HortFACT
Identification
The apple mussel scale insect passes most of its life protected beneath an elongate, waxy covering which resembles the shell of the salt-water mussel. The adult female is yellow, ovoid, and flattened; its scale covering is about 3 mm long by 1 mm wide and straight or, more commonly, curved. Asymmetry of the greyish chocolate-brown scale is due to off-centre addition of waxy material after each moult. The rare adult male eventually emerges from its scale as a minute, fragile, pinkies-cream insect with white wings.
Up to 80 shiny white eggs are produced by each female during late summer. The female's body becomes completely shrivelled during their formation, and she dies shortly after laying them. They are about 0.3 mm long, elongate-ovoid, and occupy virtually all the space beneath the dead female's scale.
Crawlers, or first-instar nymphs, which hatch in spring, are elliptical, about 0.4 mm long, and yellow. When they move out from under the old scales and disperse to colonise suitable areas of wood and fruit they may be wind-blown to new host plants. They settle by inserting their fine stylets (mouthparts) into the plant tissues. They then secrete a series of long, paired, hair-like waxy filaments, which make them appear rather like woolly aphis. They soon moult into the sedentary second instar. The old external skeleton forms the first section of the scale covering. The immobile second-instar female is similar in appearance to a small adult. It secretes the first off-centre section of the scale, thus giving it an elongate appearance. A further increase in the size of the female's scale covering accompanies the third moult to the adult stage, whereas males pupate after only two nymphal instars.
Host plants
Apple trees are commonly infested, and pears, hawthorn, plum, willow, poplar, gooseberry, and currant will support populations. Some deciduous shelter trees can also support this insect. The relative abundance of male apple mussel scale insects varies considerably between different hostplant species.
Damage
Apple mussel scale sucks the sap from its host plants by means of its very long stylets. The insect may form dense encrustations on fruit or on the bark. Feeding on the bark debilitates the tree, and portions may gradually decline and die. Feeding on fruit leads to its disfigurement, though generally without staining the flesh or skin.
Distribution
Apple mussel scale is widely established in temperate regions. It is distributed throughout lowland New Zealand where pome (pip) fruits are grown, and is especially prevalent in home gardens and orchards not receiving frequent insecticide sprays.
Life cycle
| Egg |  |
| Crawlers | |
| Instars | |
| Adult | |
| Month | |
| Note: Coloured bars indicate periods of peak activity in each of the life cycle stages | |
In the South Island one generation is completed annually; a partial second generation can occur in warmer North Island localities. Stages of the life cycle are well synchronised with the seasonal growth of the host, and the rate of development depends on local climatic conditions. Very little overlap occurs between the life stages.
The eggs overwinter under the old scale covering, which affords them considerable protection. In Canterbury most hatch during a 14-day period between late September and early November. The large swarms of small, free-living crawlers disperse widely. Many crawlers settle and develop under old empty scales, which can remain attached to the wood for several years. At each moult of the insect more material is added to the scale shell. Populations may be bisexual, when small males are present, or pathenogenetic, when the females produce female eggs without fertilisation by a male. On apple trees the insect is predominantly pathenogenetic. Growth of the insects during summer ensures that a generation reaches maturity between January and March. Egg development takes place during January to October.
Common name
Lepidosaphes ulmi is commonly referred to as 'mussel scale' in Britain and Europe. In New Zealand 'apple mussel scale' has been adopted as the standard common name. However, in the United States this species is referred as 'oystershell scale', a name which in New Zealand and Europe is used for a different scale insect pest, Quadraspidiotus ostreaeformis (Curtis).
W. P. THOMAS