I. Introduction: San Jose
scale (SJS) was first introduced from China into California in about 1870, and by 1895 had
spread throughout the U.S. and Canada on nursery stock. Currently, this insect is
generally kept under control with conventional pesticides so that it is mainly a pest of
larger, poorly pruned standard-sized trees that do not receive adequate spray coverage.
SJS is still common throughout the U.S. and Canada, and is capable of killing mature trees
if not controlled. This was the first insect to develop resistance to a pesticide in the
U.S. (to lime sulfur in 1908) and has been responsible for the death of thousands of acres
of apples since it was introduced.
II. Hosts: SJS attacks most cultivated fruits including apple, pear, quince, plum,
apricot, sweet cherry, currant, and gooseberries. It also attacks many species of
ornamental trees and shrubs; osage orange is often heavily infested in the West and serves
as a reservoir for reinfestation.
III. Description: Most of the life cycle of this insect is spent under a secreted
waxy covering that protects the soft, sessile insect from predators and to some extent
even insecticides. Young scales have smaller, very light-colored coverings that darken to a sooty black or ashy appearance as they grow
larger and mature. When viewed under magnification, this covering looks like a miniature
volcano, with the shape of a very low cone with a circular ridge at the apex, inside of
which is a nipple-like elevation. Female scales develop almost perfectly circular
coverings with the nipple in the center, but male scales develop an oblong shape with the
nipple near one end. Similar species of scale found on fruit, such as lecanium scale,
oystershell scale and scurfy scale, all have non-circular coverings distinctive to their
species. The lemon-yellow, newly born nymphs (called "crawlers") (photo 1)
are barely visible to the eye, and resemble mites except they have only three pairs of
legs and possess a pair of antennae. The full grown female scale covering is about 1/16
inch (1.5 mm) in diameter. Under this covering, the female scale is pale yellow in color
and rather baglike in form, with no discernible head or legs. Male scale coverings are
about half the size of female coverings. Adult male scales are strikingly different, with
well-developed legs, antennae, and a single pair of wings (photo 2). They are a
dark yellow to cinnamon brown color and have a thin dark brown band extending across the
thorax between the wing bases. Males are rarely seen except in pheromone traps (photo
IV. Biology: SJS overwinters as partially grown immatures on the
trunks and scaffolds of the tree, with the majority being in the first nymphal instar.
Extremely low temperatures in the winter may cause high mortality to these overwintering
stages. Most of the nymphs can tolerate temperatures above -10EF (-23.3EC). The nymphal scales remain dormant under their waxy coverings
until the sap begins to flow in the spring, and then continue to feed until bloom when
they become mature. At this time, the winged male scales come out from under their waxy
coverings to search for females and mate. Adult females do not leave their scale covering,
but produce a pheromone to attract the males for mating, which takes place under the waxy
covering. After mating, females continue to live for about another six weeks, producing
crawlers at a rate of about ten per day. A single female may eventually produce 150 to 500
crawlers during this period. First generation crawler production by the overwintering
females is synchronized, and generally occurs four to six weeks following bloom, about 30
days after first male flight. The time interval between first and peak crawler emergence
is only about seven days for this overwintering generation.
Crawlers are very active for the first few hours after being born and may travel
considerable distances before finding a suitable feeding site on the trunk, limb, twig, or
fruit. Within 24 hours following birth, crawlers insert their beaks through the bark to
feed on the sap. They then become sessile and begin secreting waxy filaments from the
body, which harden along with shed skins into the cap-like scale covering. In about
another three weeks, crawlers undergo their first molt and lose their legs. Females
undergo a second and final molt in three to four weeks and become adults. Males go through
four stages, including two non-feeding final stages known as prepupae and pupae, during
which wing pads and short, thick legs develop. The main means of dispersal of SJS is on
the feet of birds, on clothing, by the wind, and on farm machinery.
