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The San Jose Scale is a tiny homopteran insect that is a well known pest of a wide variety of fruit and ornamental trees and shrubs, including many in the family Rosaceae such as Apple, Pear, cherries, Peach, Apricot, Plum, Almond, currants, and Quince. In the western United States, Osage Orange is often heavily infested and serves as a reservoir for reinfestation of cultivated fruit trees. Native to eastern Asia, San Jose Scale was introduced to California from China around 1870 and within a few decades was widely established in the U.S. and Canada through the movement of infested nursery stock.

San Jose Scales feed by sucking plant juices from twigs, branches, fruit, and foliage. Although an individual scale insect cannot inflict much damage, a single female and her offspring can produce several thousand scales in one season. If uncontrolled, they can kill the host tree as well as make the fruit unmarketable. Although scales lives primarily on the tree bark, the first indication that San Jose Scale is in the orchard may be small red spots on fruit or leaves.

Scale insects are quite unlike the insects that are familiar to most people. Most of the complex life cycle of this soft insect is spent beneath a protective waxy covering that it secretes. These scales start out small and very light colored, but darken to a sooty black or ashy appearance as the insects grow larger and mature. Under magnification, this covering looks like a miniature volcano, having 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, whereas male scales develop an oblong shape with the nipple near one end.

The fully grown female scale covering is around 2 mm in diameter and composed of concentric rings. Under this covering, the female scale is yellow, soft, and globular, with no discernible head or appendages. Male scale coverings are smaller, about 1 mm in diameter. Adult males (which are rarely seen except in pheromone traps) have well-developed legs, long antennae, and a single pair of wings. They are a dark yellow to cinnamon brown color and have a thin dark brown band extending across the thorax between the wing bases. After developing through larval and pupal stages, the male matures and backs out from his scale around 4 to 6 weeks after birth to search out females, which produce a sex pheromone attractant. Adult males fly for only a few days and are capable of mating immediately with the females, which remain under their scales.

Females produce live young, not eggs, within 4 to 6 weeks after mating. A single female may produce first instar young (known as "crawlers") for 6 to 8 weeks at a rate of about 10 per day. The six-legged lemon-yellow crawlers are tiny, just  0.2 mm X 0.1 mm, and superficially resemble larval spider mites except that they bear a pair of antennae and a bristle-like sucking beak that is almost three times the length of the tiny, oval body.

After exiting from beneath the female's cover, the crawlers move over the plant. They can be carried to other trees by the wind, on the feet of birds, on the clothing of farm workers, or on orchard equipment. Within a few hours they settle on the bark, leaves, or fruit, tuck in their legs and antennae, insert their long sucking mouthparts into the host, and begin feeding and forming a scale covering. After settling, the crawler secretes a white waxy covering which hardens into a scale. The scale turns from white to black and then to gray and goes through several molts before maturing. The sexes of these immature stages can be distinguished after the first molt, although the scales covering them are identical. The females are smaller and rounder than the males (males are more elongate) and lack eyes, legs, and antennae. The males have eyes, but no legs or antennae. Following the second molt, males pass through two non-feeding instars prior to the final molt to winged, legged adults.

San Jose Scales overwinter on bark in the "black-cap" phase of the first instar. Development resumes as spring temperatures exceed 10° C (50° F). In warmer climates, mated females may also survive the winter. First generation crawler production by all overwintering stages is synchronized and usually occurs within 4 to 6 weeks following plant bloom.

(Mague 1980; Hoyt 2010)

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