Oystershell scale - Description
	Adults - Eggs - Crawlers - Settled immatures - Damage

The name 'scale' describes the characteristic protective wax covering of this group of insects. Most life stages of oystershell scale have a sub-circular, moderately convex, grey or grey-brown scale which grows with the insect's development. By turning over the scale, the soft-bodied 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 oystershell scale. San Jose scale, Quadraspidiotus perniciosus (Curtis), has similar circular grey scales and the female is often a deeper yellow than oystershell scale. 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. The females of greedy scale, Hemiberlesia rapax (Comstock), latania scale, H. lataniae (Signoret), and oleander scale, Aspidiotus nerii BouchÈ have convex fawn scales covering the insect. All these scale insects can be seen and compared in the insect key and the damage key.

Oystershell scale is known overseas as the European fruit scale. Moreover, the mussel scale in New Zealand is called the oystershell scale in other parts of the world!

Adult The adult male and female scales look very different. The adult female grey-brown scale is about the size of a pinhead (1.3 -1.8 mm long). The immobile body of the female underneath is pale 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 0.5 -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 lays eggs beneath its scale covering.

Egg Oystershell scale produces eggs (oviparous) and this distinguishes it from its close relative, San Jose scale, which produces live young (viviparous). The eggs are yellow and oval shaped, and found beneath the female scale cover. Although laid singly, they are often present in groups of up to 12 while they develop. They soon hatch into the mobile 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 (with the eggs) 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, 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 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 wax cover, which soon turns darker; it moults to the second instar and the scale cover becomes grey-brown. 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, and fruit. Oystershell 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 square cm, 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 stunt growth and affect crop yield. Oystershell scales occur on most parts of the host tree, including the fruits (but not the leaves), thanks to dispersal by the mobile crawlers. On fruits, such as apple, the settled scale insects cause red blotches and early settlement and feeding may cause pitting of the fruit surface. Late-fruiting pipfruit varieties are more likely to be infested with oystershell scale because crawler dispersal continues from January through to March. With only one generation per year, adult oystershell scale insects never occur on the fruit (unlike San Jose scale), where only first and second instar scales can be found. Unlike many sap-sucking insects, armoured scale insects such as oystershell scale do not produce honeydew. Although oystershell scale on the fruit is not a quarantine problem in itself, its close similarity to San Jose scale means that it is equally unacceptable on exports from the southern regions of New Zealand where it occurs.