Aspidiotus perniciosus, San Jose Scale


Synonyms:  Aspidiotus peniciosis (Comst.)

                     Quadraspidiotus perniciosus

Host: This is a widely distributed pest of temperate fruits in Himalaya. It was accidentally introduced into India at around 1910. Now it attacks apple, cherry, plum, peach, apricot, pear, walnut, almond, strawberry, chestnut, grapes, raspberry and many wild temperate plants.

Damage: Both nymphs and adults cause damage by sucking sap from the leaves, branches and fruits. Infested fruits show a puncture mark and a halo-like purple discoloration around it. Heavily infested fruits shed their leaves and gradually die. Lighter infestation causes leaves to turn yellow and cracks to develop on the bark, exuding gum.

Life cycle: Female insect is yellowish orange in colour, oval in shape, 2 mm long, without appendages and with a black pustule. It is covered with a waxy scale that is round, convex and raised a little in the middle. Female body shows segmentation of abdomen and a pointed proboscis for sucking sap. Nymphs in various stages of development are also covered with a waxy scale for protection.

Female is viviparous and has longevity of about two months, during which it can produce thousands of nymphs that are called “crawlers”. Males are free-living, very tiny being only one mm long and having well developed eyes, antennae, appendages and wings but vestigial mouth parts and therefore survive only for a day. They emerge from the scale, mate with the female which remains stationary under the scale, and die.

The crawlers are only 0.2 mm long and crawl to find tender shoots where they thrust their proboscis to lead a stationary life. They secrete a waxy covering over their body and metamorphosis starts. Legs and antennae are folded beneath the body and disappear after the first moult. Scale gradually becomes darker and after 2-3 weeks turns blackish. This stage is known as black-capped stage. Adulthood is achieved after two moults and approximately in 6 weeks. Males regain their eyes and appendages after the second moult but the female continues to be bag-like and attains sexual maturity.

Overwintering takes place in nymphal stage and dormant nymphs again become active in March. Males emerge sometime in April and females start reproducing in mid-May. Total life cycle takes 35-40 days in summer but is prolonged in winter.

Distribution: The pest is widely distributed in the apple-growing areas of the world. The pest has been recorded from Southern Europe, Algeria, South Africa, Zimbabwe, Congo, Iraq, Turkey, India, Pakistan, China, Korea, Japan, Australia, New Zealand, USA, West Indies, Mexico and South America.

Control: In nurseries fumigation with methyl bromide kills all stages. During dormant period diazinon or parathion 0.025% can be sprayed. Bordeaux mixture is an effective age-old practice. Esso Tree Spray is effective as an emulsion in water.

Biological control carried out by the Commonwealth Institute of Biological Control (CIBC) has been successful in containing the pest. Two hymenopterous parasites, Prospaltella perniciosi and Aphytis diaspidis were introduced from California in 1960s and effectively controlled the pest. Another parasite, Aspidiotiphagus sp. is also effective.

Two predators, Chilocorus circumdatus and C. kuwanae, cause a lot of damage to the pest stages.