HortFACT - Oystershell Scale Life Cycle
Identification
The oystershell scale spends most of its life protected by a grey waxy covering. The cover of the adult female is grey, subcircular, slightly convex and about 1.3 mm in diameter.
The insect underneath is round, yellow and flattened and about 0.9 mm in diameter. It remains attached to the host plant by means of its stylets (mouthparts). The female is immobile and has no legs, wings, antennae or eyes. The adult male, however, has all these structures with two large white wings and a small yellow body, but no mouthparts.
The female lays yellow ovoid eggs, approximately 0.2-0.3 mm long. These hatch into yellow, flattened, oval crawlers (first instars) which can leave the protection of the scale cover to walk or be carried by wind to new feeding sites on wood or fruit. They insert their stylets into the plant to feed. As they form their first covering ("white cap") they lose their legs, antennae and eyes.
The scale cover of the second larval instar is larger and darker than that of the first but as the instar progresses the scale cover of the male become more elongate. The male passes through prepupal and pupal instars with developing wings, legs, antennae and eyes before it emerges as an adult. There is no pupal stage in the female life cycle.
Although microscopic examination is to be preferred for the separation of San Jose scale (Quadraspidiotus perniciosus (Comstock)) and oystershell scale there are two characteristics that can be useful.
1. San Jose females produce live young, while oystershell females lay eggs
2. Males of San Jose scale can appear at any time of the year (except winter) on wood or fruit but males of oystershell scale only appear in October and never develop on fruit on the tree.
Host Plants
The host range of oystershell scale includes apple, pear, plum (both European and Japanese), peach, nectarine, cherry, willow, silver birch, matagouri and broom.
Recent work has shown that some plant species and cultivars within a group are more susceptible to scale than others and can therefore act as a source of infestation, especially if planted as a shelter belt near orchards. For example, the lombardy poplar Populus nigra italica is resistant to oystershell scale but some of its hybrids with Populus deltoides are susceptible, for example the hybrid P.deltoides x nigra 'Tasman'. There is also considerable variation in resistance among the willow species with some of the new selections of Salix matsudana x alba being susceptible while others such as 'Tangoio' and 'Moutere' being resistant. The crack willow Salix fragilis is susceptible.
Damage
Oystershell scale is regarded as a pest more because of its resemblance to San Jose scale than from concern about the damage it causes. Oystershell scale does not have the same debilitating effect as San Jose scale on tree growth but it can affect the appearance of the fruit. Scales can settle anywhere on the fruit surface but are most commonly found around the calyx. Apple varieties such as 'Golden Delicious' often exhibit red marks on the skin where the scales have been feeding.
Distribution
Oystershell scale is restricted in its distribution to the Southern half of the South Island and is a problem only in the fruitgrowing areas of Centerbury and Otago.
Life Cycle
| Egg |  |
| Crawler | |
| 1st instar larva | |
| 2nd instar larva | |
| Adult female | |
| Adult male | |
| Month | |
| Note: Coloured bars indicate periods of peak activity in each of the life cycle stages | |
In contrast with San Jose scale which has 3-3.5 generations per year in New Zealand, oystershell scale has only one. Females commence egglaying at the end of November and produce most of their eggs from December to February. Crawlers appear from December to April. These settle to form white caps. Although most of these first instars are present from February to April a few can still be found in June. Second instar larvae start to appear in early February and remain in this stage throughout the winter. The males pass through the prepupal and pupal instars relatively quickly starting in late August and emerging as adults over about 35 days during October. They live only a few days. The females moult into the adult stage during September and outlive the males by six to eight months.
Female crawlers in particular often settle under the scale of the mother, resulting in dense populations under an accumulation of scale covers representing many past generations. Male crawlers often settle in more isolated sites.
Common Name
In New Zealand Q. ostreaeformis is called oystershell scale (as in UK and Europe) but in USA and Canada it is referred to as the European fruit scale. In North America, "oystershell scale" refer to Lepidosaphes ulmi (Linnaeus) (apple mussel scale in New Zealand).
