Advice for the Home Gardener from the Help Desk of the
UC Master Gardener Program of Contra Costa County
UC MG Help Desk Response: Thank you for contacting the UC Master Gardener Program Help Desk about the problem with your citrus and also thanks for sending that great picture as it allows for a clear diagnosis. Your citrus is infected with scale – but don't worry, scale can be managed.
Scale is an insect that sucks on plant juices. Left unchecked it could cause damage to or reduce the vigor of your plant. Regarding whether the quality of your fruit will be impacted - I don't believe so. The problem with the scale on the rinds is that it is primarily unsightly. If you were going to leave the rind on for your use of the fruit then you would want to scrub the surface to remove the scale bodies..
The scale on your citrus stems and fruit is an armored scale. From your picture I am unable to identify the particular species of armored scale but that does not matter because management would be the same.
What you see residing on your plant are females and immature who do not have wings and therefore simply reside there – feeding.
Understanding the basic life cycle will help to understand the pest management practices:
Adult females produce eggs that are usually hidden under their bodies. Eggs hatch into tiny crawlers (immatures) which settle down and begin feeding within a day or two after emergence. Settled immatures may spend their entire life in the same spot without moving as they mature into adults.
Chemical treatments are most effective when the crawlers are just emerging in spring. See the link at the end of the narrative (below) for additional information on monitoring when crawlers are crawling.
Management:
Provide plants with good growing conditions and proper cultural care, especially appropriate irrigation, so they are more resistant to scale damage. You can prune off heavily infested twigs and branches, if they are limited to a few parts of small plants.
Horticultural oil is effective in spring or summer on deciduous plants when sprayed soon after most crawlers have emerged and most scales are in the young immature stage. Late spring and summer are also the times to spray avocado, citrus, and many other broadleaf evergreens. Horticultural oil kills the scales through suffocation.
Where plants can be sprayed, complete spray coverage of infested plant parts with horticultural oil at the proper time provides good control of most scales. Horticultural oils (e.g., Bonide Horticultural Oil and Monterey Horticultural Oil) are specially refined petroleum products, often called narrow-range, superior, or supreme oils.
Other non-persistent, contact sprays for garden include insecticidal soap (e.g., Safer Brand Insect Killing Soap Concentrate II), neem oil (Bayer Advanced Natria Neem Oil Concentrate, Green Light Neem, Garden Safe Brand Neem), canola oil (Bayer Advanced Natria Multi-Insect Control), and other botanical (plant-derived) oils.
Thoroughly cover with spray the plant parts where scales occur, typically on twig terminals and the underside of leaves. Follow the product label with respect to target plant, timing etc. Do not apply oil or other insecticides when it is foggy, freezing (under 32°F), hot (over 90°F), when relative humidity is above 90%, or if rain is expected in the next 24 hours. Especially at locations with hot weather, be sure plants are well irrigated before spraying foliage.
Below is a link to the UC Integrated Pest Management website that provides additional information on Scale lifecycle and management.
Armored scale do not produce honey dew (a sugar based excrement) so disregard the information about monitoring honeydew and controlling ants.
http://ipm.ucanr.edu/PMG/PESTNOTES/pn7408.html
Help Desk of the UC Master Gardener Program of Contra Costa County (EDC)
Note: UC Master Gardeners Program of Contra Costa's Help Desk is available almost year-round to answer your gardening questions. Except for a few holidays (e.g., last 2 weeks December), we're open every week, Monday through Thursday for walk-ins from 9:00 am to Noon at 2380 Bisso Lane, Concord, CA 94520. We can also be reached via telephone: (925) 608-6683, email: ccmg@ucanr.edu, or on the web at http://ccmg.ucanr.edu/Ask_Us/. MGCC Blogs can be found at http://ccmg.ucanr.edu/HortCoCo/ You can also subscribe to the Blog (//ucanr.edu/blogs/CCMGBlog/)