Elongate Hemlock Scale
About Elongate Hemlock Scale
Elongate hemlock scale (Fiorinia externa) is an insect pest that affects several species of coniferous trees. They are native to Japan and China but have become widespread in the eastern United States. In the U.S., scale populations tend to be denser and have a greater impact on the host than in their native range where synchronized natural predators prevent high infestations (McClure, 1986). They are of particular concern to Christmas tree farmers because infestations are unsightly and can cause symptoms like yellowing, also known as chlorosis (Figure 1), needle drop, and occasionally tree death (Van Driesche et al., 2013). They also present issues concerning the export and import of trees across state lines.
Figure 1. Mottled yellowing on the top of hemlock needles (inset), corresponding with elongate hemlock scales on the underside.
Elongate Hemlock Scale Description
Elongate hemlock scale is an armored scale insect that lives on the underside of needles, particularly on hemlock, fir, and pine trees. These insects have multiple stages, which are barely visible to the eye (Figure 2). A single needle can contain many individuals of this scale insect all in different stages. Beginning in their nymphal stages, female adult elongate hemlock scales are immobile and secrete a brown, waxy covering that fades along the length to yellow. The covering is oval-shaped and about 2 millimeters long. Females will lay eggs underneath their own covering, and these rows can be visible through the covering. Immature nymphs are yellow and flat. Males produce a white waxy cover with the insect visible as a yellow tip and are about 1.5 millimeters long (smaller than females). Adult males live only a short while and are small and brown with a single pair of wings.
Quick Elongate Hemlock Scale ID
- Female adults are soft-bodied, legless, wingless, and found under a waxy cover that is light brown. Females are about 2 millimeters long.
- Male adults are light brown, 1.5 millimeters long, and have legs and wings, but are weak flyers.
- Crawlers are soft-bodied, have legs, and range from 0.1 to 1.0 millimeters.
Figure 2. EHS eggs overwinter under the protective waxy covering of their mother. Eggs hatch into crawlers, which become immobile indefinitely for females and until adulthood for weakly-flying males. A natural outbreak of the insect-killing fungus, Conoideocrella luteorostrata, in elongate hemlock scale on Fraser fir is also shown. Photo credits: Egg photo, Pennsylvania Department of Conservation and Natural Resources; adult male photo, Pennsylvania Department of Agriculture; and crawler photo, Eric. R. Day, Virginia Polytechnic Institute.
Elongate Hemlock Scale Biology and Life Cycle
Elongate hemlock scales begin their life as a group of light-colored eggs protected under an oval-shaped waxy cover. When the weather warms seasonally around May, their eggs begin to hatch and emerge as soft-bodied, legged crawlers that are about 0.1 millimeters long and lemon-colored. Crawlers are the only mobile life stage and the primary life stage that spreads to new plants. Since scales have staggered reproductive cycles through the year, more than one stage can be observed at the same time (Kosztarab, 1996). They can be carried by the wind or birds. Adult males are about 1.5 millimeters long, light-colored, and have visible legs and wings. They are mobile but weak flyers. The most visible stage is the adult female, which is around 2 millimeters long, covered in a brown to yellow waxy cover, and immobile.
Elongate Hemlock Scale Hosts and Distribution
Scales are most commonly found on eastern hemlock ( Tsuga canadensis) and Carolina hemlock ( T. caroliniana) but have been observed on many species, including spruce ( Picea), fir ( Abies), and yew ( Taxus) (McClure and Fergione 1977). In Christmas tree operations, this is a pest of true fir ( Abies spp.) that can also be found on Douglas fir ( Pseudotsuga menziesii), spruce, and pine ( Pinus) trees (Sidebottom, 2016).
Figure 3. State-level elongate hemlock scale range in 2021 compiled from available reports.
Elongate hemlock scale was first reported in the United States from New York in 1908; it was likely imported from Japan (Sasscer, 1912). Since then, this insect has spread through 21 states and Washington, D.C. in the eastern United States (Figure 3).
Elongate Hemlock Scale Management
Integrated pest management, or IPM, uses a combination of techniques to keep the population of a pest below a reasonable limit. If we identify the pest and use our knowledge of its lifestyle to deploy a variety of control techniques, we can establish a balance that maintains plant health in the long term with limited negative side effects. The main components of IPM are identification and evaluation of the issue, actions to reduce pests and prevent further outbreaks, and long-term monitoring to adjust the strategy as necessary. Below are some strategies that can be integrated into a pest management system for elongate hemlock scale.
Providing plants with good growing conditions (e.g. spacing) and proper cultural care (e.g. proper pruning) can help reduce elongate hemlock scale. Appropriate irrigation reduces stress and increases tolerance to scale insect damage. Also, over-fertilization can increase scale reproduction, resulting in higher scale densities (McClure, 1980).
