Wood Borers

• Perhaps the most destructive of all insect pests are the wood borers, also referred to as bark beetles. While these insect pests do not usually kill the tree, they rob the leaves and roots of valuable nutrients, making them susceptible to attacks by other insects or disease. Most of the damage done to trees is the result of the larvae burrowing into the bark and disrupting the trees' transport system. While some of the borers are tree specific, others will attack a variety of trees.
  
  

Leaf Eaters

• Insects like the elm span worm, gypsy moth, leafminer and tent caterpillar prefer to attack the leaves of trees and shrubs, causing them to turn brown. These insects -- some in their larval form -- do not often kill the tree or shrub, but they can strip most of its leaves, slow its growth and make it more susceptible to plant diseases. Infestations of these insects are usually controlled by spraying.
  
  

Sucking Insects

• Cicadas are one of the main types of sucking insects that affect trees and shrubs. If your tree exhibits dead twigs, malformed leaves or large numbers of yellow leaves, sucking insects may be to blame. Aphids and lacebugs are also included in this group of insects as are several varieties of scales. While some of these insects are specific to a single variety of tree, most will feed on and weaken numerous species.
  
  

Evergreen Pests

• Several insect species prefer evergreen trees to deciduous trees. Among those are sawflies, which are actually small wasps that feed in large groups on spruce and several varieties of pine. They are capable of totally stripping the limbs of trees in only a few days. Spruce gall adelgids are small aphidlike insects that suck the sap from several types of spruce trees. Spider mite infections are usually very noticeable due to pronounced webbing on infected trees.