Utah Pests News Spring 2009

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Surge of Black Pineleaf Scale in Northern Utah

  Many trees in the Salt Lake Valley are severely infested, as shown on this Austrian pine above.

According to the Oregon Dept. of Forestry, scale infestations are measured by the average number of scales per inch of needle.  Low/endemic populations occur at 0.5 scale/inch of needle, growth loss occurs at 4 scales/inch, and mortality occurs at 20+ scales/inch.

Arborists and USU Extension agents have noticed an upsurge in the incidence of black pineleaf scale in the last 3 to 5 years on Scotch, Austrian, and ponderosa pines in northern Utah.  “I think this is huge problem for the Salt Lake City metro area,” said Hal Jensen, certified arborist and member of the Utah Community Forest Council.  “Some trees are so infested that they are approaching death, from city parks in Draper, Sandy, West Valley City,” to lighter infestations elsewhere.

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.



Infested trees may die within 5 years (top). Pruning is not an option in scale control. Infested twigs retain only the current years’ growth (bottom).

Leaf-inhabiting armored scales feed by inserting their straw-like mouthparts into the needle tissue and removing nutrients and contents of mesophyll cells.  Where scales feed, the foliage becomes yellowed with localized necrosis. Under heavy and prolonged infestations, needles that are normally retained for 5 years will drop in 1 to 2 years.  New needles are sparse, stunted, and chlorotic.  Branches die back and the tree may eventually be killed.  Often, infestations will predispose trees to attack by bark beetles.

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.


Know Crawler Emergence...

    wrap double-sided sticky tape tightly around an infested needle cluster;  
    shake a limb over white cloth;  
    examine needles regularly in June/July with a hand lens  
   In general, crawlers begin hatching at approximately 1068 degree days, or when greenspire linden or northern catalpa blooms.  
Horticultural oil, insecticidal soap, and some insect growth regulators (IGRs) are foliar insecticides that are softer on the beneficials.  Note that oil should not be applied to drought or heat-stressed plants, and that fatty acids in soaps can be inactivated by Utah’s hard water, so add a buffer or conditioning agent. IGRs such as Distance (pyrifoxyfen) or Talus (buprofezin) disrupt molting and work best on earliest instars.  These products also have sublethal effects in that surviving females lay fewer eggs the following year.

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.

-Marion Murray, IPM Project Leader