Introduction to Scale Insects

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.   

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.

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.

• Natural enemies or Pollinators and Beneficial Insects (“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.

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 enemies. Since 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.

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.