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Scale insects on Minnesota trees and shrubs

Quick facts

  • Scale insects are common on many trees and shrubs.
  • Although natural enemies help keep them in check, scales sometimes can become numerous and seriously injure plants.
  • Identification of the scale species is important to choose the most effective way to manage them.
  • There are a variety of low impact and residual insecticides available to treat scales; correct timing is critical for successful management.

How to identify scales

Sticky dew-like layer on a green leaf
Honeydew produced by soft scales
A wasp feeding on the honeydew produced by scales
Wasp feeding on honeydew

There are two types of scales: soft scales or armored scales.  Both types live under a covering they produce.

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.

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 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.
  • An exception to this are pine needle scales; they have bright white shells in contrast to the green needles on which they feed.

Biology of scale insects

Scale insects have three stages in their development: eggs, crawlers, adults.

  • They first start as eggs laid under the shell of the females.
  • The eggs hatch into nymphs, commonly called crawlers.
  • They move from under the cover of the scales to colonize other sites on the plants.
  • There are two instars (growth stages) of nymphs.
Lecanium scale eggs laid under the shell of the female scale
Lecanium scale eggs laid under the shell of the female scale.
Lecanium scale crawlers on the foliage of the tree before they migrate back to the stem.
Lecanium scale crawlers on the foliage of the tree before they migrate back to the stem.

The crawler stage of scale insects is the primary method of dispersal. The crawlers are small enough to be blown by wind to other plants and are also able to be transferred on feet of birds and other animals that frequent the trees on which they feed.

Soft scales

  • Crawlers typically move out to feed on foliage and the second instar (growth stage) nymphs return to twigs and branches to mature and build their own protective shells.
  • Retain their tiny legs and are able to move but very slowly.

Armored scales

  • Travel to new plant tissues (rarely leaves) and settle down to feed, creating new coverings near the adult scales.
  • Lose their legs and become non-mobile, remaining in the same location for the remainder of their life.

When fully grown, female scales mate with winged males (rarely seen) and lay eggs under their protective shell cover and die.

Different scales overwinter in different stages.

  • Soft scales overwinter as nymphs or adults
  • Armored scales overwinter as eggs under the hard shell of their mothers.

This difference in overwintering habits as well as timing of the crawler stages of each of the scale species is very important to determine the best management methods as well as the best timing of management.

Injury caused by scales

Scale numbers can increase rapidly after just a few seasons.  A plant can literally be covered with scales before the problem is noticed.  Injury doesn’t usually become obvious until scale populations reach outbreak levels.  

Sooty mold growing on honeydew on a tortoise scale infested pine.
Sooty mold growing on honeydew on a tortoise scale infested pine.

Armored scale insects are typically more damaging to their hosts than soft scales. 

  • Their feeding on branch tissues kills individual cells. 
  • Cell death disrupts the transportation of materials through the tree limb and can cause the branch to die. 
  • In heavy infestations armored scales feeding on branch tissues can cause the death of entire plants. 
  • Limb death is often the first indicator that there is a scale problem on the plant.
Crabapple tree in decline from infestation of armored scales.
Crabapple tree in decline from infestation of armored scales.

Armored scales that feed on foliage kill individual cells as well.  Damage is observed as purpling and yellowing of foliage and loss of foliage to which the scales are attached.   

Soft scales are frequently identified by the honeydew they produce on infested plants. 

  • Honeydew supports the growth of sooty mold on the leaves of plants, but it is not directly damaging to plants. 
  • In severe soft scale infestations damage occurs on individual limbs before it impacts the entire canopy. 
  • Observable soft scale insect damage can range from deformed leaves and branch tip dieback to limb death and loss of sections of the canopy.

How to protect your trees from scales

Scale insects are usually kept below damaging numbers by natural enemies like lady beetles and tiny parasitic wasps.  However, when biological control is not sufficient, scale numbers can become abundant, requiring management. 

It is important to properly identify the scale species present. This will tell you when scale crawlers are most likely to be active.  The crawler stage is when scales are most vulnerable to management.

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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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Author: Jeffrey Hahn, Extension entomologist

Reviewed in 2021

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