Quadraspidiotus perniciosus (Comstock)
Timothy J. Smith
San Jose scale is an introduced pest that has been a potentially serious problem in Washington orchards for almost 100 years. It has a wide host range, attacking not only all tree fruits, but also a number of related native hosts. It is most troublesome in older orchards where spray coverage is difficult and in orchards that are near ornamental or native hosts that maintain uncontrolled populations. Large populations of this pest can seriously injure the tree, and even small populations may attack and cull high percentages of the fruit.
This scale overwinters on the tree in an immature stage, surviving in higher numbers after relatively mild winters. They complete their development in the early spring. They are most vulnerable to control measures during this maturation period. This over-wintering and early spring stage of the scales development is often called the “black cap.” Winged males emerge from under their waxy scale and mate with the females, which remain under their protective covering. Males live only a few days, and are attracted to the female by a pheromone. Each female then produces several hundred live young (crawlers) over a six week period. These crawlers disperse onto the twigs and fruit during June and July. These settle, develop a waxy scale cover, and feed by sucking sap from the tree. This stage is called teh “white cap.” These mature, and produce a second generation of crawlers, often in much higher numbers than the first generation, during August through October. These scale often move onto the fruit. One scale can cause a fruit to be discarded during the packing process, and presence of scale is often considered a quarantine violation in international trade.
San Jose Scale is controlled best during the delayed dormant period of tree development. Summertime “control” sprays are an effort to reduce scale infestation of fruit, and should never be considered an effective way to manage this pest. Sprays during the post-bloom period are most effective if applied during the time that crawlers are moving about on the tree, prior to their formation of the waxy shell.
Horticultural mineral oils are the most important control materials. These products are usually applied in the early Spring, shortly after the tree has commenced growth, but prior to blooming, after the overwintering scale have broken their dormancy. The infested trees are usually sprayed with a 1.5 to 2 percent suspension of oil in water carrier per acre adequate to fully cover the tree “to drip,” even behind bark scales, crevices, pruning stubs, and other hard to contact areas of the tree. The volume of water/pesticide mix varies from 60 to 400 gallons per acre, depending on tree size. Usual spray volume per acre is about 200 gallons. In almost all instances, the oil is mixed with an insecticide to improve the percentage control. This mixture is more effective than either component alone. This oil-insecticide mixture is also a very important part of the integrated mite management process.
See Crop Protection Guide for Control product details.
Organic growers often spray with a fish oil emulsion in the spring, instead of horticultural mineral oil.
Cultural Control Practices
Planting orchards on size controlling rootstocks and pruning to keep trees in an open habit to improve spray coverage are the most effective cultural control practices. Small, young, well-sprayed trees rarely have San Jose scale problems.
There are some natural predators and parasites that attack, but do not control this pest. Biological control is only supplemental to chemical control
The chart below is a projection, based on predicted and average temperatures, of the San Jose Scale Crawler activity in infested lower elevation orchards in North Central Washington. It will be periodically updated, as temperatures affect the rate of development of Scale.