HortFACT
Identification
The name 'scale' describes the characteristic protective wax covering of this group of insects. Most life stages of San Jose scale have a circular, slightly convex, grey scale resembling a barnacle, which grows with the insect's development. By turning over the scale, the insect itself can be examined. Apart from the adult male and the crawler, all stages are immobile.
Adult San Jose scales are sexually dimorphic (the male and female differ considerably). The body of the female is yellow, circular, and flattened, and its scale is about 0.9 mm long. The aphid-like male, about 1.1 mm long, is also yellow but has prominent antennae and is winged as an adult, after emergence from the scale covering. The female does not lay eggs, but produces up to 400 live young called 'crawlers'.
Crawlers are about 0.3 mm long, circular, and yellow, and can be found alongside the female under her scale before they disperse. They walk over the wood, stems, leaves, and fruit of the host plant to colonise new areas, and are also carried by the wind to new hosts . After finding a suitable feeding site the crawler inserts its stylets (piercing mouthparts) into the plant and begins to produce its covering. This first settled stage is called a 'white-cap' after the colour of its cover; it soon moults to the second instar (stage).
The immobile second instar appears as a smaller version of the adult female. However, during development the males and their covering become increasingly elongate in comparison with the females. Adult females are produced when the second instars moult. Second-instar males pass through pre-pupal and pupal stages before emergence as winged adults.
Other related scale insects may be found on the same hosts with San Jose scale. Oyster-shell scale, Quadraspidiotus ostreaeformis (Curtis), has almost circular scales and the female is pale yellow. Mussel scale, Lepidosaphes ulmi (Linnaeus), has very elongate scales and females are up to 3 mm long. Scales of the genus Parlatoria are also elongate, but the female's body is mauve. Female red scales, Aonidiella aurantii (Maskell), have a flat red-brown scale. The females of greedy scale, Hemiberlesia rapax (Comstock), latania scale, H. Iataniae, and oleander scale, Aspidiotus nevii have convex fawn scales.
Host plants
There is a wide range of host plants, mainly deciduous trees. San Jose scale is a serious pest of apples, pears, and stone fruits, and also occurs on grapes, kiwifruit, and walnuts. Willows, birches, elms, mountain ash, and other common ornamental trees serve as reservoir hosts for this pest outside orchards.
Damage
San Jose scale inhabits mainly the bark of its host tree, where encrustations of scales are often formed. The feeding of these scales, whose density often exceeds 100 per cm2, has a gradual debilitating effect on the branch. Very large populations may cause the death of the limb, leader, or entire tree; this usually takes several years. Scales occur on all parts of the host tree, even the leaves, owing to colonisation by the mobile crawlers. On fruits, such as apple, the settled scales cause red blotches and pitting. Some crawlers of the first generation (see life cycle chart) move on to the fruit in November-December, but most damage results from crawlers of the second generation (February-March). Late-fruiting apple varieties may be damaged by third generation scales.
Distribution
Except for areas of high altitude and latitude, San Jose scale occurs in all major deciduous fruit-growing areas of the world. In New Zealand it is distributed throughout the North Island, and is present in Nelson and Marlborough in the South Island. Further south, oyster-shell scale is dominant although San Jose scale occurs in restricted localities.
Life cycle
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| Crawlers | |
| Instars | |
| Adult | |
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| Note: Coloured bars indicate periods of peak activity in each of the life cycle stages | |
There are three to three-and-a-half generations each year. Except for adult males, which have not been found in winter, all stages are present throughout the year. Overwintering is accomplished mainly by settled first- and second-instar scales . Occasionally a few crawlers are released by some females during the winter. By August almost all scales are second instars, resulting in a well synchronised first generation of adults in September-October and crawler release reaching a maximum in November. In February the crawlers of the second generation are released, and during late summer there is considerable overlap between stages during the third generation.
The rate of development depends on temperature, and is very slow in winter. There is an interval of about 70 days between the peaks of the first two summer generations. In summer the first and second instars are completed in 40-45 days. Females begin producing crawlers about 30 days after becoming adult, and then live for a further 40-60 days.
C. H. WEARING