San Jose scale
Diaspidiotus perniciosus
55 host plants
Last updated
Tiny circular scales cluster on twigs and branches of stone fruits, with a reddish area often developing at each feeding site and extending internally. Large populations cause tree vigor loss with thinning and yellowing of foliage. This introduced pest has been a potential orchard problem for decades; infestations develop most readily in poorly managed orchards near ornamental or native hosts.
Well-managed orchards maintain populations below damaging levels through consistent monitoring and timely intervention. Inspect twigs in early spring and prune infested branches. Dormant oil spray in early spring coats emerging beetles. Remove dead wood and avoid stress conditions.
Quick Reference
San Jose scale Egg hatch/crawler 1st gen typically begins around 2644 GDD₃₂. As of April 3, 2026, Puget Sound stations range from 1070 to 1180.6 GDD₃₂, approximately 1463 units before the expected threshold.
Regional Season Tracker
GDD₃₂ accumulation across 7 Puget Sound stations · as of Apr 3, 2026| Station | GDD₃₂ | Current Stage | Next | To Go |
|---|---|---|---|---|
| Issaquah / East King | 1,181 | Pre-season | Egg hatch/crawler 1st gen | 1,463 |
| Seattle / UW | 1,171 | Pre-season | Egg hatch/crawler 1st gen | 1,473 |
| Kent / Auburn | 1,111 | Pre-season | Egg hatch/crawler 1st gen | 1,533 |
| Olympia / Tumwater | 1,106 | Pre-season | Egg hatch/crawler 1st gen | 1,538 |
| Bellingham / Whatcom | 1,101 | Pre-season | Egg hatch/crawler 1st gen | 1,543 |
| Tacoma / Puyallup | 1,075 | Pre-season | Egg hatch/crawler 1st gen | 1,569 |
| Sequim / Rain Shadow | 1,070 | Pre-season | Egg hatch/crawler 1st gen | 1,574 |
Source: UC Davis IPM: base 51°F upper 90°F; crawler emergence 405 DD; 600-700 DD optimal treatment window. Sources: UC Davis IPM: UC Davis IPM. About GDD₃₂ →
Monitoring & Action
Double-sided sticky tape on twigs starting late April (WSU HortSense)
Feeds on cell contents of bark, leaves, and fruit. Heavy infestations cause branch dieback and can kill trees. On fruit, feeding produces red halos around the scale. No honeydew production (armored scale).
Cultural Controls
- Provide proper plant care.
- Healthy plants are more tolerant of insect damage.
- Prune and destroy localized infestations, when practical.
- Natural predators and parasites help control scale populations, but may not prevent damage in severe infestations.
- Avoid use of broad-spectrum insecticides which kill beneficial insects.
- By late April, begin monitoring for crawlers by using double-sided sticky tape.