largest scale insect, this species reaches 12 mm (1/2
inch) in length. Color ranges from dark brown to pink-orange
and older scales are covered with a white waxy powder. This scale
produces large amounts of honeydew and sooty mold. Treat with
dormant oil in April or treat crawlers in October with two treatments
seven to ten days apart.
Fletcher Scale (Parthenolecanium fletcheri)
Mature scales are dark brown, shiny and very convex. Arborvitae
and yew are the most frequently attacked hosts, but pachysandra
and Eastern red cedar are also susceptible. Honeydew excreted
by the scales supports unsightly, sooty molds. There is one generation
per year with crawlers active in late June through early July.
There are several effective parasites which are best conserved
by using dormant oil treatment in late April.
Cottony Maple Scale (Pulvinaria innumerabilis)
Large, conspicuous, white egg sacs are produced on the twigs
and small branches of host plants. During summer, immature scales
feed on leaves, but they migrate to twigs as fall approaches.
Honeydew excreted by the scales supports unsightly, sooty mold
growth. Cottony maple scale is most common on silver maple, but
also found on other maples, boxelder, linden, black locust, red
mulberry, white ash, apple, beech, cherry, dogwood, elm, hickory,
holly, honeylocust, peach, plum, sycamore, willow and others.
There is one generation per year with crawlers active in late
June through July. Many maples are sensitive to oils--both dormant
and summer. Check labels carefully or treat crawlers.
Oak Kermes Scale (Kermes galliformis)
Mature scales are tan, globular and hard. They are easily
mistaken for galls or buds. Oak is the only host. This species
is not particularly injurious to host trees, but, if necessary,
treat crawlers between late May and mid-July, or use a dormant
oil in April.
Adapted from the Virginia
Cooperative Extension and the University of Massachusetts Extension,
are poisonous! Read and follow all safety precautions on labels.
Handle carefully and store in original containers out of reach
of children, pets or livestock. Dispose of empty containers
immediately, in a safe manner and place. Pesticides should never
be stored with foods or in areas where people eat.
When trade names are used for identification, no product endorsement
is implied, nor is discrimination intended against similar materials.
Be sure that the pesticide you intend to use is registered for
the state of use.
The user of this information assumes all risk for personal injury
or property damage.
information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
of Rhode Island Cooperative Extension provides equal program