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).

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).