Part IX: Scale Insects
by Ron Westby
In predator-prey relationships, one axiom stands before all
others: What can’t be seen (that is, recognized)
generally won’t be eaten! And insects seem to have all
but cornered the market on effective disguises. Moths blend
in with tree bark . . . katydids resemble leaves … and
inchworms mimic twigs (when alarmed).
However, the slyest members of the
whatyou-see-may-not-be-what-you-get set are definitely the
scale insects. These crusted con artists look more like
bird droppings or disease organisms than bugs . . . and are
often ignored, even by gardeners. That’s too bad, because
scales are related to aphids and can become just as
abundant . . . often literally encrusting the stems of
trees and shrubs.
Young scales, called crawlers, are fairly mobile; but they
soon grow sessile under the protection of their tough outer
shell. These insects do their damage by sipping the sap
from a host plant (usually orchard trees and ornamentals),
a process that can cause a gradual decline in vigor of the
plant or tree, stunt its growth, and even kill it. And the
foliage-killers are so adept at hiding that they often
exist unnoticed till winter causes the leaves to drop from
their deciduous-tree hosts.
GROWING UP TOUGH
Of the 4,000 or so species of scale insects, few ever
exceed 3 cm (1.2″) in length, and most range in size from a
barely discernible 1 mm to a tiny 1.5 cm. The most
prevalent types, in the U.S., are the soft scales (family
Coccidae) and the armored scales (family Diaspididae).
Despite the names, both of these develop hard shells, but
the soft scales grow theirs as an extension of their
exoskeleton, whereas the protective “bubbles” of the
armored group are not attached to the bugs’ bodies.
In general, all scales have similar (and fairly simple)
life cycles. They go through an incomplete metamorphosis
(egg to nymph to adult). In the nymph stage, scale insects
are equipped with legs, but when they reach adulthood,
these appendages drop off, and the creatures become
sedentary. A female lives out her life beneath a protective
shell, sucking plant juice and laying eggs (about 1,000 a
year!) till there’s little left of her. An adult male-with
a set of feeble wings able to propel him only a few feet at
a time, and with no digestive tract-exists only to
copulate, after which, in most cases, he will die.
TOP: Hemispherical scales are among the
most common of soft scale insects. MIDDLE: Oystershell
scales are oblong, armored scale in sects. BOTTOM:
Two typical armored scales. The outer shell has been
removed from the one on the left to expose the legless
insect beneath.
A FEW SPECIFICS
About the most notorious (and prolific!) scale insect in
the U.S. is the armored San Jose (Quadraspidiotus
perniciosus). This minuscule black or brown bug is
particular ly injurious to orchard trees and ornamentals,
and an army of them-if not controlled-can easily destroy an
entire tree. The San Jose’s armored, oblong cousin, the
oystershell scale (Lepidosaphes ulmi), enjoys the
same diet and is equally lethal.
Citrus, avocados, ferns, and greenhouse plants are fair
game for the hemispherical scale (Saissetia
hemisphaerica), a brown, softshelled member of the
crusted clan. Black scales (Saissetia oleae) are
widely distributed soft scales and are destructive to
olives, citrus, pears, apples, grapes, and figs. All in
all, the North American species of the scale family are too
numerous to list, but they’re so minute that, without a
microscope, you’d often be hard put to tell one “breed”
from another anyway.
ERADICATION
Scales are difficult to get rid of, but in many instances
they can be kept in check by natural predators and
parasites. Ladybugs, such as the vedalia (Rodolia
cardinalis),which was imported into California from
Australia in 1860 to combat cottony-cushion scales (Icerya
purchasi ), and various chalcid wasps (Aphy tis
lingnanensis, A. melinus, and Prospaltella perniciosi) are
often loosed in scale-infested orchards and greenhouses.
And these insects have proved pretty effective at cleaning
up the crusty bug population.
Then again, if you’ve got the time, you could try
hand-crushing scales. But, if you do so, remember that
females lay their eggs underneath their shells . . . so
you’ve got to really put the pressure on in order to
destroy them!
As a last resort, oil sprays can be used against the pesky
insects. The so-called dormant sprays are oil solutions
that are applied to deciduous trees during their dormant
season. These are most effective when sprayed in early
spring after the young scales hatch and before buds on the
trees open. Lighterweight “summer oils” are generally used
on citrus and evergreens during their growing season.
One last thought. Although scales may seem to lead
relatively dreary existences, you’ve got to admit they’re
pretty darn good at the two things they do best: living off
our food-bearing trees (and plants), and procreating! So
the next time you spot what appears to be a rash on your
prized citrus tree, better take a closer look to make sure
it’s not a colony of those elusive masters of disguise . .
. the scale insects.
