Our fruits and ornamental plants are always at a higher risk of being attacked by pests. One such pest attacking our fruits and commercially important shrubs is the San Jose scale. It belongs to order Hemiptera and family Diaspididae. Its scientific name is Quadraspidiotus perniciosus. It is native to China but was introduced to San Jose, California from Japan. Currently this pest is found all over the Southern Canada, United States, India, South Africa and New Zealand. It is mainly a major pest of cultivated fruits and a large number of ornamental shrubs and trees like pear, plum, peach and cherry. Citrus fruits are sometimes also attacked by this pest.
It attacks all the fruits and ornamental shrubs of the orchards. Both adults and the nymphs suck the sap from the wood and leaves of trees reducing growth and economic yield. Developing fruits are also attacked by San Jose scale and they turn gray with mottled blemishes and poor in quality. If the infestation by this insect is left unchecked then death of trees may occur later on resulting in complete reduction of fruit crop. Terminals are the first to die. Infested fruits are characterized by presence of a deep reddish purple ring surrounded by spots.
Adult females are aperterous or wingless, 2mm in diameter and covered with waxy covering of the scales secreted by body. Adults are oval and measure about 1 mm in length. Both adult males and females may be identified by presence of a raised nipple at the top of the scale cover. Nymphal scales are light colored but become dark as they transform into adults. Young nymphs without scales are known as crawlers and are yellow in color resembling mites. Nymphs particularly the first instar under diapause in the winter months. After moulting twice in the months of March and May they emerge out as adult males and females. Females are viviparous and are known to produce 8-10 nymphs per day from late May onwards. Egg laying lasts for 6 weeks. The total number of nymphs produced by a single female on suitable host plants may be up to 400.
Nymphs are tiny, yellow colored crawlers which soon after their birth wander here and there in search for the suitable host plant. Once they find a suitable host plant they settle on it. After settling nymphs insert their piercing and sucking type of mouth parts in the hosts plants and start sucking the sap. They secret a a white waxy material. This stage is called as white cap stage. Four generations have been observed in a year. Summer generations usually overlap and the crawlers are observed through the summers and the fall. Growth is completed in 30-40 days and 2-3 overlapping generations are found in each season. The insects are very prolific and if all the nymphs of a single female survives then there will be 30,000,000 nymphs in a single season. Crawlers are transferred from one place to the other through bird's feet of the agency of other insects. Man may also play an important role in dispersal of San Jose scale.
Many species of parasitic wasps and ladybird beetles act as predators of San Jose scale helping to keep a check over their population. It is a polypahgous insect and is also known to attack about 150 species of apple. Use of pesticides can be beneficial for killing of both adults and nymphs. Dormant oil sprays should be recommended.
It is true prevention is always better than cure.