A Matter of Scale
We all run into a few bumps on the road. As a horticulturist,
you're also likely to run into a few bumps on your plants. Those bumps
may turn out to be scale. These tiny insects can wreck havoc on nursery
stock through direct feeding, production of honeydew with the companion
black sooty mold, and transmitting pathogens. Small and cryptic, they
are often difficult to spot, increasing the risk for movement of scale
when plants are transported.
Scale refers to a very large superfamily of insects
called Coccoidea. According to the ScaleNet website there are 28 families
of scales comprised of 7,355 species. Not all scale insects are pests.
Scale insects have been exploited for lacquer and shellac, candle wax,
and used as biological control agents of noxious weeds. They produce a
rich carmine color used as a natural dye for cloth and in the food and
beverage industry. I recall the look of horror on a workmate's face when
I told her, cochineal, the ingredient listed in her favorite all-natural
lemonade, referred to the scale insect on Opuntia cactus, Dactylopius.
Most scale insects are female. Mature females are wingless and often secrete
a hard shell-like covering for protection. The males are rare, small,
non-feeding, and short-lived but look more like other insects as they
have wings. With a few notable exceptions, the first immature stage, or
the first "instar" females are generally the only stage that
disperses on plant material. All other stages remain attached to the plant
surface, sessile. Females lay eggs or crawlers under their secreted scale
covering or in a cavity under their bodies.
Scale identification can be a bit of a challenge given an insect not much
more descriptive than a pimple. The undisputed web resource for the scale
geek in all of us is ScaleNet.
There are identification tools for scales of quarantine importance. Contained
within is a list and descriptions of families of scales, mealybugs, and
soft scales. For those stumbling over scale terminology, there is a glossary.
Included in the glossary are 18 different entries starting with anal,
giving one an idea of the nature of scale identification. The key is designed
for slide-mounted specimens. If you know the scientific name or care to
browse species links, images of the scale as they would appear to the
naked eye, hosts, distribution, and life history are included. Or use
the "Query" tool at the ScaleNet website. Many think by plant
host and that search query is available, listing scales found on that
host. Much easier to use is the National
Collection of Scale Insect Photographs, a site which also allows a
search by host, even by state or city! A line of potentially helpful images
will appear. Suomi's Scale
Insects on Ornamentals is web accessible, has great images and lifecycle
charts, and focuses on common scale species in the Pacific Northwest.
Still like a book? Johnson and Lyon's seminal guide,
Insects that Feed on Trees and Shrubs, lends itself to quick identification
of scale problems by matching images of hosts and pests, and it is thick
with information on species of concern. Many of us take council with Cranshaw's
Garden Insects of North America when scale guidance is needed.
Should you be looking for trouble, monitor your plants for damage;
adult and crawler stages; and also for activity of biological control
agents. If a scale has taken a shine to your plants, look for the tell-tale
signs of honeydew and sooty mold. Look for ant activity. Ants, fond of
honeydew, will fight off scale natural enemies to protect the source.
Look closely, with a hand lens. Near buds, along veins, in the dark cracks
of bark, underneath the leaves, these insects conceal themselves well.
Note details. A round hole in the side of a scale covering may evidence
the emergence of a parasitic wasp.
Crawlers test the sight of even young eyes. Use double-sidedsticky
tape to capture crawler emergence. As they emerge from their protective
cover, they stick to the adhesive tape encircling either side of the infestation.
The application of soaps and oils, insect growth regulators, and other
foliar insecticides are often timed for this occurrence.
Hide the welcome mat. Scrutinize new plant material received as if it
were a future in-law. Need another reason to quarantine new stock? Watch
your parents. Scion material and mother stock can be a source of joy or
headaches depending on whether you propagate pests with your plants. Meticulous
attention to their scale-free status pays dividends. Wind, equipment,
and clothing can move scale insects. Pruning and rouging may be an effective
tactic in the landscape or on a limited number of nursery plants.
Natural enemies often keep scale under control. Factors
that disrupt biological control include ants that tend, move, and protect
scale; dust; and long-residual, broad spectrum insecticides. In protected
culture, some growers have programs with augmentative releases of lady
beetles such as the Mealybug destroyer, Cryptolaemus montrouszieri,
for mealybugs, and Chilocorus and Rhizobius for soft scale.
There are various strategies to slay a scale. Many target
the vulnerable young crawlers. This allows managers to choose a greater
range of insecticides including those of lower toxicity, such as soaps,
oils, and insect growth regulators. Correct scale species identification
helps predict whether the scale will have only one or multiple generations
of crawlers emerging each year. Some kill scale while they sleep, covering
the overwintering stage with suffocating dormant oil. Thorough contact
by foliar sprays is important. Another tactic is application of systemic
insecticides that are drawn into the plant, managing multiple feeding
stages of the scale. It helps to think a little ahead on this one as time
may be needed for some plants to translocate the insecticide to infested
parts of the plant. Systemic insecticides are used against many scale
species but generally have been less successful with hard scales and pit
Insects on Shade Trees and Shrubs website includes a Guide to Assessing
Scale Infestation. It contains a nice decision matrix for management
actions, particularly in landscapes where scale populations may be more
Common Scale Families
According to the USDA-ARS website, there are 56 introduced species of
mealybugs in the United States, 45 of these species are pests. The most
likely plant host for mealybugs is a grass or composite. Many mealybugs
are associated with greenhouse production or interiorscapes. Citrus mealybug,
Planoccoccus citri, is a common pest in such situations. The longtailed
mealybug, Pseudococcus longispinus is another familiar species
in protected systems distinguished by two long waxy filaments forming
Medeira mealybug, Phenacoccus madeirensis is
one of the more recently introduced mealybugs to the United States. It
is very polyphagus, feeding on 42 plant families. Pink hibiscus mealybug,
Maconellicoccus hirsutus, is a species with a host range of morethan
200 genera. Like Florida, California has begun a biological control program
for this recent introduction.
