Glenn Kohler – Forest Entomologist, Washington Department of Natural Resources
Oak pit scales (OPS) are sucking insects that feed on the sap of oak species around the world. The insects
themselves are no larger than the head of a pin and they are covered by a hard “shell”. Once they start feeding they are stationary, which often causes the bark to swell and grow around them, leading to “pits” (Figure 1). Heavy infestations can slow tree growth and cause branch dieback, and after several years may cause mortality. This is especially true in oaks stressed by other factors such as drought. Common symptoms of OPS infestation are delayed leaf expansion in spring, clumping of foliage, and dead leaves that persist on branches in the fall (Figures 2 and 3).
The golden oak scale was introduced from Europe and can now be found on numerous oak species throughout North America, including Oregon white oak (Quercus garryana) in Oregon and Washington.
Oak pit scales in Washington
Very little is known about OPS effects on Oregon white oak because damage is rarely reported. Most of what is known is based on observations and research in California, where Oregon white oak does not grow.
In the spring of 2011, landowners in the Columbia River Gorge area, from Lyle to Washougal, reported branch dieback and some mortality in Oregon white oaks. Some landowners recalled seeing symptoms as far back as 2009. The Washington State Department of Natural Resources (DNR) and Washington State University (WSU) Extension found OPS to be the primary cause of oak damage in Skamania and Klickitat Counties. Crown symptoms have been recorded as far north as the South Puget Sound area, but mortality is rare outside of Klickitat County.
In 2013, DNR conducted a survey of OPS populations and damage throughout the range of Oregon white oak in Washington (Figure 4). Thirty-seven plots were established from the San Juan Islands to Klickitat County. Over 1,000 branches were examined for damage and the presence of pit scales, with the condition of the tree and symptoms of the infestation noted. Evidence of wasp parasitism on the pit scale was also recorded by looking for tiny exit holes made by adult wasps emerging from the scales. Analysis is ongoing, but OPS presence was confirmed at 34 of the plots. To date, the golden oak scale is the only species of OPS that has been identified in Washington.
Identification and management
Landowners should look for evidence of pit scales on twig bark of symptomatic oaks (magnification may be needed). The adults are 1 to 2 millimeters wide, green to brown in color, circular, flattened, and usually surrounded by a ring of abnormal bark swelling. On heavy infestations, the previous year’s growth will appear pimply in texture and tip dieback may occur. Adult female pit scales produce live young called crawlers in spring and summer. The tiny crawlers are the only mobile life stage that can colonize new growth. Young OPS settle to feed on fluids under twig bark and do not move for the rest of their lives. There is one generation per year.
Drought conditions may predispose oaks to more severe damage and mortality from OPS. In areas where drought is rare, such as Puget Sound, OPS are typically a minor pest of little concern. Open grown oaks and those on the edge of stands may be more susceptible to damage because they are exposed to desiccating wind and heat that can wilt and dry leaves on OPS-infested twigs. Oaks stressed by damage from disease, defoliating insects, wounding, or soil compaction may be more susceptible to OPS damage. While rains bring relief from drought, above normal spring precipitation may actually contribute to increases in OPS populations. Sucking insects often become more abundant when host trees have increased access to water. For this reason, watering or fertilizing infested oaks is not recommended.
Natural enemies, such as parasitic wasps, and changing environmental conditions can contribute to an eventual decline of OPS populations. While waiting for nature to take its course, landowners can protect high-value oaks by applying insecticide. Proper timing and selection of insecticide are critical for effective OPS control. Please refer to the WSU Extension Hortsense website (http://pep.wsu.edu/hortsense/) for more information on managing OPS. Contact Glenn Kohler, forest entomologist with Washington DNR (firstname.lastname@example.org; 360-902-1342), or your county’s WSU Extension office (http://ext.wsu.edu/) for more information on proper timing and use of insecticides.