Scale insects on Minnesota trees and shrubs

Quick facts 

  • Scale insects feed on many plants and appear as small bumps on the foliage, twigs and branches of many landscape trees and shrubs in Minnesota.
  • They blend in with the bark and leaves of plants and usually do not move around.
  • The scale cover is a waxy secretion that protects the insects.
  • Pesticides might not reach the insect because the scale acts as a barrier.
  • Proper identification of the scale species is important to choose the most effective pesticide and to determine the best time for spraying. 

How to identify scales

Sticky dew-like layer on a green leaf
Honeydew produced by soft scales

Scale insects have either soft scales or armored scales.

Both types live inside the scales they produce. Scales are not a part of the insect, but are a protective barrier.

Soft scales

  • Convex and about 1/8 to 1/4 inch long.
  • Feed on the phloem tissue of trees and shrubs.
  • Excrete excess sap from the phloem as a by-product called honeydew.

Honeydew

  • If you see honeydew, look closely for signs of soft scales.
  • Honeydew will stick leaves together and attract many insects (ants, wasps and flies) that feed on the excretions. 
  • Other insects like aphids and wooly aphids can also produce honeydew.

Armored scales

Brown ridge-like scales made by scale insects on a brown bark
Oystershell scales. Armored scales can blend in with the color of branches and stems
  • Smaller than soft scales (about 1/16 to 1/8 inch long) and usually oval or somewhat elongate.
  • Feed on individual plant cells (instead of the phloem) and do not produce honeydew.
  • Many armored scales match the color of the bark of their host tree and are not spotted until the damage is noticeable.
  • Pine needle scales are easier to find as they have bright white shells in contrast to the green needles on which they feed.

How to protect your trees from scales

Check for crawlers

White cottony patches on green pine needles
Pine needle scales are easy to identify on infested plants due to their white coloration

Start to look for the presence of crawlers just before they are expected to appear.

  • Shake infested branches over a white sheet of paper or white paper plate to look for reddish or yellowish colored crawlers.
  • Place double-sided tape on branches where scales are present. Check the tape for crawlers.
  • Check for crawlers at least once a week until they are discovered.

Using pesticides

A wasp feeding on the honeydew produced by scales
Many insects feed on the honeydew produced by soft scales

Natural enemies like lady beetles and tiny parasitic wasps control scale insects well. But, if numerous scale insects are present, chemical control might be required.

It is important to properly identify the scale species that is present. This will tell you the approximate time when scale crawlers are active.

The best time to treat most scale insects is after eggs hatch when the crawlers are first active and are searching for feeding sites, but before they begin producing their shells.

  • Apply horticultural oils in late fall and early spring (before bud break) to suffocate overwintering nymphs.
  • Spray crawlers with
    • Insecticidal soaps
    • Horticultural oils
    • Insect growth regulators: pyriproxifen and buprofezin
    • Residual pesticides: acephate, pyrethroids and carbaryl
  • Systemic pesticides are effective for scale control on soil or bark:
    • Use dinotefuran as a soil application or as a bark treatment in the spring.
    • Apply imidacloprid in the fall.

Combining systemic treatment with a crawler spray is the most effective method. Timing of these applications when the crawlers are active is crucial for effective control.

Pesticides and bees

To protect bees, take following precautions when using pesticides:

  • Be careful when applying systemic pesticides to hardwood trees and shrubs that are attractive to bees. Some examples are: linden/basswood, crab apple, sugar maple, as well as juneberry (serviceberry), pagoda dogwood, nannyberry viburnum and many other shrubs. 
  • Apply systemic pesticides after flowering to reduce pesticide exposure to bees.
  • If this timing does not match with the time when crawlers are active, use another control method.
  • Do not use systemic pesticides that are applied to the soil when bee-attractive flowers are planted next to trees or shrubs.

CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.

Scales on conifers

Open the drawers to find pictures and more information about the types of scales found on conifers in Minnesota.

  • Pine needle scale
  • Black pineleaf scale 
  • Small spruce bud scale
  • Fletcher scale
  • Pine tortoise scale
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Scales on deciduous trees

Open the drawers to find pictures and more information about the types of scales found on deciduous trees in Minnesota.

  • Oystershell scale
  • Scurfy scale
  • Lecanium scale
  • European elm scale
  • Magnolia scale
  • Cottony maple scale

       

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    Jeffrey Hahn, Extension entomologist

    Reviewed in 2018

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