San Jose Scale
Quadraspidiotus perniciosus (Comstock)  
San Jose scale crawler
San Jose scale adult male
San Jose scale pheromone trap
San Jose scale fruit injury
San Jose scale wood injury
San Jose scale tape trap

 

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 3).

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 Monitoring Guide.

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.


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