Diaspidiotus perniciosus / Quadraspidiotus perniciosus
English names: San José Scale, Californian scale
Nordic names: San José skjoldlus (DK), San José-sköldlus (SE)
Major host plants
Main hosts are apples, peaches, pears, plums and Rubus. The scale can attack many other deciduous trees and shrubs, making over 150 hosts in total.
All surface parts of young host plant tissue are infested. Scales are mostly found on the stem and branches of woody plants. In severe infestations, leaves and fruits may also be penetrated. A characteristic violet-red halo appears in the tissue around the young scales. Haloes increase in size as they mature and may coalesce. The red cortical tissue swells with accumulating sap. Bark often cracks and exudes gum, resulting in a surrounding dark-brown gelatinous area. Heavy infestation causes growth inhibition and loss of yield.
The scale is indigenous in the Far East from where it has spread world wide.
A map can be downloaded from EPPO´s website. See instructions here.
Most individuals of Q. perniciosus overwinter in the first larval stage. In February to March the larva moults, and then the male scales take an elongated shape while the female scales remain round. The female gradually increases in size through two developmental stages until the scale cover reaches about 2 mm in diameter. The male and female mate, and later the female gives birth to larvae that are mobile and able to crawl to infest new host tissue within the first 24 hours. Females can lay from 50-400 larvae over a period of 6 to 8 weeks. One life cycle takes from 45 to 80 days.
The first instar crawlers are the main dispersal phase but are not normally carried more than a few kilometres by the wind. International spread is liable to occur through human transport of planting material of host trees and shrubs, or fruits.
Detection and inspection
Positive identification of Q. perniciosus requires microscopic identification of the female.
Larva: Depending on stage and sex, the larva is a round to elongated, white to black, fixed scale, or tiny mobile yellow organism with three pairs of legs.
Female: The female is circular, grey and about 2 mm in diameter. Removing the scale reveals the insect body which bears characteristic ornamentations on the posterior part.
Male: The adult male only has forewings present, the hind pair being reduced to slender halteres linked to the wing bases by hooklets.
Pest status and importance
Q. perniciosus damages trees by injecting toxic saliva and, in the absence of control, young apple and pear trees, for example, can be killed within 2-3 years. Fruit quality and marketability are greatly reduced by scale attacks. Heavy infestation causes loss of yield.
Source of information
See further information here:
Author: Jorma Rautapää
Editor: Dorthe Vestergaard