San Jose Scale
Contact: Eric Day, Manager, Insect Identification Laboratory
ENTOMOLOGY PUBLICATION 444-223, August 1996
San Jose Scale
Plants Attacked
Apple, pear, peach, plum, Osage orange, or ornamental trees and
shrubs
Description of Damage
San Jose scale is one of the most destructive scale insects.
Scale insects feed on plant juices by means of long, hollow,
needle-like mouthparts inserted into the tissue of the host. The
host tissue near the feeding site often turns red or purple (this
is most evident on bark by making a thin slice into the bark with
a knife). When feeding on apple fruits, the small brown scale
has a characteristic red halo around it. Slow progressive
killing of infested twigs and stems often results. Heavy
infestation may cause death of the tree. Damaging stages are
nymphs and adult females. Plant parts attacked are fruits,
twigs, stems, and branches.
Identification
The immature and adult females live under a small circular brown
or smoky black cover up to 1/16th of an inch (1 to 2 mm) in
diameter. The scale has dark concentric rings that can be seen
under a magnifying glass. A few elliptical-shaped scales (larger
male immatures) may be interspersed among the otherwise round
scales. The San Jose Scale, Quadraspidiotus perniciosus
(Comstock), is in the order Homoptera family Diaspididae.
Life History
San Jose scales start life as crawlers which are tiny, free-
living insects that move about the plant looking for a place to
settle. The crawlers can be moved to another plant by wind or by
hitching a ride on a bird's leg. This is the only time that the
scale can move from plant to plant. After the scale settles, it
quickly starts to secrete its waxy covering and remains in place
for the rest of its life. There are at least three generations
per year in Virginia. The first crawler emergence is in late May
through early June. The date of the first crawler appearance
varies by up to two weeks from year to year and also between
areas within a given year. The second generation crawlers occur
in late July, with the third generation in late summer or early
fall; but due to overlapping of generations there is some
variability to exactly when it occurs. By the end of the season,
generations are no longer discrete and all stages are present.
Crawlers usually settle very close to where they hatched and
heavily infested plants have bark crusted with scales. If the
crawler is a male, it will develop like the female early in its
life, but during the later stages of development the male scales
form what is known as a pupal stage from which emerges a tiny
two-winged insect capable of flight. After mating with the
female, the male dies. Adult females remain under cover and lay
eggs. Crawlers appear about 50 days after the first male flight.
Immature male and female scales overwinter and mature in the
spring.
Control
Apply superior dormant oil in late winter for the overwintering
stages. During the growing season, treat when crawlers are first
seen and again 7-10 days later. To observe crawlers, wrap black
electrical tape around a scale infested branch with the sticky
side outward. Check each day for lemon-yellow crawlers caught on
the tape. When the tape losses its tackiness, replace it.
Although variable, the crawler dates in Virginia are usually
about June 10-15 and late July. See the Virginia Pest Management
Guides for specific insecticides recommended for control.
Remarks
Females produce a sex pheromone to attract the winged males.
Synthetic pheromones have shown some possibilities for monitoring
and control. If pheromone traps are used for monitoring, expect
crawler emergence about one month after the first male catch
(traps should be hung at bloom or shortly before). Sprays
directed toward male flight can be effective as crawler sprays,
but often male activity occurs during bloom when sprays may not
be applied. Insecticides should not be used at bloom because
this will disrupt pollination by bees.
Prepared by Eric R. Day and Douglas G. Pfeiffer. Dept. of
Entomology, Virginia Polytechnic Institute and State University,
Blacksburg, Virginia 24061-0319