San Jose Scale

Female San Jose scales give birth to living young that 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. Later the covers turn various shades from gray to black.

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. 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. Males molt a total of four times. Following the final molt, adult male scales emerge from the scale covering as tiny, yellow winged insects. They mate with the females who remain under the scale covering. After about 2 months, 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.

If heavy scale infestations are left unchecked, trees may be seriously damaged, resulting in reduced vigor, thin foliage, cracked or dying branches, and the eventual death of the tree. Young trees may be killed before fruiting. Infested fruit develop a reddish purple ring surrounding each spot where a scale settles.

San Jose scale is the most common and the most damaging of the scales found in apple orchards. Dormant season treatments are the key to controlling this pest. The only other effective treatment time is in May. Natural enemies can contribute significantly to control when not disrupted by insecticides.

Natural enemies that feed on San Jose scale include two predaceous beetles: the twicestabbed lady beetle, Chilocorus orbus, 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 can disrupt this natural control and scale populations increase rapidly.

Biological control and approved oil sprays are organically acceptable methods, but San Jose scale is rarely a pest in organic orchards.

Monitor for San Jose scale during the dormant period by checking prunings to make sure scale hasn't developed in tree tops. Also check fruit at harvest for the presence of scale.

Due to the damage potential of this pest, annual dormant sprays are recommended in most areas. Oil sprays work the best on the black cap stage, so apply them in early January. Control heavy populations of San Jose scale by applying an insecticide plus 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 to monitor for male San Jose scale flights and double-sided sticky tape for monitoring crawlers in April and May. Time a treatment, using a 51°F lower threshold and 90°F upper threshold, for 600 to 700 DD after the beginning of the male flight or 200 degree-days after crawler emergence begins.

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

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