Obscure Scale
Contact: Eric Day, Manager, Insect Identification Laboratory
Publication 444-226, August 1996
Obscure Scale
Plants Attacked
Primarily oaks, chestnuts, pecan and other
hickories, but also several other ornamental trees. It is not a
pest of forest trees.
Description of Damage
Heavily infested trees will have large numbers of scales on twigs
and branches. Scales may also be found on exposed roots and on
the trunk of young trees. Scale insects feed on plant sap with
their long thread-like mouthparts (stylets), which are several
times longer than the insect itself. The continual drain of sap
from the scale's feeding and the disruption of the photosynthetic
and respiratory functions of the bark due to encrustation (see
below) weaken the infested tree. Infestations seldom kill the
tree directly, but can cause extensive die-back of twigs and
branches which decreases the tree's aesthetic value. Weakened
trees also are more susceptible to secondary infestation by other
insects and diseases which can kill.
Identification
As its name implies, obscure scale is difficult to detect,
particularly when it occurs on trees with dark-colored bark. The
protective cover above the insect is typically gray but is often
the same color as the bark. This cover will have a diameter of
no more than 3mm when the insect is mature. At times, a small
black cap is evident near the center of the cover. All
developmental stages except the immature female stage are pink in
color. Immature females are light yellow to cream. The first
stage of development following egg hatch is called the crawler
stage. This developmental stage is the only one other than the
adult male stage that possesses legs and is able to walk.
Crawlers tend to settle under their maternal cover or under
covers of past generations and begin forming their cover. This
encrusting behavior results in patches of scale covers several
layers deep that resemble roughened areas on the bark. The
obscure scale,Melanaspis obscura (Comstock), is in the order
Homoptera: Diaspididae.
Life History
On pin oak in Maryland, eggs are laid from the end of June
through the beginning of September. Crawlers emerge from the
first of July through mid-September; however, crawler activity
peaks from mid to late July. Crawlers settle, begin feeding, and
form their protective cover within an hour. Males will feed and
grow until the following April at which time they discontinue
feeding and enter a pre-pupal and pupal stage. Adult males will
emerge from the pupal stage throughout the second half of May,
mate with adult females, and then die.
The female feeds throughout her entire life until she lays eggs
that following summer. Approximately two weeks prior to crawler
emergence, adult females will add a small flap to their cover
through which crawlers may exit. There is one generation per
year. The scale's development on white oak occurs one month
later than that on pin oak.
Control
Several factors contribute to the difficulty of controlling
obscure scale: (1) the scale's waxy cover provides protection
against pesticide exposure; (2) crawlers tend to settle under the
protection of older scale covers, thus producing a layering or
encrusting effect; and (3) crawler activity extends over a long
period of time. Dormant oil can be applied to the overwintering
stages (second-instar males and females on pin oak; settled
crawlers on white oak). During the growing season, when dormant
oil cannot be used, insecticide treatments must be correctly
timed at either the unprotected adult males or unprotected
crawlers. Crawlers emerge approximately two weeks following the
formation of the exit flap on the adult female covers. If using
exit flaps to predict crawler emergence, one must be sure that
the covers possessing the flaps are of the present generation.
This can be done by removing the cover and determining if there
is a living female beneath. Although difficult, male emergence
may be monitored by placing sticky traps within the canopy during
May. An example of a sticky trap is an ice cream lid coated with
a tacky substance such as petroleum jelly, and hung from infested
branches.
See the Virginia Pest Management Guide for specific insecticides
for control. Average treatment dates given for the control of
crawlers on red oaks (e.g., pin oak) is mid-July, and for those
on white oaks (e.g., white oak) is mid August. Care should be
taken when applying insecticides because they may deplete the
scale's natural enemy populations.
Remarks
A serious pest for landscape oaks! Very difficult to control
once the scale population becomes established. Early detection
of infested trees is necessary for best control and infested
twigs and branches should be pruned from the tree and destroyed
before scale populations reach injurious levels. Consideration
should be given to the adult male stage rather than the crawler
stage when applying contact insecticides, because this stage
occurs over a much shorter period of time. Chemical applications
at this stage will require fewer treatments and will have a less
detrimental effect on the natural enemies of obscure scale.
