Scale insects are a serious threat to plant health.
Scale insects are small, highly modified animals that have little resemblance to most insects. Because of their small size and often cryptic appearance, large numbers may be present without being noticed.
While a few scale insects are little cause for concern, large numbers may be harmful to the host plant. By closely watching your trees and shrubs, we can often catch a scale infestation in its early stages and take appropriate action.
How do scale insects damage my trees?
Scale insects injure plants as they feed. Long, threadlike mouthparts are inserted into the host plant and used to suck plant sap from the tissues. If large numbers are present, the insects can remove so many nutrients from the plant that it does not have enough left over to carry on its own metabolic activities.
Large amounts of honeydew, a sugary waste product, may also cover leaf and other surfaces located beneath a scale infestation. A fungus called “sooty mold” will use the honeydew for food and can cover leaves, sidewalks, patios or other surfaces, giving them a discolored appearance.
Two Categories of Scale Insects
Armored scales: Secrete a thick, waxy covering over the tops of their bodies, but do not produce honeydew. This covering, combined with their own cast skins, serves to protect the insect from the environment as well as from contact pesticides. Therefore, it is difficult to control scale insects with most contact insecticides after the waxy covering has formed. Examples of armored scale includes: pine needle scale, oystershell scale, euonymus scale, and obscure scale.
Soft scales: May produce waxy secretions and excretes excess sap as honeydew. The wax is granular and does not serve as a protective barrier as do the armored scale secretions. Examples of soft scales insects include: lecanium scale, Fletcher scale, cottony maple scale, and magnolia scale.
There are many different scale insects and their life cycles vary. As a general rule, however, armored scales usually spend the winter in the egg stage or as mature females. The eggs are located beneath the waxy scale covers of the female scale insects. The eggs usually begin to hatch during late May or early June.
Life Cycle of Scale Insects
Soft scale insects overwinter as young nymphs attached to the twigs of the host plant. The nymphs complete development, and the females lay their eggs during late spring. The eggs begin to hatch later in the year than those of armored scale insects; usually during late June and July.
Newly emerged scale insects are called crawlers. They receive this name because unlike mature scale insects, they have the ability to crawl and move from place to place. The newly emerged crawlers are in the dispersal stage of scale insects. The crawlers move to different portions of the host plant, insert their mouthparts and begin to feed. Armored scale crawlers then begin to build the waxy coverings over their bodies. The covering enlarges with each molt.
The crawlers of most soft scale species leave the twigs where the eggs were located and move to the leaves to begin feeding. The crawlers feed on sap from the leaves all summer, but return to the twigs to overwinter. They leave the foliage before the leaves drop in the fall. If they do not, they will fall from the tree with the leaves and eventually die.
Signs & Symptoms
Severe scale insect infestations cause:
- Premature leaf drop and branch dieback
- Canopy thinning
- Undersized and sometimes yellow-mottled leaves
- Black leaves, bark, sidewalks and other surfaces from sooty mold growth