by Karen Weiland, Advanced Master Gardener
Scale insects are sap-feeding insects named for the scale or waxy, shell-like covering that hides their bodies. There are more than 60 different kinds that occur in Indiana and yet are many times not noticed or ignored until tree or shrub branches start to die. When you look real close at these dying branches they may be covered with small bumps which are hiding scale insects that are damaging the plant by sucking out the plants sap.
Scale insects are very small with their color, shape, texture and other features varying with each specie. They are generally divided into two categories: Soft scale, which produces a soft, thin, cottony, powdery or waxy layer over themselves that cannot be separated from the insect body. Soft scale insects excrete a sugary liquid called honeydew. You will find items placed beneath an infested tree will often become covered in a sticky mess of honeydew. This substance may encourage the growth of a black fungus called sooty mold which can shade leaves and reduce plant growth. Armored scales have a hard, shield like cover which is composed of shed skins and wax that hides the body but is not actually attached to the insect itself. Armored scales do not produce honeydew.
After scale eggs hatch, they may look like dust on the surface of the plant. Nymphs, which are tiny, immature scales and adult females for most species will feed at the same location for the rest of their lives. Adult males, which are tiny, gnat-like insects, fly to new females to mate.
Scale insects are not easy to control with traditional contact insecticides because of the covering that protects their bodies. Parasitoids (parasitic wasps) and natural predators such as lady beetles, can feed on and greatly reduce scale insect populations. Physically removing scale insects on small trees and bushes is possible but impractical on larger plants. Branches or stems that are heavily infested can be pruned out and discarded.
If you decide to use a chemical control, you need to keep in mind that this material can kill the scale’s natural enemies which provide lasting control in your landscape. Systemic insecticides can be injected into the plant, or are applied as a foliar spray, drench or soil injection under an infested tree or bush. This type of insecticide circulates through the plant and can control both the young crawlers and adult scales with less impact on natural enemies and less risk of pesticide drift. Horticultural oil may be an alternative. It works by smothering scales and can kill them after they have settled while the scale body is still somewhat clear. After the oil dries, it is not toxic to natural enemies that can fly back to an infested plant and feed on the remaining scales. Always read and follow label directions on mixing, using and application safety.
As always, Happy Gardening!!
More information about gardening and related subjects can be found online at www.purdue.edu. The Purdue University Cooperative Extension Service can be reached at 499-6334 in LaGrange Co., 636-2111 in Noble Co., 925-2562 in DeKalb Co. and 668-1000 in Steuben Co.
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