San Jose Scale

Female San Jose scales lay eggs that hatch immediately and the young emerge from under the edge of the scale covering. These tiny yellow crawlers wander in a random fashion until they find a suitable place to settle. Immediately upon settling, the crawlers insert their mouthparts into the host plant and begin feeding and secreting a white waxy material (white cap stage); eventually the waxy covering turns black and is known as the black cap stage.

San Jose scales overwinter predominantly in the black cap stage, although in mild years some adult mated females may also survive. In late January, these nymphs resume their growth, molting two (females) or four (males) times before becoming adults in March. Immature male and female scales are indistinguishable until the first molt. At this time, the male scale covering begins to elongate, while the female's remain circular. Yellow-winged adult males emerge to mate with the females. The adult female remains under the scale covering, which is gray and circular; the body under the shell covering is yellow. After mating, females produce eggs, which are hidden under the covers. Crawlers begin to emerge from the females, usually in April; peak emergence is generally in early May. There are usually four generations a year. Summer generations overlap and crawlers are present throughout summer and fall.

San Jose scales cause injury by feeding on twigs, branches, and fruit. They may also inject a salivary toxin while feeding. Infested fruit and wood develop a reddish purple ring (halo) surrounding each spot where a scale settles. Fruit infested by San Jose scale is often bumpy; in extreme cases, pears may be severely misshapened and stunted. Presence of either the insect or red ring on fruit causes it to be culled from fresh-market shipments. Pears may also be rejected for cannery use because insect feeding often causes sunken areas that cannot be removed by peeling. Heavy population can cause gumming and kill twigs, branches, and entire tree if left uncontrolled. Young trees may be killed before fruiting.

San Jose scale has many natural enemies that can frequently keep the pest under control if not disrupted by applications of broad-spectrum insecticides. Many orchards that have not used broad-spectrum sprays for 2 or 3 years do not have San Jose scale problems. The best time to spray is during the dormant season. The scale is monitored as part of the pruned wood sample during the dormant season and with pheromone traps in spring.

Natural enemies that feed on San Jose scale include two predaceous beetles: the twicestabbed lady beetle, Chilocorus ortus, and another small beetle, Cybocephalus californicus. A number of small chalcid and aphelinid wasps parasitize this scale. These predators and parasites may be helpful in reducing scale populations, but insecticides used during the growing season for other pests may disrupt this natural control.

Organically acceptable methods include biological control and approved oil sprays.

Monitor for San Jose scale during the dormant period by checking prunings to make sure scale haven't developed in tree tops (see DORMANT TO DELAYED-DORMANT SAMPLING for details).

Because of the damage potential of this pest, annual sprays of oil during the dormant or delayed-dormant period are recommended in most areas. Control heavy populations of San Jose scale by applying an insecticide with the oil spray during the delayed-dormant period.

If inadequate control is achieved with the dormant spray, treatments are also effective when applied soon after the emergence of the crawlers in May. Use pheromone traps in March (place in orchard in mid-March in Delta-growing regions and late March in north coast orchards) to monitor male San Jose scale flights, or double-sided sticky tape wrapped around tree branches for monitoring crawlers in April and May. This is also a good way to monitor effectiveness of a dormant or delayed-dormant treatment. To time treatment, accumulate degree-days using a lower threshold of 51°F and an upper threshold of 90°F. The optimum time for spring spraying is 600–700 after the beginning of the male flight or 200 DD after crawler emergence begins.

Calculate degree-days for San Jose scale in pears for your location using the San Jose scale pest model. To learn more about using degree-days to time insecticide applications, watch the degree-days video.

At harvest, assess your IPM program by monitoring fruit in the bins for presence of San Jose scale or red halo left on fruit. Sample 200 fruit per bin from 5 bins per orchard (or 20-acre block in large orchards). (See HARVEST FRUIT SAMPLE for more information.)

• Degree-day calculator
• Degree-day table
• Using degree-days to time insecticide applications