Written By: Dr. Ash Sial, Department of Entomology, University of Georgia
Scales are a large group of insects (superfamily Coccoidea) in order hemiptera that are minute to small in size and sexually dimorphic – males and females are distinctly different in appearance (Daly et al. 1998). They have unusual lifecycle; females have incomplete metamorphosis (egg-immatures-adult), whereas males have complete metamorphosis (egg-immatures-pupal state-adult). Not all scales have both male and female sexes – some are hermaphrodites. They usually have waxy or scale like covering depending the family. Several families of scale insects including soft scales (Coccidae), mealybugs (Pseudococcidae), armored scales (Diaspididae), and giant scales (Monophlebidae) have been cited as pests of blueberries (Marucci 1966, Milholland and Meyer 1984, Antonelli et al. 1992).
Soft scales secrete a waxy covering that is part of their body. Of the soft scales, Indian wax scale, Ceroplastes ceriferus (Fabricious) (Fig. 1), Terrapin scale, Mesolecanium nigrofasciatum (Pergande) (Fig. 2) and European fruit lecanium, Parthenolecanium corni (Bouché) (Fig. 3) can cause economic damage to blueberries. Recently, Azalea bark scale, Eriococcus azaleae Comstock (Eriococcidae) was also reported to feed on blueberries (Walton et al. 2006).
The armored scales do not secrete honeydew; they concentrate and incorporate anal secretions into the scale cover (Foldi 1989). Among the armored scales, Lesser Snow scale, Pinnaspis strachani (Cooley) (Fig. 5) and Putnam scale, Aspidiotus ancylus (Putnam) (Fig. 6) can cause the most damage to blueberries.
Mealybugs are morphologically different from the other scales insects because they possess functional legs throughout their lifecycle. The infested plants look snowy because of the white waxy body filaments. The blueberry mealybug, Dysmicoccus vaccinii Miller & Polavarapu has been reported to infest blueberries. Based on circumstantial evidence, blueberry mealybug has been implicated as a vector of the Ringspot virus, the causal agent of the Red Ringspot disease in blueberries (ScaleNet).
The cottony cushion scale, Icerya purchasi (Fig. 7) can be distinguished easily from other scale insects. The mature females (actually hemaphrodites) have bright orange-red, yellow, or brown bodies (Ebeling 1959). The body is partially or entirely covered with yellowish or white wax. The most conspicuous feature is the large fluted egg sac, which will frequently be two to 2.5 times longer than the body. The egg sac contains about 1000 red eggs (Gossard 1901). Cottony cushion scale has a wide host range and was reported to feed on blueberries in Bacon County (GA) earlier this year.
Scale insects insert their piercing-sucking mouthparts into the plant tissues and siphon the plant sap. As they feed, soft scales and mealybugs excrete large amounts of a sweet, sticky liquid referred to as “honeydew” which provides an excellent medium for the growth of black fungus called sooty mold. Accumulation of honeydew and sooty mold on foliage interferes with photosynthesis which can reduce plant vigor and slow plant growth. If the feeding occurs on the fruit, grade loss can occur due to the presence of unsightly honeydew and sooty mold. Sooty mold usually weathers away following the control of scale infestation (Buss and Turner 2004).
Honeydew also attracts ants and when ants are observed, plants should be closely examined for scale infestation. If active scale populations are suspected in a blueberry orchard, double-sided sticky tape should be put around canes in close proximity to the eggs sacs. Tape should be changed at least every other week and looked under the microscope to determine when eggs begin to hatch which is extremely important because crawlers (the first instar nymphs) are the only mobile stage and are readily controlled by oil or most insecticides.
The best strategy to manage scale insects is to prune the old wood annually. Dormant pruning of old, weak canes and scale infested wood removes a large pool of eggs and prevents the scales from increasing their population density. The most scale populations have historically remained below economic threshold as a result of natural biological control. However, the increased pesticide use against spotted-wing drosophila over the past couple of years might have disrupted the natural biological control in blueberry orchards leading to higher populations of scales and other secondary pests.
