Contents: By Damage and Image
A Matter of Scale
We all run into a few bumps on the road. As a horticulturist, you're also likely to run into a few bumps on your plants. Those bumps may turn out to be scale. These tiny insects can wreck havoc on nursery stock through direct feeding, production of honeydew with the companion black sooty mold, and transmitting pathogens. Small and cryptic, they are often difficult to spot, increasing the risk for movement of scale when plants are transported.
Scale refers to a very large superfamily of insects
called Coccoidea. According to the ScaleNet website there are 28 families
of scales comprised of 7,355 species. Not all scale insects are pests.
Scale insects have been exploited for lacquer and shellac, candle wax,
and used as biological control agents of noxious weeds. They produce a
rich carmine color used as a natural dye for cloth and in the food and
beverage industry. I recall the look of horror on a workmate's face when
I told her, cochineal, the ingredient listed in her favorite all-natural
lemonade, referred to the scale insect on Opuntia cactus, Dactylopius.
Still like a book? Johnson and Lyon's seminal guide, Insects that Feed on Trees and Shrubs, lends itself to quick identification of scale problems by matching images of hosts and pests, and it is thick with information on species of concern. Many of us take council with Cranshaw's Garden Insects of North America when scale guidance is needed.
Crawlers test the sight of even young eyes. Use double-sidedsticky tape to capture crawler emergence. As they emerge from their protective cover, they stick to the adhesive tape encircling either side of the infestation. The application of soaps and oils, insect growth regulators, and other foliar insecticides are often timed for this occurrence.
Natural enemies often keep scale under control. Factors that disrupt biological control include ants that tend, move, and protect scale; dust; and long-residual, broad spectrum insecticides. In protected culture, some growers have programs with augmentative releases of lady beetles such as the Mealybug destroyer, Cryptolaemus montrouszieri, for mealybugs, and Chilocorus and Rhizobius for soft scale.
There are various strategies to slay a scale. Many target the vulnerable young crawlers. This allows managers to choose a greater range of insecticides including those of lower toxicity, such as soaps, oils, and insect growth regulators. Correct scale species identification helps predict whether the scale will have only one or multiple generations of crawlers emerging each year. Some kill scale while they sleep, covering the overwintering stage with suffocating dormant oil. Thorough contact by foliar sprays is important. Another tactic is application of systemic insecticides that are drawn into the plant, managing multiple feeding stages of the scale. It helps to think a little ahead on this one as time may be needed for some plants to translocate the insecticide to infested parts of the plant. Systemic insecticides are used against many scale species but generally have been less successful with hard scales and pit scales.
Purdue's Scale Insects on Shade Trees and Shrubs website includes a Guide to Assessing Scale Infestation. It contains a nice decision matrix for management actions, particularly in landscapes where scale populations may be more easily tolerated.
Common Scale Families
Medeira mealybug, Phenacoccus madeirensis is one of the more recently introduced mealybugs to the United States. It is very polyphagus, feeding on 42 plant families. Pink hibiscus mealybug, Maconellicoccus hirsutus, is a species with a host range of morethan 200 genera. Like Florida, California has begun a biological control program for this recent introduction.
Mealybugs are not only indoors in the Pacific Northwest, they can also be found outdoors. The ground mealybug, Rhizoecus sp., on Phormum is a good example of a species which successfully overwinters in our mild climate.
One of the most recent finds in Oregon included Eulecanium excrescens on Styrax. Around the country entomologists report finding various soft scale species including European peach scale, P. persicae; frosted scale, P. pruinosum; calico scale, E. cerasorum; magnolia scale, Neolecanium cornuparvum; spruce bud scale, Physokermes piceae; hemispherical scale, Saisettia coffee; black scale, S. oleae; and nigra scale, Parasaissetia nigra
Armored or Hard Scales
One hard scale species found in the PNW is pine needle scale, Chionaspis pinifoliae. Pine, spruce, fir, hemlock and Douglas fir can all host this species. Adult females are elongated, white, and armored with a yellow area at the front end. Those growing juniper should be aware of the juniper scale, Carulaspis juniperi.
For more scale species listings, see Scale species listed by host type
Scale have disappointed many a grower, posing as part of their plant's anatomy and feeding on the profits. With a little effort, it is possible to set up an IPM program that is effective and tips the balance in your favor in the battle with the scale.
This information is from an original article in the Digger magazine but is updated on occasion to correct links or add information.
Futch et al. 2000. A Guide to Scale Insect Identification, University of Florida Extension
Gyelshen et al. 2006. Field Guide to Identification of Scale Insects on Holly. University of Florida. June 2013 revision.
Sadof, C. Scale Management. Purdue Extension Entomology.
Hard Cover Resources:
D. Miller and J. Davidson. 2005. Armored Scale Insect Pests of Trees and Shrubs. Cornell University Press. 456 pp
2007 PNW Insect Management Handbook, C.S. Hollingsworth, ed. Oregon State University.
W. Cranshaw (2004). Garden Insects of North America. Princeton University Press, Princeton, NJ. 656 pp
W.T. Johnson and H.H. Lyon (1991), Insects That Feed on Trees and Shrubs, 2nd ed., Cornell University Press. 560 pp
Orginal publication: 10-31-2007
Author: R.L. Rosetta, Extension Nursery Integrated Pest Management, Department of Horticulture, Oregon State University