There are two to four generations a year with considerable overlap of the broods
because of the long reproductive life of the females. The summer generation is usually
completed in five to seven weeks. Adult males emerge over a much longer period because of
this overlap, and usually begin flying during late July and continue through the month of
August into September. Second generation crawlers are usually present by early August. A
third generation occurs in some states south of Pennsylvania and a fourth generation
sometimes occurs in North Carolina.
SJS is parasitized by species in the chalcidoid (Hymenoptera) families Aphelinidae and
Encyrtidae. It has been successfully controlled by Encarsia (= Prospaltella) perniciosi (Tower). The most successful species are aphelinid ectoparasites in the
genus Aphytis. Adult Aphytis females drill through the scale
"shell" with their ovipositors and deposit an egg on the surface of the host's
body, but underneath the scale covering. After the parasitoid feeds on and kills the scale
host, it pupates beneath the empty scale covering, chewing a hole to escape as an adult.
V. Injury: Heavy SJS infestations of the bark contribute to an overall decline in
tree vigor, growth and productivity, and early season injury may result in small, deformed
fruit. Fruit and leaves of apple, peach, and pear may also become infested. Feeding on the
fruit causes a distinctive reddish-purple discoloration that reduces the quality of the
apple at harvest (photo 4). This discoloration may also be seen in wood from
feeding on the bark of trees (photo 5).
VI. Monitoring: If SJS injury to fruit (photo 4) was detected at harvest
last season, look for scale-infested branches (photo 5), especially in the tops of
trees, while conducting dormant pruning. Use a pen knife to cut through the bark of
branches with a sooty black or ashy appearance to check for purple discoloration in the
wood (photo 5).
At the pink stage of apple bud development, install a minimum of two pheromone traps (photo
3) in each scale infested orchard to monitor adult male (photo 2) emergence.
Monitor traps daily until the first male scale is caught (biofix) and then weekly
thereafter until flight stops. Count and record the number of male scales caught each week
and remove insects from the trap with a probe or pen knife. It is more efficient to
replace the trap, rather than to remove the insects, if large numbers of males are caught.
A new trap and lure should be installed at the end of June to monitor the second flight of
adult males. After first trap catch of each generation, monitor and record daily maximum
and minimum temperatures and accumulate degree days [base 50EF (10EC)].
Crawler (photo 1) emergence
generally occurs after an accumulation of 300 to 350 degree days (DD) greater than 50EF (167-194 DD greater than 10EC) after the first adult
catch of each generation. The most reliable method for detecting crawler emergence is to
wrap black electrician=s tape (sticky side out) around tree limbs that are encrusted with
dark scale coverings (photo 6). A thin film of petroleum jelly may be spread on the
tape surface to enhance crawler capture. Inspect the tape traps twice weekly for the
bright yellow crawlers.
VII. Management: An insecticide application is recommended at green tip to
half-inch green to control overwintering scale if one percent of the fruit harvested the
previous season showed the characteristic red spotting injury (photo 4). This
application should be made as a dilute spray since thorough coverage is critical for
effective scale control. Make sure sprayers are properly calibrated to deliver sufficient
spray volume to the tops of trees to provide adequate coverage. To control the crawler
stage, apply insecticides when crawlers are first detected with tape traps and again in
about 10 days. The potential for scale infestation can be greatly reduced by following
proper annual pruning practices which will facilitate thorough spray coverage. Prunings
from scale infested trees should be removed from the orchard and burned.
Modified from text prepared by D. G.
Pfeiffer, L. A. Hull, D. J. Biddinger and J. C. Killian in the Mid-Atlantic Orchard
Credits: Text prepared by H. W. Hogmire and S. C. Beavers, West Virginia University, modified from
the original text in the Mid-Atlantic Orchard Monitoring Guide (original text by D.
G. Pfeiffer, L. A. Hull, D. J. Biddinger and J. C. Killian). Prepared in April, 1998.
Web Site Author: Alan