If scale insect pressure is high, winter pruning should be followed by dormant oil applications before the bloom. If the temperatures are high enough for insect development to occur, insect growth regulators can be applied in combination with oil earlier in the season. Achieving 100% control of scales using pesticides is a major challenge because adult females and eggs are protected from virtually any pesticide. The crawlers (first instars nymphs) are the only mobile and susceptible stage, and therefore timing of chemical applications to target the crawler activity periods is critical.
In order to control heavy scale infestations, high spray volumes (100-200 gallons per acre) should be used to ensure thorough coverage of all parts of blueberry bushes. It is not advisable to apply oil sprays at or below 32°F, but rather at temperatures above 50°F under calm conditions. Also, oil should not be applied after fruit set because it will remove the bloom and the resultant spotted berries will be unmarketable for fresh markets.
Recent studies conducted to evaluate efficacy of different insecticides against scales indicate that insecticide applications made after harvest and hedging the bushes in August performed better than the same insecticides applied in November. So, if scale infestation is observed in an orchard and the population density is high enough to justify spray application, spray applications should be made after harvest in July/August (Fig. 8).
For further details regarding scale control in blueberries, please refer to Southeast Regional Blueberry Integrated Management Guide at
Antonelli. A., E. Elsner, and C. Shanks. 1992. Arthropod managernent. pp. 55-75. Jn Pritts, M. P., J. E Hancock, and B. Strik, eds., Highhush Blueberry Production Guide. Northeast Regional Agricultural Engineering Servil’e Bulletin 55, 199 pp.
Borror, D.J., C.A. Triplehorn, and N.F. Johnson.1992. Order Homoptera (cicadas, hoppers, psyllids, whiteflies, aphids, and scale insects), In Borror, D.J., C.A. Triplehorn, and N.F. Johnson (ed.), An introduction to the study of insects, 6th ed. Harcourt Brace College Publishers, New York, NY.
Buss, E.A., J.C. Turner. 2004. Scale Insects and Mealybugs on Ornamental Plants. EDIS. University of Florida. http://edis.ifas.ufl.edu/MG005. (December 12, 2006).
Daly, H.V., J.T. Doyen, and A.H. Purcell III. 1998. Order Hemiptera (bugs, leafhoppers, etc.), pp 433-434. In Daly, H.V., J.T. Doyem, and A.H. Purcell III (ed), Introduction to insect biology and diversity, 2nd ed. Oxford University Press, New York, NY.
Ebeling W. 1959. Subtropical Fruit Pests. University of California Press, Los Angeles. 436 p.
Foldi, I. 1989. 220.127.116.11 The scale cover, pp. 43-54. In Rosen, D., ed., Armoured Scale Insects, Their Biology, Natural Enemies and Control. Vol. A. Elsevier, Ams.terdan1, the Netherlands, 384 pp.
Gossard H.A. 1901. The cottony cushion scale. Florida Agricultural Experiment Station Bulletin 56:309-356.
Marucci, P E. 1966. Insects and their control, pp. I 99-236. In Eck, P. and N. F. Childers, eds., Blueberry Culture. Rutgers University Press, New Brunswick, New Jersey, 378 pp.
Milholland, R. D. and J. R. Meyer. 1984. Diseases and arthropod pests of blueberries. The North Carolina Agricultural Research Service, Raleigh, 33 pp.
Polavarapu, S., J.A. Davidson, and D.R. Miller. 2000. Life history of the putnam scale, Diaspidiotus ancylus (Putnam) (Hemiptera: Coccoidea: Diaspididae) on blueberries (Vaccinium corymbosum, Ericaceae) in New Jersey, with a world list of scale insects on blueberries. Proc. Entomol. Soc. Wash. 102(3): 549-560.
Walton V., R. Rosetta, J. DeFrancesco, W.Q. Yang, B. Strik. Scale insects on Blueberries, what you should know. http://oregonstate.edu/dept/nurspest/Azalea_Bark_Scale.pdf