Figure 70. Waxy filaments ("cotton") created by Prescott scale in Arizona.
Symptoms/Signs: The most noticeable symptom is the appearance of branch flagging
in spring. Upon closer inspection old egg masses can often be found
on flagged twigs, often at branch nodes.
Figure 71. Branch damage ("flagging") caused by Prescott scale in the White Mountains, Arizona.
Biology: In April and May, flightless but mobile females and winged
males emerge from overwintering pre-adults. Following mating, eggs
are laid, often at branch nodes. Eggs are laid in groups, covered
with a fluffy white wax, which makes them look like tiny cotton
balls. During May and June first stage larvae, called crawlers,
settle in temporary feeding spots. The second stage larvae settle
in permanent feeding locations in July and August. They feed beneath
scales at the base of needles and in cracks and crevices of twigs,
particularly around the first and second nodes. During fall the
third or pre-adult stage develops and then overwinters. The life
cycle is completed in 1 year.
Affected twigs exhibit lesions, brownish areas
in the phloem. A fungus, Cenangium spp., is sometimes associated
with the lesions. Apparently the principal factor in natural control
is branch dieback. Few natural enemies have been found outside of
a species of ladybird beetle and a green lacewing.
Effects: Branch flagging is the most common
symptom. It occurs primarily in younger trees, less than 75 years
old and is most conspicuous in dense stands. Both lateral and terminal
shoots can be affected. Mortality rates averaged 1-2 percent in
past outbreaks.
Similar Insects and Diseases: Bark beetles
known as twig beetles also cause
twig dieback in ponderosa pine. However upon close inspection one
can find the galleries of these insects either in the inner bark
or the pith of affected branches.