Sap-Sucking Insects, Gall Formers and Mites
FIELD GUIDE TO INSECTS AND DISEASES OF ARIZONA AND NEW MEXICO FORESTS
Prescott Scale
Matsucoccus vexillorum (Morrison)

Host:  Ponderosa pine

Figure 70. 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. 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.

References:  24, 63, 65

Figure 72. Figure 72. Seasonal life history of Prescott scale.