Surge of Black Pineleaf Scale in Northern Utah
The black pineleaf scale (Nuculaspis californica) is an armored scale, concealing its body under a removable, waxy covering. It feeds only on needles, and most pine species are susceptible, as well as Douglas-fir. Considered to be a native insect occurring throughout North America, black pineleaf scale is primarily a pest of economic importance in western U.S.
This scale is kept in check by two wasp parasitoids (Prospatella sp. and Physcus varicornis) that have three or more generations during the scale’s single one. Pineleaf scale is sensitive to sudden cold temperatures, and early fall frosts can also prevent outbreaks. Jensen has observed that trees in the lower part of valley are more heavily infested than those higher up on the benches, where temperatures are cooler.
The current outbreak in Utah was probably triggered by a combination of factors including mild fall weather, dust (which encourages scale and discourages predators), and urban stressors such as prolonged moisture deficit, soil compaction, and root injury. Use of pesticides that inadvertently killed the parasitoid wasps could also have played a role.
Mike Marett, Sandy City Urban Forester, has seen infested trees in his city increase to the hundreds in the last 5 years. “I had to remove 12 to 14 specimen trees last year, and only one had secondary bark beetle attack.” He has struggled with control and has tried a variety of options such as dormant oil, foliar sprays to the crawlers, and Mauget injections of systemics.
We know from observation in neighboring states that there is probably only a single generation per year in northern Utah. Scales overwinter as immatures, and in early June, winged adult males emerge and in their short life spans, seek out females. Mated female lay up to 40 eggs under their coverings in late spring. Egg hatch occurs over several weeks starting in late June and early July, after new needles have completed growth. Yellow crawlers are soon visible with a 10-20x hand lens, and are active through mid to late July. They migrate to current season needles, or become airborne, where they are delivered to nearby hosts. By early August, crawlers will have settled to immobility for the remainder of their lives.
Managers should take an IPM approach in controlling black pineleaf scale, which includes a combination of maintaining tree health, promoting the wasp parasitoids, and foliar or systemic insecticides for severe infestations. Foliar insecticides must be timed to the crawler stage because the waxy covering on adults is impervious to liquids.
A systemic may also be applied as an injection or soil drench. Safari (dinotefuran) is a newer neonicotinoid that has shown good results on armored scales (however, it is expensive). It is highly water soluble and moves quickly through the plant tissue. It should be applied in early spring. Imidacloprid (Merit) is not effective against armored scales because the amount taken up through the xylem and into plant mesophyll and parenchyma cells is not a lethal dose.
Jerry Goodspeed, USU Extension horticulture agent in Weber County, noted that with years of remedial care, trees can recover. “The worst infestation I saw was about 100 pines on a golf course in SLC in the summer of 2005. Most of the trees eventually recovered after oil treatments, supplemental insecticides, and improved irrigation.” USU Extension will continue to work with city managers and arborists to bring the epidemic to a manageable level. A balance must be found among the wasp predators and the scale, which may take several years of treatment. In the meantime, an early fall frost can result in significant mortality, speeding up the process.