In this section:
San Jose Scale
Comstockaspis perniciosus
Affected
Stone and pome fruit
Description
Colonies of this scale appear as dense, greyish encrustations on the branches and twigs of trees.
The insect itself is concealed beneath a tough, shell-like, waxy scale formed by secretions from its body. The scale covering is grey, with a pale nipple-like prominence in the centre. The mature female scale is about 1-2 mm in diameter; the male scale develops beneath a smaller oval shaped scale cover. If the scale covering is lifted, the yellow flattened pear shaped body of the insect can be seen.
Where and when to look
The best place to look for scale is in sheltered spots such as the forks of branches, tiny cracks and crevices in the bark, and between the bud and stem on young branches. Even heavy infestations can be inconspicuous, because the scales colour is so well matched with that of the older tree bark.
Adult San Jose scale are not mobile, once the nymphs have inserted their long tubular mouthparts into the tree tissue they remain in that spot for life. The nymphs are only capable of crawling a few metres from their mothers scale which they emerge from.
Damage
San Jose scale impede the flow of sap in the tree and they also secrete a poisonous substance into the plant cells. Tree symptoms are abnormal yellowing of the leaves and unseasonal leaf drop, gumming and cracking of the bark, followed by dieback of the infected branches. Branches in the tops of trees are usually the first to be injured. Unchecked, vigorous infestations on mature trees permanently affects their structure, vigour and productivity. On young wood a bright red discolouration often shows up, especially on peaches. An undetected infestation can kill young trees within three years.
Control
When infestation is severe and dieback is obvious the affected limbs should be pruned out. Pruning should be sprayed immediately and then burnt.
Winter oil should be applied to stone and pome fruit trees during dormancy. During winter the scale population is at its lowest and consists mainly of young scales which are the least difficult to kill. Thorough coverage of every tree is essential to achieve good control. Hand-jetting is preferable to drive past spray units which do not give sufficient coverage to kill scale sheltering in bark crevices, forks of limbs and the lower trunk. Do not apply winter oil to trees close to budswell as it suffocate the trees.
A single winter oil spray is usually sufficient for light infestations. Well established infestations may require a follow up spray using summer oil and applying as late as possible before budburst. Do not apply more than one winter oil spray per season.
A number of chemicals can be used for the control of San Jose scale.
