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. If trees have received repeated aerial dormant applications, they can build up on the undersurface of branches. Limbs with large scale 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, 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 produce live young. The young crawlers settle on shoots, feed and become adults or overwinter as partially grown scales. In California there are four 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 infest branches, shoots, leaves, and fruit. Adults and nymphs suck plant juices and cause considerable damage. They seriously weaken branches and main scaffold limbs, causing permanent injury to mature trees. Crawlers settling on fruit may cause fruit spotting.

A number of natural enemies help keep San Jose scale populations suppressed. Dormant sprays are recommended to manage scale populations, followed by regular monitoring to see if populations are increasing and to assess the presence of biological control. Broad-spectrum pesticides applied during the summer often destroy natural enemy populations, resulting in increased scale infestations; avoid such treatments where possible.

Many parasites and predators feed on San Jose scale. These include Aphytis spp., Encarsia perniciosi, andlady beetles (Chilocorus orbus and Cybocephalus californicus). However, in situations where heavy scale populations exist, biological control will not prevent severe damage, so annual sprays are usually needed. Use San Jose scale pheromone traps to help monitor scale parasites during the growing season.

Oil sprays and biological control by native scale parasites are organically acceptable management tools.

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 presence-absence monitoring, see DORMANT SPUR SAMPLING and the monitoring form (PDF).

Growing season

Monitoring from late bloom until petal fall 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 and placing sticky tape in the trees in April. For more information see PHEROMONE TRAPS and record results on a monitoring form (PDF). 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 degree-days 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 prune 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.

Fruit damage sample

In mid-July, take a fruit damage sample to assess the overall effectiveness of the current year's IPM program and to determine next year's needs. For details, see FRUIT EVALUATION AT HARVEST. Record results on a monitoring form (PDF).

• Degree-day calculator
• Degree-day table
• Final fruit damage evaluation form for caterpillars, scale, and aphids (PDF)
• Dormant spur sampling form for San Jose scale, European fruit lecanium, aphids, and mites (PDF)
• Sampling form for insects monitored using pheromone traps and degree-days (PDF)
• Using degree-days to time insecticide applications