Several natural enemies of hemlock scales are present under natural conditions. For example, the cosmopolitan parasitoid wasp, Encarsia citrina (Figure 4), regularly kills 90% of the hemlock scale population in Japan and is well established in the United States (Abell and Van Driesche, 2011). The predatory beetle, Cybocephalus nipponicus (Figure 4), also preys on these scales in their range in the United States (Mayer et al., 2008). In addition, generalist predators, such as lady beetles and lacewings, are commonly found on coniferous trees. Lady beetles, lacewings, E. citrina, and other natural enemies of scale insects are also commercially available. Lastly, entomopathogenic fungi, such as Colletotrichum fioriniae (Marcelino et al., 2008) and Conoideocrella luteorostrata (Hywel-Jones 1993), can infect and kill nymphs and adult scales (Figure 2).
Figure 4. Two natural enemies of EHS, including the parasitoid wasp Encarsia citrina (top; shown parasitizing an elongate hemlock scale female) and the small black beetle Cybocephalus nipponicus (bottom; males have a yellowish pronotum). Photo Credit: Beetle photo, Pennsylvania Department of Conservation and Natural Resources
Timing of the application is important for effective chemical control because scale insects become less susceptible to insecticides as they age. Several insecticides with different active ingredients kill armored scales when targeting them at the crawler stage. Horticultural oils and reduced-risk insecticides, such as buprofezin, pyriproxyfen, and spirotetramat, are recommended because of their efficacy and compatibility with beneficial insects. Other active ingredients, such as bifenthrin, imidacloprid, dinotefuran, and acephate, also kill armored scales; however, they are harmful to beneficial organisms and can cause spider mite outbreaks. Many products are commercially available with the mentioned active ingredients, but federal laws indicate that the site of application must be listed on the pesticide label.
Abell, K., Van Driesche, R. (2011, Dec 1). The use of Cohorts to Evaluate the Impact of Encarsia citrina (Hymenoptera: Aphelinidae) on Fiorinia externa (Hemiptera: Diaspididae) in the Eastern United States. Florida Entomologist, 94(4):902-908. doi.org/10.1653/024.094.0426
Kosztarab, M., (1996, Jan 1). Scale insects of northeastern North America: identification, biology, and distribution. Virginia Museum of Natural History. Pp 502. ISBN-10: 1884549012
Marcelino, J., Giordano, R., Gouli, S., Gouli, V., Parker, B.L., Skinner, M., TeBeest, D. and Cesnik, R. (2008). Colletotrichum acutatum var. fioriniae (teleomorph: Glomerella acutata var. fioriniae var. nov.) infection of a scale insect. Mycologia, 100(3), 353-374.
Mayer, M., Dorsey, T., Scudder, T., Bronhard, L. (2008). Mass Release and Recovery of Cybocephalus nipponicus (Coleoptera: Cybocephalidae) on Elongate Hemlock Scale, Fiorinia externa. New Jersey Department of Agriculture.
McClure, MS., Fergione, MB. (1977, Dec 1). Fiorinia externa and Tsugaspidiotus tsugae (Homoptera: Diaspididae): distribution, abundance, and new hosts of two destructive scale insects of eastern hemlock in Connecticut. Environmental Entomology 6: 807-811. doi.org/10.1093/ee/6.6.807
McClure, MS. (1980, Feb 1). Foliar Nitrogen: A Basis for Host Suitability for Elongate Hemlock Scale, Fiorinia Externa (Homoptera: Diaspididae). Ecology. (61)1 p 72-79. doi.org/10.2307/1937157
McClure, MS. (1986). Population Dynamics of Japanese Hemlock Scales: a Comparison of Endemic and Exotic Communities. Ecology, 67(5), 1986, pp. 1411-1421. Ecological Society of America
Sasscer, E. R. (1912). The genus Fiorinia in the United States. (No. 16). US Government Printing Office.
Sidebottom, J., (2016, Oct 1). Elongate Hemlock Scale. Christmas Tree Notes. North Carolina State Extension. Retrieved from content.ces.ncsu.edu/elongate-hemlock-scale
Van Driesche, RG., LaForest, JH., Bargeron, CT., Reardon, RC., Herlihy, M. (2013, Sep). Forest Pest Insects in North America: A Photographic Guide. U.S. Department of Agriculture. Retrieved from: forestpests.org/book/
Authors: Carlos Quesada, WVU Extension Service Assistant Professor and Entomology Specialist; Matt Kasson, Associate Professor, WVU Davis College of Agriculture, Natural Resources and Design; Brian Lovett, Post-Doctoral Fellow, West Virginia University; and Hosanna Barrett, Undergraduate Researcher, West Virginia University