Scale insects affecting conifers
Family: Coccidae, Diaspididae, Matsucoccidae
Description, life history, and host range
Several scale insects can attack coniferous trees in Colorado. The two most damaging species are the pine needle scale (Chionaspis pinifoliae) and striped pine scale (Toumeyella pini). Other species that attack conifers include the pine tortoise scale (Toumeyella parvicornis), black pineleaf scale (Nuculaspis californica), juniper scale (Carulaspis juniperi), pinyon needle scale (Matsucoccus acalyptus). All scale insects produce a waxy cover for protection. The armor of soft scales remains attached to the body of the insect, while hard scales produce a cover that is not attached to the insect’s body. Hard scales are generally less susceptible to insecticides than soft scales.
Scale insects feed on phloem or xylem using their long stylets to extract plant fluids. In addition to weakening the plant and reducing its growth, wounds produced by feeding of scale insects also provide an entry route for plant pathogens. Mature female scales lack wings, while the males are winged, which allows them to find mates. During egg laying, the female decreases in size as eggs are deposited under the scale covering. The crawlers (nymphs) are the most vulnerable life stage since they do not produce a protective scale covering until they complete their development. The exact host range, appearance and life history depend on the species.
- Scales are insects in the order of Hemiptera and two major families are pests of conifers: soft scales (Coccidae) and armored scales (Diaspididae).
- Scales have waxy or hard cover that protect them from predators, prevents desiccation, and provides protection from contact insecticides.
- The pine needle scale and striped pine scale are two significant pest species of conifers in Colorado. Several other species of scale insect can injure trees to a lesser degree.
- Symptoms of infestations include yellowing of foliage, the accumulation of scales, and the presence of honeydew or sooty mold.
- Cultural practices such as proper tree care and removing infested branches can be effective management approaches. Chemical treatments are also available for controlling scale insects and tend to be most effective when they target crawlers.
Infestation of striped pine scale. Note the abundant black sooty mold, which grows on the honeydew excreted by these insects. Image credit: R. Scott Cameron, Advanced Forest Protection, Inc., Bugwood.org
Infestation of pine needle scale on mugo pine (Pinus mugo). Note the yellowing of needles near the scale feeding sites. Image credit: John. A. Davidson, Univ. MD, College Park, Bugwood.org
Adult of striped pine scale. Note the white line running down the back of the insect. Image credit: Steve Clarke, USDA Forest Service, Bugwood.org
Adult pine needle scale (right) with the white scale cover lifted off (left). Image credit: John. A. Davidson, Univ. Md, College Pk, Bugwood.org
Multiple life stages of striped pine scale. Image credit: William M. Ciesla, Forest Health Management International, Bugwood.org
Heavy infestation of pine needle scales on mugo pine. Image credit: Scott Tunnock, USDA Forest Service, Bugwood.org
Description, life history, and host range continued…
Pine needle scale
Adult females are about 3.2 mm (1/8 inch) long, dark orange, and remain sessile after settling on needles of a host tree. The wingless females produce 8 mm (1/3 inch) long coverings that are white, oval or tear-drop shaped, and yellow at the apex. Mature males are 1 mm (1/25 inch) long but are rarely observed. The armor produced by hard scales often remains in place after the scale insect dies. Eggs are laid under scale coverings and are rust or brown in color. Crawlers emerge from the eggs and are very small, typically oval, and light purple, red to brown, or yellow when they are close to molting. As the crawlers molt, they combine the cast skins with waxy secretions to form the protective white covering.
In the fall, about 20-30 eggs are laid under a scale covering which will serve as the overwintering site. In some instances, a female pine needle scale can survive winter and lay eggs the following spring, which extends the egg-hatch period. After settling, the female pine needle scale remains sessile for the rest of its life. After hatching, crawlers migrate to needles and begin feeding, and the dispersal of crawlers to uninfested trees is driven by wind. This first generation of pine needle scales mature by early July, and eggs of the next generation can be produced through sexual or asexual reproduction. The pine needle scale can produce two generations each year in Colorado at lower elevations, and only a single generation at higher elevations. The pine needle scale feeds on the needles of most pine, spruce, and fir species.
