Scale, which has been showing up with increasing frequency in Sacramento Valley walnut orchards for about 10 years, is attracting interest from growers in how to identify and manage it. Recent confirmation that it is associated with the fungal disease, Botryosphaeria, which also has been spreading to more walnut orchards, has added to the interest in this pest.
Armored scales have a cover that is separate from the body. They suck plant juices from the inner bark by inserting their mouth parts into twigs and branches but rarely cause economic damage.
In the case of soft scales, the cover is its body wall. These scales suck plant juices from leaves and twigs. Low to moderate populations don’t appear to be damaging. However, heavy populations reduce terminal growth and vigor, resulting in smaller nuts and poor kernel quality. The secreted honeydew may cover nuts and favor the growth of sooty mold, increasing the chances for sunburn damage.
In the July issue of the Sacramento Valley Walnut News newsletter, University of California Cooperative Extension Advisors Dani Lightle (Glenn, Butte and Tehama Counties) and Richard Buchner (Tehama County) offer several possible reasons why growers are seeing more scale in their orchards.
Usually, a secondary pest, like scale, emerges as a problem when conditions within the system have changed to favor the pest, the advisors point out. Perhaps the parasitoids or other biological control agents that had been regulating scale populations have become disrupted by changes in pesticide use. Then, again, broad spectrum insecticides that are no longer being used may have been controlling scale better than thought at the time. Changes in such cultural practices as tree spacing and canopy management, may also be playing a role.
Here are tips for identifying the various scale species.
It’s becoming common in walnut orchards. In high populations, it is found in crusted layers on older branches and scaffolds.
Mature walnut scale, usually the males, appears to have a daisy-like outline.
Females lay eggs underneath their protective cover before dying. The eggs hatch into small, yellow crawlers, which can be seen only with a magnifying lens. They crawl, hitch-hike on the feet of birds or are carried by the wind to new feeding sites. They secrete the protective waxy cover, and remain there, completing two generations annually.
This year, growers also have been finding this type of scale in their orchards. Unlike walnut scale, which can colonize older wood, frosted scale typically is found only near the actively growing tips of walnut shoots.
Frosted scale has a domed appearance, similar in shape to an army helmet. In the spring, frosted scale are covered briefly with a white waxy coating, which disappears the rest of the year. Like walnut scale, frosted scale lay their eggs under their protective cover. The crawlers move to new (green) shoots and leaf growth where they feed through the rest of the summer. In the fall, crawlers move back onto woody permanent growth and overwinter there. In spring, the scale rapidly develops into adults, which mate and lay eggs for the next and only new generation of the year.
Over the years, several other species of scale have been found in walnut orchards, although none of them appear common at this time in the upper Sacramento Valley.
San Jose scale, an armored scale, can be distinguished from walnut scale by its smooth body margin. By contrast, the body margin of the walnut scales is a scalloped pattern. Natural predators appear to keep San Jose scale in check.
European fruit lecanium is a species closely related to frosted scale. It is indistinguishable from frosted scale for most of its lifecycle. However, unlike frost scale, it doesn’t develop the frosty coating in the spring.
Italian pear scale usually lives underneath lichen or moss. Most blight spray control programs control lichens and, consequently, Italian pear scale.
Good control of scale hinges on timing of the treatment. A delayed dormant spray is the traditional timing, Depending upon the pesticide, it has done a good job of controlling this pest while also causing less harm to beneficial parasitoids, the advisors note.
During the growing season, the scale cover, which helps protect against predators, also protects the scale from many pesticides. However, scales are very susceptible to pesticide application during the crawler stage when they are exposed. The best timing depends on the species and seasonal conditions.
To monitor for scale, the advisors recommend wrapping a piece of double-sided sticky tape around a branch where you see scale populations. Remove the sticky tape weekly and check for the presence of crawlers (usually on the margins of the tape) using a magnifying lens. Replace the tape with a new piece each week.
The July, 2014, issue of the Sacramento Valley Walnut News includes photos of walnut scale, frosted scale and crawlers and information about lifecycle stages. More details are available at the UC IPM web site: www.ipm.ucdavis.edu