Scale insects are one of the more damaging insect pests of landscape trees and shrubs. The difficulty in identifying and controlling Scale insects stems from the fact that there are two general groups of Scale insects, soft scales and armored scales, and that this insect doesn’t even look like an insect at all. Additionally, the lifecycle of each species may dictate different timing of treatments to assure Scale insect control.
Scale insects are unique in that they spend most of their life as an immobile insect hidden under a tough skin or waxy coating that is impenetrable by insecticides. Scale insects damage trees and shrubs by sucking plant juices. Host plants gradually lose health and vigor as a result of the feeding done by Scale insects. At first the foliage on infested branches will be stunted. Eventually, twigs, then entire branches will dieback.
There are two general groups of Scale insects, soft scales and armored scales. Soft scales excrete a sugary liquid called honeydew. Ants and hornets are often attracted to the honeydew on plants infested with soft scales. A black fungus called sooty mold will often grow on the honeydew that drops on whatever is below infested branches. Armored scales do not produce honeydew and are much more inconspicuous than soft scales.
TREES AND SHRUBS ATTACKED BY SCALE INSECTS
Scale insects can be found on many different landscape trees and shrubs. A short list of common Scale insects and their hosts includes:
Magnolia Scale on Magnolia, Locust, or Tulip-tree.
Oystershell Scale on Purpleleaf Plum, Ash trees, or Maple trees.
Euonymus Scale on Euonymus shrubs, vines, or ground cover.
Obscure Scale on Oak trees.
Cottony Maple Scale on Maple trees.
Lecanium Scales on Maples, Locust, Crabapples, Sycamores, or Linden trees.
Pine Needle Scale on pine or spruce trees.
Tuliptree Scale on Tuliptree or Magnolia.
Calico Scale on Maple, Dogwood, Sweetgum, Magnolia, or Locust trees.
Pine Tortoise Scale on Scotch Pine or Austrian Pine.
SYMPTOMS OF SCALE INFESTATION
Trees or shrubs infested with Scale insects will initially exhibit stunted growth. As the Scale infestation builds twigs, then entire branches will dieback. Scale insects may be found on the foliage, twigs, or trunks of infested plants.
HOW TO CONTROL SCALE INSECTS
Insecticide control of Scale insects can be done with foliar treatments or with soil drenches of systemic insecticides.
Dormant oil treatments, applied in the fall or early spring, will smother some of the adult Scale insects and their eggs. Highly refined oil sprays can even be used in the summer to control newly hatched Scale insects – called crawlers.
Foliar insecticide treatments for Scale insect control should be directed at newly hatched Scale insects. While they are in the crawler stage Scale insects are susceptible to foliar applied insecticides. The treatments should be timed to when the Scale insect eggs will be hatching. This timing varies according to what species of Scale insect is infesting the tree or shrub.
Systemic treatments need to be applied well in advance of when the Scale insect eggs will be hatching. The systemic insecticide needs to be given time to be absorbed by the plant so it can be moved in the plant to where the Scale insects are feeding. Different systemic insecticides are used depending on if the target insect is a soft scale or armored scale.
Scale insects cause the most damage on weak or stressed plants. Make sure infested plants are watered, mulched, and, if necessary, fertilized. If possible prune out heavily infested branches.