Striped pine scale
Adults of striped pine scale have round, light brown or red-brown bodies with black markings and one or more white stripes down the center of the dorsal (back) surface. This species is a soft scale that overwinters as mature females or as fertilized immature females. A single female can lay hundreds of eggs under the scale covering. Mature female scales remain on twigs, while winged male scales emerge from their protective covering in late summer and die soon after mating. The eggs hatch over several weeks beginning in early June. Crawlers are usually orange or brown and females tend to remain on twigs, while male scales migrate to needles and become enclosed in papery covering as they mature. This species produces one generation each year in Colorado. The striped pine scale is becoming an increasingly important pest of Scotch, pinyon, Austrian, and lodgepole pines along the Front Range.
Feeding of scale insects generally causes decreased tree vigor, yellowing of foliage, stunted growth of needles, and needle drop or dieback. Soft scales excrete honeydew, which can facilitate the growth of sooty mold and is attractive to ants, bees, and wasps. The accumulation of sooty mold and scales causes infested trees to become discolored. Prolonged infestations can kill young trees or branches of larger trees, which increases their susceptibility to secondary insect infestations or diseases. Infestations of striped pine scale are sometimes associated with increases in adelgid infestations (Pineus spp.).
To scout for crawlers, branches can be inspected visually or shaken over a white piece of paper from late April to late May. Another scouting method involves sealing a small amount of plant material infested with scales in a clear plastic bag in early spring. The bag can then be placed in a shaded area outdoors and checked daily for the presence of crawlers.
Predators such as lacewing larvae, certain species of lady beetles in the genus Chilocorus, birds, as well as parasitic wasps such as Encarsia aurantia can attack scale insects. The natural enemies complex can be highly diverse in natural ecosystems, and research indicates that populations of pine needle scale are much lower in natural ecosystems than in managed ecosystems such as nurseries, tree farms, and ornamental landscapes. One proposed approach to managing scale insects involves manipulating plant species diversity in managed ecosystems to increase the diversity and abundance of natural enemies. Poorly timed insecticide applications or using broad-spectrum insecticides can reduce the diversity of natural enemies and make outbreaks of scale insects more likely.
In ornamental plantings and nurseries, proper plant care such as appropriate irrigation and pruning heavily infested branches is recommended. Minimizing stress on trees will help keep scale populations low and will decrease the tree’s susceptibility to other pests.
Horticultural oils are relatively harmless toward natural enemies and can be applied as dormant oils in winter or during the growing season from late winter to early summer to target crawlers. Applications of dormant oil should thoroughly cover all plant structures above ground. The infested plant structures, especially shoot terminals and the underside of leaves, should be thoroughly wetted with the spray when applied during the growing season.
Systemic insecticides can provide season-long control of soft scale crawlers and adults, and soil applications or trunk sprays are recommended when using systemic insecticides for managing these pests. However, these insecticides are toxic toward beneficial insects and should not be applied during or shortly before flowering. It is also recommended to wait until nearby plants have completed flowering before applying foliar sprays or soil applications.
Baker, J. 2016. Striped Pine Scale. North Carolina State University – Extension. Available https://content.ces.ncsu.edu/striped-pine-scales#:~:text=Striped%20pine%20scales%20are%20generally,papery%20covering%20as%20they%20mature.
CSU. (n.d.). Scale Insects on Pines. Colorado State University – Plant Talk Colorado. Available https://planttalk.colostate.edu/topics/insects-diseases/1418-scale-insects-pines/#:~:text=In%20Colorado%2C%20the%20pine%2Dneedle,laid%20underneath%20the%20mother%20scale.
Kabashima, J. and S. Dreistadt. 2014. Pests in Gardens and Landscapes: Scales. University of California – Agriculture and Natural Resources. Available https://ipm.ucanr.edu/PMG/PESTNOTES/pn7408.html
PNW Handbooks. (n.d.). Landscape pests- Scale insect. Pacific Northwest Pest Management Handbooks. Available https://pnwhandbooks.org/insect/hort/landscape/common/landscape-scale-insect
Sutherland, C. 2006. Scale Insects & Their Relatives. New Mexico State University – College of Agriculture and Home Economics. Available https://plantclinic.nmsu.edu/documents/o-07-scaleinsects.pdf
Tooker, J., and L. Hanks. 2000. Influence of Plant Community Structure on Natural Enemies of Pine Needle Scale (Homoptera: Diaspididae) in Urban Landscapes | Environmental Entomology. 29 (6): 1305-1311. Available https://academic.oup.com/ee/article/29/6/1305/381897
USDA. 2011. Pine Needle Scale. United States Department of Agriculture – Forest Service. Available https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5351723.pdf
USU. (n.d.). Soft Scales. Utah State University – Extension. Available https://extension.usu.edu/pests/ipm/notes_orn/list-treeshrubs/soft-scales