Although minuscule compared to their victims, tiny insects can devastate---even kill---a large tree or shrub in a single growing season. Whether they're sucking, chewing or boring, immediate action is needed to maintain the health and vitality of your trees and shrubs. Proper identification will help you form your plan of attack to protect your trees and shrubs, since different types of insects may require different types of control.
Tent caterpillars and gypsy moths are large enough to remove by hand. Insert a broomstick or sturdy stick into the center of the nest. Twist the sticky web fibers around your stick and remove the mass of webbing and caterpillars. Scrape the nest into a 5-gallon bucket containing soapy water.
Oil sprays work by suffocating insects. Effective against common ornamental tree and shrub pests such as scale insects, aphids, mites and adelgids, many horticultural oils can also be used on fruit and vegetable crops. According the Colorado Cooperative Extension, "Oils pose few risks to people or to most desirable species, including beneficial natural enemies of insect pests...Toxicity is minimal, at least compared to alternative pesticides, and oils quickly dissipate through evaporation, leaving little residue."
For severe infestations that are resistant to manual or non-toxic control methods, there are several effective chemical sprays. Permethrin, a synthetic insecticide, is effective against borers, caterpillars and beetles. Permethrin, however, is not selective and will kill beneficial insects as well.
Bacillus thuringiensis (Bt) is a selective, biological caterpillar control. It is an effective and environmentally responsible choice because it does not harm other insects, fish, birds or animals. Bt is applied as a foliar spray. Control is somewhat slow---the caterpillars sicken and die over a 3 to 5 day period.
Hemlock woolly adelgids have a waxy outer surface that resists contact insecticides. Adelgids are best treated with a systemic insecticide. Imidacloprid is the most common adelgid systemic and it is applied as a soil drench or foliar spray. The insecticide is taken up by the shrub or tree's roots or absorbed through the leaves. Control is long lasting and thorough. Systemic insecticides are also recommended for aphids.
Birds and beneficial insects are the most environmentally friendly method of tree and shrub insect control. Birds play an important role in integrated pest management. In the words of the Smithsonian National Zoological Park, "Birds are technologically advanced, highly motivated, extremely efficient, and cost-effective, insect-pest controllers."
Smaller but no less important, ladybugs, lacewings and mantids are formidable predators, feeding on aphids, mites and scale insects.