Mealybugs are not only indoors in the Pacific Northwest,
they can also be found outdoors. The ground
mealybug, Rhizoecus sp., on Phormum is a good example
of a species which successfully overwinters in our mild climate.
Eriococcid or felt scales, are named for the felt-like waxy ovisac that
encloses the body of the female. Two species found in the Northwest are
the European elm scale, Gossyparia spuria, named for its host,
elm, particularly American and rock elm, and the azalea
bark scale, Eriococcus azalea, which can be found on several
different hosts including azalea, rhododendron, andromeda, hawthorn, poplar,
willow, and blueberry.
The soft scales are commonly identified scales on ornamental plants and
can be quite variable. Some, such as brown soft scale, Coccus hesperidum,
are extremely common throughout United States. This scale is flattened
to slightly convex. The much rounder lecanium scale is also found in landscapes
and nursery production. European fruit lecanium, Parthenolecanium corni,
is a native species and on many different woody plant hosts. One can find
oak lecanium, P. quercifex, on our native white oaks but often
held in check by parasitic wasps. Conifers get soft scale too. Fletcher
scale, P. fletcheri, sometimes infests yew in nurseries. It can
also infest juniper, arborvitae, and baldcypress.
One of the most recent finds in Oregon included Eulecanium
excrescens on Styrax. Around the country entomologists report
finding various soft scale species including European peach scale, P.
persicae; frosted scale, P. pruinosum; calico scale, E.
cerasorum; magnolia scale, Neolecanium cornuparvum; spruce
bud scale, Physokermes piceae; hemispherical scale, Saisettia
coffee; black scale, S. oleae; and nigra scale, Parasaissetia
Wax scales are definitely on the regulatory radar in the Pacific Northwest.
Recently Barnacle scale, Ceroplastes cirripediformis, was found
in one nursery in Oregon. The crop was destroyed.
The bane of many holly growers, cottony
camellia scale, Pulvinaria floccifera, is also called cottony
taxus scale. It produces long, narrow and unavoidably obvious ovisacs
on the leaf undersides. Cottony maple scale, P. innumerabilis,
can produce quite a spectacular colony of females hunched over with their
cottony ovisacs. It can infest many genera of woody plants besides maples.
Margarodid scale are very diverse and quite striking in appearance. This
family of scale has been found in Oregon on Nandina, infested with
the cotton cushion scale, Icerya purchasi. Adult females produce
a long, fluted ovisac. Like its cousin the sycamore scale, Stomacoccus
platani, all stages of this scale can move.
Armored or Hard Scales
These tiny scales do not produce honeydew. And their covering is separate
from their body. They can build up to high populations, reducing the vigor
of plants and causing die-back. Oystershell scale, Lepidosaphes ulmi,
has been well-named based on resemblance to a mollusk. This species can
infest a wide range of woody genera. A Scale with a similarly wide appetite
is San Jose scale, Quadraspidiotus perniciosus. Infestations can
build up rapidly as this species has multiple, overlapping generations
in some locations. Euonymus scale, Unaspis euonymi, is a worry
for those growing that key host.
One hard scale species found in the PNW is
pine needle scale,
Chionaspis pinifoliae. Pine, spruce, fir, hemlock and Douglas
fir can all host this species. Adult females are elongated, white, and
armored with a yellow area at the front end. Those growing juniper should
be aware of the juniper scale, Carulaspis juniperi.
For more scale species listings, see Scale
species listed by host type
Scale have disappointed many a grower, posing as part
of their plant's anatomy and feeding on the profits. With a little effort,
it is possible to set up an IPM program that is effective and tips the
balance in your favor in the battle with the scale.
A database of the Scale Insects of the World.
Systematic Entomology Laboratory Coccoidea Web Page
US National Collections of Scale Photographs
Insects on Ornamentals. D. Soumi. Washington State University EB1552E
Insects of Trees and Shrubs in Minnesota, Univ. of Minn. Extension
Insects, Virginia Cooperative Extension, pictorial key
UC IPM On-line, Pests of the landscape and garden.
and Mealybugs on Ornamental Plants, University of Florida Extension
A Guide to
Scale Insect Identification, University of Florida Extension
Affecting Ornamental Plants. University of Florida Extension
Insects on Shade Trees and Shrubs. Purdue Extension
Hard Cover Resources:
D. Miller and J. Davidson. 2005. Armored Scale Insect
Pests of Trees and Shrubs. Cornell University Press. 456 pp
2007 PNW Insect Management Handbook, C.S. Hollingsworth,
ed. Oregon State University.
W. Cranshaw (2004). Garden Insects of North America.
Princeton University Press, Princeton, NJ. 656 pp
W.T. Johnson and H.H. Lyon (1991), Insects That
Feed on Trees and Shrubs, 2nd ed., Cornell University Press. 560 pp