oystershell scale

Oystershell scale, an armored (hard) scale
Photo: Whitney Cranshaw, Colorado State University, Bugwood.org

Updated: April 18, 2022

Key Points

  • Considered pests, scale are sucking insects that consume sap or plant cell contents. They are categorized as either armored (hard) scale or soft scale, and this distinction determines the damage they can cause and how they are best controlled.
  • Low populations tend to go undetected. High populations can cause plant damage, such as leaf yellowing, plant stunting, or branch dieback. Large numbers of soft scale also contribute to sooty mold growth.
  • Understanding their life cycle is key to managing them effectively, with the crawler stage being the most vulnerable to insecticides.

What is scale?

Scale are small soft-bodied insects that cover themselves with layers of either flexible or stiff wax. These covers help them blend into their environment, prevent them from drying out, and hide them from predators. For gardeners, this shield also blocks most contact insecticides from touching the insect itself.

Typical wax colors are brown, gray, or white, though the insect underneath can be more colorful - rosy-red or purplish. Cover shapes are usually round, rice-shaped, or oystershell-shaped, and are often smaller than a third of an inch (8-9 millimeters). Scale enlarge their covers as they grow, though, in some species, male and female covers can be different sizes, shapes, and colors.

Scale are sorted into two main categories:

  • Soft scale have a leathery or gum-like pliable wax that stays firmly attached to their bodies. Some will move between leaves and twigs depending on the time of year and their life cycle. They feed on juices in vascular tissues - the sap in the phloem, either under bark or in leaf veins. They excrete honeydew.

Soft scale

Examples of soft scale

  • cottony camellia scale

    Cottony camellia scale

  • Azalea Bark Scale. Photo: John Davidson, University of Maryland

    Azalea bark scale

  • Calico scale

    Calico scale

    Soft scale are easy to pluck off and squish.
    Photo: Joe Boggs, OSU Extension

  • Armored (hard) scale have a shell-like, stiffer, or parchment-like wax that does not attach to the scale’s body. They tend to remain in one place once settled. They feed on the contents of individual plant cells, either under bark or in foliage. They do not produce honeydew.

Armored scale

Examples of armored scale

  • white male scale covers on an euonymus leaf

    Euonymus scale

  • White prunicola scale

    White prunicola scale

Where scales feed on plants - These diagrams illustrate how the two groups of scale differ in feeding. The stylet is a straw-like mouthpart they insert into plant tissues. It can reach relatively far from where the insect is settled.

Features of scale life cycles

  • Mature female scale do not have wings. Mature males are winged in order to find mates, but they are so tiny that they are rarely seen.
  • Females lay eggs either entirely under their protective covers or under an additional secretion of fluffy, cottony wax.
  • Mature females die after laying eggs. Generally, soft scale have one generation per year, while armored scale have several, though there are exceptions.
  • When eggs hatch, the newborns are called crawlers because they actively wander around to find a place to feed. They do not have covers for protection. Once they choose a spot, they either rarely move or stay put for the rest of their lives; at this point, they are referred to as “settled.”
  • As they grow, these settled crawlers quickly develop their waxy shell and, for some species, lose their legs.

What problems do they cause?

  • Scale feed on plants by sucking up either plant juices in sap or the contents of plant cells. Although a few plants tolerate high-scale populations with no apparent damage, their feeding can weaken plants or cause localized tissue death.
  • As with many other sap-sucking insects, soft scale produces honeydew. (Armored scale does not.)
  • This sugary waste excretion falls and sticks onto leaves or any surface below the insect.
  • A fungus grows on top of the honeydew but does not infect the plant itself. Its dark color gives it the name sooty mold, and it can take a while to weather away, even after the honeydew production ends.
  • Heavy layers of sooty mold shade leaves and can weaken plants in addition to being an eyesore.

Management

Regardless of the technique used, suppression efforts for large populations of scale may require more than one year of intervention.

Physical removal

Removal of the scale is the most environmentally-friendly approach, but not always practical because scale often populates inaccessible plant parts. Refer to the mechanical section for details.   

Cultural

Prevention: Prevent plant stress by watering appropriately.

Plant placement: Don’t plant scale-susceptible species in sites with limited root space or reflected heat (such as too close to pavement or walls).

