San Jose scale - Description - Adults - Crawlers - Settled immatures - Damage

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 minute barnacle, which grows with the insect's development. By turning over the scale, the insect itself can be examined. Apart from the winged adult male and the tiny six-legged crawler, all stages are immobile.

Other related scale insects may be found on the same hosts with San Jose scale. Oystershell 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. lataniae (Signoret), and oleander scale, Aspidiotus nerii Bouché have convex fawn scales covering the insect. All these scale insects can be seen and compared in the insect key and the damage key.

Adult The adult male and female scales look very different. The adult female grey scale is about the size of a pinhead (c. 1.5 mm long). The immobile body of the female underneath is yellow, circular, and flattened. It is attached to the plant by its long flexible needle-like stylets (mouthparts), which can sometimes be seen when the body is lifted. The fragile-looking male, about 1.1 mm long, is aphid-like, being winged as an adult after emergence from the scale covering. It is yellow-brown with a characteristic dark-brown line across its back, and has prominent long antennae. The female does not lay eggs, but produces several hundred live young called 'crawlers'.

Crawler The crawlers are about 0.3 mm long, circular-oval, and yellow. Male and female crawlers look identical and can only be distinguished under a high power microscope. They have six legs and two antennae, and are often found alongside the adult female under her scale before they disperse. When they leave the protection of the female scale, 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. Male crawlers settle in greater proportion on the leaves than do the female crawlers. After finding a suitable feeding site, the crawler inserts its stylets into the plant and immediately begins to produce its covering.

Immature This first settled stage is called a 'white-cap' after the colour of its cover, which soon turns black; it moults to the second instar and the scale cover becomes grey. This immobile second instar (without legs or antennae) appears as a smaller version of the adult female. However, during development and growth, the males and their covering become increasingly elongate in comparison with the females. Unlike the males, the females have also lost their eyes. Adult females are produced when the second instars moult. Second-instar males pass through pre-pupal and pupal stages before emergence as winged adults.

Damage Scale insects use their long stylets to suck the phloem sap of the plant on branches, twigs, fruit and foliage. San Jose scale inhabits mainly the bark of its host tree, where encrustations of dense scale populations are often formed. The feeding of these scales, whose density often exceeds 100 per cm², has a gradual debilitating effect on the branch. Scale insect feeding interferes with the growth of the cambium and leads to abnormal phloem and xylem cells, and desiccation. Salivary secretions cause necrosis of the tissues. 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. There are several generations of San Jose scale per year. Some crawlers of the first generation may move on to the fruit early in the season and cause pitting, but most infestation results from crawlers of the second generation. Late-fruiting pipfruit varieties may be infested by third generation scales in the warmer parts of the country. Unlike many sap-sucking insects, armoured scale insects such as San Jose scale do not produce honeydew. San Jose scale is a quarantine pest in some markets which will not accept infested fruits.