San Jose Scale, Quadraspidiotus perniciosus (Comstock)

I. Introduction: San Jose scale (SJS) was first introduced from China into California in about 1870, and by 1895 had spread throughout the United States 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 United States 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 United States (to lime sulfur in 1908) and has been responsible for the death of thousands of acres of apples since it was introduced.

II. Hosts: The 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 and 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, having the shape of a very low cone with a circular ridge at the apex, inside of which is a nipple-like elevation. Female scale develop almost perfectly circular coverings with the nipple in the center, but male scale 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"; right photo below) are barely visible to the eye, less than 1/100 inch long (0.2 mm X 0.1 mm) 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 0.08 in (2 mm) across. Under this covering, the female scale is a pale yellow color and rather baglike in form, with no discernible head or legs. Male scale coverings are about 0.04 in (1 mm) across. Adult male scales are strikingly different with well-developed legs and antennae and a single pair of wings (left photo below). 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 (Plate 66).

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 -10ƒF. The nymphal scales remain dormant under their waxy coverings until the sap begins to flow in the spring and then continue to feed until early May 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, however, do not leave their scale covering, but produce a pheromone to attract the males with mating taking place under the waxy covering. After mating the females continue to live for about another 6 weeks, producing crawlers at a rate of about 10 per day. A single female may eventually produce between 150 to 500 crawlers during this period. First generation crawler production by the overwintering females is synchronized, and generally occurs within 4 to 6 weeks following bloom, about 30 days after first male flight. The time interval between first crawler hatch and peak crawler hatch is only about 7 days for this overwintering generation.

The 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, the crawlers insert their beaks through the bark to feed on the sap and 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 3 weeks, the crawlers undergo their first molt and lose their legs. The females undergo a second and final molt in 3 to 4 weeks and become adults. Males, however, go through four stages including the two non-feeding final stages known as prepupae and pupal stages during which wing pads and short, thick legs develop. The main means of dispersal of the SJS is on the feet of birds, on clothing, by the wind, and on farm machinery.

There are two to three 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 5 to 7 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.

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. The fruit and leaves of apple, peach, and pear may also become infested (left photo below). Feeding on the fruit causes a distinctive reddish-purple discoloration that reduces the quality of the apple at harvest. This discoloration may also be seen from feeding on the green bark of young trees as well (right photo below).

VI. Monitoring: If SJS presented a problem during the last harvest, find scaly spots while pruning to ease trap placement and later control actions.

Adult male emergence can be monitored with special sex pheromone traps (pictured above), which should be placed by pink. These adults are weak fliers and traps should be placed in infested blocks. A parasitoid of the SJS, Encarsia (=Prospaltella) perniciosi (Tower) (New Zealand image), is also attracted to the pheromone traps and because they are similar in size, may confuse trap catches. Male scale adults, however, have a characteristic body shape and a dark band across the thorax that is not found in the parasitoids. (In practice, it is easy to tell these insects apart while on the sticky trap.  Compare images of male scale with scale parasite

Crawler emergence generally occurs after a degree day accumulation of 300-350 (Base 50°F) from the first adult catch of either generation. Daily monitoring of traps until the first male is caught for a biofix is therefore important.  The most reliable method for detecting crawler emergence in an orchard is by putting dark double-sided sticky tape around tree limbs (Plate 69) that are encrusted with dark scale coverings and checking for the bright yellow crawlers twice weekly. Control measures for the SJS are recommended if 1% of the fruit at harvest from the previous season showed red spotting.

This is taken primarily from a chapter by D.G. Pfeiffer, L.A. Hull, D.J. Biddinger, & J.C. Killian on apple indirect pests, reprinted with permission from Mid-Atlantic Orchard Monitoring Guide, published by NRAES, 152 Riley-Robb Hall, Ithaca, New York 14853-5701.
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