San Jose Scale

San Jose scale, a major pest of fruit trees, is inconspicuous and usually not noticed until it builds up to large numbers on limbs. Limbs supporting large populations often ooze gum and exhibit rough bark and dieback. Dead leaves adhering to fruit spurs during dormant season indicate the presence of scale. Partially mature nymphs overwinter on limbs and trunks. In spring the nymphs develop into winged male and sessile female scale insects. Female scales have gray circular scale coverings. If the covering is removed, the lemon yellow body beneath can be seen. In May females lay eggs that hatch immediately and the young emerge from under the edge of the adult scale cover. The young crawlers settle on shoots where they feed and become adults or overwinter as partially grown scales. In California there are five overlapping generations each year. Crawlers first appear in late April and May, followed by continuous overlapping emergence from late June through December.

San Jose scale can infest branches, shoots, leaves, and fruit. Adults and nymphs suck plant juices and cause considerable damage. They have been known to seriously weaken branches and main scaffold limbs, thus causing permanent injury to mature trees. Crawlers settling on fruit may cause fruit spotting.

San Jose scale has many natural enemies that can frequently keep the pest under control if not disrupted by application 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 when low-to-moderate populations can be managed with oil sprays, which don't destroy the scale parasites. The scale is monitored as part of the shoot 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 orbus, and another small beetle, Cybocephalus californicus.

Parasites include a number of small chalcid and aphelinid wasps, including Aphytis and Encarsia (Prospaltella) sp. These predators and parasites are helpful in reducing scale populations, but broad-spectrum insecticides used during the growing season for other pests disrupt this natural control, and scale populations can build as a result. Low winter mortality due to mild temperatures will also permit a buildup of scale populations.

Oil sprays and biological control by native scale parasites are acceptable in organically managed orchards.

Dormant season

Monitor San Jose scale during the dormant season by collecting 100 spurs and examining them for live scale as well as for tiny emergence holes, which indicate parasite activity. For details on sampling, see DORMANT SPUR SAMPLING and the monitoring form (PDF).

There is a correlation between infested spurs and infested plums, however, plums harvested in mid-June don't generally need a treatment. Expect more damage in late-harvested plums and treat them with oil at greenbud or popcorn if 3 spurs out of 100 are infested. If over 10% are infested, add an insect growth regulator (pyriproxyfen) to the oil sprays. Follow the guidelines below:

Dormant Treatment Decision Table (% infested spurs)

Oil alone can be effective in controlling low-to-moderate populations (apply before January 21). If populations are high, include an insect growth regulator (pyriproxyfen-Esteem, Seize) with the oil. Organophosphates are available, but are associated with environmental problems and should be avoided. When the dormant organophosphate and oil spray is first omitted, San Jose scale populations may increase the first year, but by the second and third year, parasite populations have increased to levels where they reduce San Jose scale populations and maintain them at low levels. If you notice parasitized scale in your dormant sample, be sure to only use an insect growth regulator during the growing season.

Growing season

Monitoring with pheromone traps during the growing season will help you keep track of the appearance and development of scale populations as well as the level of parasitism (Aphytis and Encarsia) but does not tell you if treatment is needed. It mainly tells you how to time treatment for best control in spring using degree-days and pheromone traps to predict the crawler stage or sticky traps to trap the crawlers. Need for treatment is better assessed during the dormant season. Delayed dormant sprays are the preferred timing for treatment.

If inadequate control is achieved with the dormant spray, or the dormant spray is not applied, treatment is also effective when applied soon after the emergence of crawlers in May. Monitor scales by putting up pheromone traps around February 25 (see PHEROMONE TRAPS) and placing sticky tape in the trees in April. Record results on a monitoring form Place pheromone traps well within the canopy to keep them out of the wind. San Jose scale traps will attract both male San Jose scales and scale parasites (Aphytis melinus and Enarsia perniciosi). Adult male scales can be distinguished from the parasites by the presence of a dark line across their thorax where the wings attach.

When the traps begin to catch males consistently, start accumulating degree-days using a 51°F lower threshold and a 90°F upper threshold. If it is needed, apply a treatment for crawlers 600 to 700 DD after you catch the first males. Be aware that the traps may fail to catch any adults if weather is cold, rainy, or windy. Total generation time for San Jose scale is 1050 DD.

Calculate degree-days for San Jose scale in plum 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.

If May sprays are required, use a high-volume (dilute) application at 400 gallons or more per acre for best coverage; do not use a low volume application.

Take a fruit damage sample at harvest to assess the effectiveness of the current year's IPM program and to determine needs of next year's program. See FRUIT EVALUATION AT HARVEST. Record results on a monitoring form (PDF).

• Degree-day calculator
• Degree-day table
• Dormant spur sampling form for San Jose scale, European fruit lecanium, aphid eggs, and mites (PDF)
• Final fruit damage evaluation form for codling moth, caterpillars, scale, and ripe fruit rot (PDF)
• Photos of San Jose scale and its natural enemies
• Using degree-days to time insecticide applications