Diaspidiotus perniciosus

Scientific Name Diaspidiotus perniciosus (Comstock) Cockerell, 1899
Synonyms Quadraspidiotus perniciosus (Comstock) Ferris, 1938. It is still controversial whether this species is a member of the genus Diaspidiotus or of Quadraspidiotus.
Common Names English: San José scale; Spanish: Cochinilla de San José; French: Pou de San-José; Russian: Калифорнийская щитовка; German: San José Schildlaus; Italian: Cocciniglia di San José; Sweden: San José-sköldlus
Description The apterous females have a more or less round, dark gray or light brown scale with a central raised nipple and are about 1.6 - 2 mm (0.06 - 0.08 inch) in diameter. Immobile stages of the males are more oval and smaller. Male adults have one pair of yellow wings and an orange body. D. perniciosus is often found together with other closely related species from the genus Diaspidiotus which can only be reliably distinguished microscopically.

Biology

Damage

D. perniciosus can be found on the surface of all plant parts. While it prefers to settle on wood, leaves and fruits will frequently be attacked, too, especially when the infestation is severe. On leaves, necrotic spots may appear. Fruits will show a very conspicious red stain around a colony of scales, and sometimes become distorted. The red coloring is rather harmless, but deformed fruits loose in market value, and those with adhering scales are rejected by customers.

Much worse, however, is the long-term effect of large populations of San José scales on plant health. Not only will the sap loss caused by the insects, due to their huge numbers, add up to considerable amounts, but they inject toxic substances with their saliva.

Red spots similar to those on fruits will appear on young bark. It may crack and exude gum. If the infestation remains unchecked, first twigs and later on whole branches may die off. Young trees can be killed completely within 2-3 years and even older ones are not completely safe but may eventually be destroyed.

Lifecycle

D. perniciosus is ovoviviparous, i.e. the nymphs hatch while they are still under the scale of their mother, so it seems as if it had live young. Within its lifetime of 6-8 weeks each female produces 3-10 „crawlers“ per day. They move around rather lively for some hours until they find a suitable place to settle. During this phase they are easily transported by wind, birds or flying insects. At last they insert their stylets into the plant tissue, start ingesting plant sap and from that moment on remain stationary.

They begin secreting wax and forming their scale, which at first is white („white cap stage“) and darkens later. When they molt, their exuviae are integrated into the growing covering. After two nymphal stages, the female is sexually mature. It permanently remains under the scale. The male passes through two further instars, called prepupa and pupa, before it emerges. It has not even mouthparts and dies soon after it has - led by pheromones - found a female and mated. Depending on temperature the whole life cycle is completed in 1-2.5 months.

There are up to five generations per season that overlap considerably, so all stages are found side by side. In cold climates, the first instar larvae can survive even hard winters in diapause and continue their development in spring. The other stages are mostly killed by frost.

Occurrence

Additional Crop Information

D. perniciosus is very polyphageous and a pest on almost all important deciduous fruit trees (among many others apple, almond, apricot, blueberries, currants, cherry, mulberry, nectarine, peach, pear, plum, quince) as well as numerous woody ornamentals. It prefers Rosaceae.

Agricultural Importance

D. perniciosus probably originates from the Far East. It has been spread with nursery stock; first to the USA, where it appeared in California in 1873, and then onwards throughout the world. It is found in the Palearctic and Nearctic regions wherever its host plants are grown. Considered one of the most serious pests of tree crops worldwide as it may kill off large numbers of trees within a comparatively short time.

Control

Integrated Crop Management

Natural enemies play an important role in control of this pest. Avoid indiscriminate use of broad-spectrum insecticides. Monitor carefully by examining prunings for overwintering scales and using pheromone traps to catch males in spring. Use only nursery material that is definitely free of scales, as young trees are particularly susceptible.

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