Reduce fertilizer use: Overfertilizing produces excessive succulent growth that increases scale populations.

Mechanical

Remove with a brush: A strong spray of water, combined with a soft scrub brush, can manually dislodge many of the scale covers or remove their protective wax. Once exposed, they are vulnerable to predators and desiccation.

Pruning: Branches that have drastically declined or already died should be pruned out. Plants that resprout readily and have multiple main stems can have entire branches laden with scale removed.

Isolate or replace: Potted plants with moderate scale populations should be isolated. Any landscape or potted plants that are too heavily infested - especially perennials or small shrubs - are best replaced, as it will likely cost about as much in time and materials to treat as it would to simply remove and replace.

Biological

  • Natural enemies (“beneficials”) are organisms that hunt and consume pests. For scale, they include tiny parasitic wasps and flies, lady beetle adults and larvae, insect-specific fungi, and predaceous mites. Minimizing pesticide use will conserve them and allow them to both limit an existing outbreak and help prevent them from recurring.
a scale with a hole in the top from a beneficial insect
Parasitoid emergence hole; scales with such a hole in their cover are dead
Photo: Lorraine Graney, Bartlett Tree Experts, Bugwood.org

Chemical

If you do not wish to use pesticides and are not obtaining good control from manual removal efforts alone, remove and replace the infested plants. Plant removal is strongly recommended when the infested plant is an invasive species. See Invasive Plants to Avoid Buying

  • Identify which species is present so you know how many generations occur and when control measures will be most effective.
  • Apply the fewest treatments at key times to protect natural enemiesSince crawlers (hatchlings) roam around with no protective shell, they are much more vulnerable to minimal-toxicity pesticides than settled scale.
  • For species with overlapping generations, monitoring and treatment will need to occur several times a year to be effective.
  • For trees or large plants or if using a restricted-use insecticide is needed, hiring a professional pesticide applicator is necessary.  

There are two main windows of opportunity for you to apply pesticides to control scale on outdoor plants - during crawler activity and during the dormant season when juveniles overwinter. Both may be needed for achieving good control.

Always follow product label instructions, and remember that dead scales do not fall off right away; you won’t necessarily see immediate improvement after an application.

Growing season: targeting crawlers

  • Use horticultural oil at a 1% dilution rate. 
  • Coverage must be thorough, coating all bark or leaf surfaces (upper and lower).
  • Repeat applications will be needed if the window of crawler activity is long. Follow product label instructions regarding how long to wait between sprays.

Dormant season: targeting overwintering stages

  • Use a dormant oil application upon leaf drop in late fall or before bud-break in early spring. Check the product label for the proper temperature range for application. 
  • Coverage must be thorough, coating all bark surfaces (and foliage for evergreens).

Horticultural oil tip: Test a small area first on evergreens, and do not use oil on plants with blue foliage (like certain junipers, spruces, and cypresses) as this will remove the leaf wax that gives them their color.

Hiring a professional pesticide applicator

Some pesticides are restricted-use, meaning that only certified applicators may obtain and apply them, so you may need to hire a credentialed landscaper or arborist. A professional should also apply any treatment - even minimal-toxicity products - that would need to be applied over your head. A combination of dormant oil applications and the use of systemic or growth-regulating insecticides is the most effective approach.

They may recommend an Insect Growth Regulator (IGR), which interrupts maturation and prohibits the scale from reproducing.

  • The peak of first-generation crawler activity is when IGRs should be applied. This can be applied in combination with a standard (1%) or weakened (0.5%) concentration of horticultural oil.
  • IGRs are slow-acting, so visible results will not be immediate.
  • Two or three applications will probably be needed to cover a long crawler activity period for certain scale species.

The other option for professional application is a systemic insecticide, which is absorbed by the plant and then ingested by the scale as they feed.

  • It should be applied at the first sign of crawler activity and may control scale throughout the rest of the season.
  • If this application occurs after the bloom of the scale’s host plant, it should have minimal impact on pollinators.
  • For infestations of late-blooming trees and shrubs, however, consider alternative treatments to avoid harming pollinators.

Author: Miri Talabac, Horticulturist & Coordinator, HGIC
2021