Mussel 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 mussel scale have an elongate, convex, grey-black to brown scale which grows with the insect's development. It's shape resembles that of a saltwater mussel. By turning over the scale, the soft-bodied insect itself can be examined. Apart from the rare winged adult male and the tiny six-legged crawlers, all stages are immobile.

Other related scale insects may be found on the same host plants with mussel scale. San Jose scale, Quadraspidiotus perniciosus (Curtis), has circular grey scales and the female is a deep yellow colour. Oystershell scale, Quadraspidiotus ostreaeformis, is sub-circular and similar to San Jose scale, though the females are often a paler yellow. Scales of the genus Parlatoria are 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 Bouché have convex fawn/brown scales covering the insect. All these scale insects can be seen and compared in the insect key and the damage key.

Mussel scale is known overseas as both mussel scale and oystershell scale, whereas in New Zealand we reserve the name oystershell scale for Quadraspidiotus ostreaeformis.

Adult The adult male and female scales look very different. The adult female's greyish to chocolate brown scale is 2-3.5 mm long by 1 mm wide, and straight, or more commonly, curved. Asymmetry of the scale cover is due to off-centre addition of waxy material and the exoskeleton (exuvia) after each moult. The immobile body of the female underneath (1.25-1.5 mm long) is creamy-white to reddish-yellow, ovoid, and flattened. It is attached to the plant by its long flexible needle-like stylets (mouthparts), which can sometimes be seen (under a microscope) when the body is lifted. The fragile-looking male, about 1 mm long, is aphid-like, being winged as an adult after emergence from the scale covering. It is very light bluish-pink with black eyes and a characteristic light yellow-brown bar across its back. It has prominent long antennae, white wings, and pale yellow legs. Males are rare or unknown in New Zealand populations of mussel scale. No authentic records have been located. Their incidence overseas varies with the host plant. The adult female lays eggs beneath its scale covering.

Egg Mussel scale produces eggs (oviparous) in summer and it is the eggs which overwinter under the female scale covering. The female's body becomes completely shrivelled as the eggs are produced and she dies soon afterwards. The eggs are white, elongate-oval shaped, and about 0.2-0.3 mm long. They are often so tightly packed like sardines inside the mother scale that they lose their ovoid shape. The colour of the eggs changes to a creamy yellowish-brown a few days before hatching. They hatch into the mobile crawlers in spring.

Crawler The crawlers (or first instar nymphs) are about 0.3 mm long, elliptical, and creamy-white to yellow. They have six legs and two antennae, and may be 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, leaves and fruits 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 scale covering.

Immature The first settled stages secrete a series of long, paired, hair-like waxy filaments which make them look woolly. They soon moult to the immobile second instar (without legs or antennae) and the old external skeleton forms the first section of the scale covering. This creamy-white second instar (c. 0.8 mm long) secretes further waxy covering, elongates, and appears as a smaller version of the adult female. Adult females are produced when the second instars moult. Growth still continues as an adult. 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, leaves and fruit. Many of those that settle on the leaves do not survive. Mussel scale inhabits first the young twigs and terminal shoots of apple trees and then spreads at higher densities to the larger branches, where encrustations of dense scale populations may be formed. The feeding of these scales 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. Mussel scales occur on most parts of the host tree, including the fruits, thanks to dispersal by the mobile crawlers. They can often be found on twigs at the base of the leaves and amongst the leafscars in winter. On fruits, such as apple, the settled scale insects may be present in groups and early settlement and feeding may cause pitting of the fruit surface. Mussel scales do not normally cause red blotches on the fruit which is characteristic of feeding by San Jose and oystershell scale insects. Although there is only one generation per year, adult mussel scale insects can occur on the fruit (unlike oystershell scale). Crawlers which settle on the fruit early in the season continue to develop through to adult during the summer, although this would appear to be suicidal for the overwintering eggs which the female produces. Unlike many sap-sucking insects, armoured scale insects such as mussel scale do not produce honeydew. Mussel scale has worldwide distribution and does not have quarantine significance. It is sufficiently different in appearance that it need not be confused with the major quarantine scale pest, San Jose scale.