Homeowners often notice their trees are under attack from defoliating insects. Many caterpillars, mites and beetles feed on the foliage of trees. Severe insect infestations may cause trees to suffer leaf damage or be completely defoliated. Cultural control methods may be combined with chemical control products to prevent or control damage from defoliating insects.
Japanese beetles are a common pest of many ornamental, fruit or shade trees. These beetles are metallic green with light brown wing covers. Japanese beetles often cause severe damage to trees because they typically feed in large groups. Japanese beetles usually begin their feeding in the later part of June and continue into early fall. Bagworm caterpillars are another common defoliator of many spruce, cedar and arborvitae. Bagworms are characterized by the brown silky bags they make from twigs and silk and place on host trees. This pest feeds on host trees for approximately six weeks, leaving their bag only to feed. Sucking pests such as aphids and spider mites often damage a wide variety of trees. Feeding on the leaves of trees with sharp mouthparts, these pests damage leaves and cause reduced tree vigor.
Japanese beetles consume flowers and buds on trees, skeletonize leaves and cause leaves to develop a lacy appearance. This insect has a very large host range, attacking over 300 species of plants. Japanese beetle larvae also damage trees by feeding on the roots. Trees with bagworm infestations often suffer severe feeding injuries. Evergreen needles may be completely stripped and entire leaves of susceptible trees are often consumed by this caterpillar. Large numbers of bagworms can totally defoliate a tree in a short period of time. Trees that cannot produce more foliage quickly often die as a result of repeated defoliation. Spider mites and aphids suck sap from tree leaves, causing them to stipple, yellow and become distorted. These types of insects excrete a sticky substance when they consume large amounts of sap. This substance is known as honeydew and it causes a condition called gray sooty mold to develop on host trees. Heavy coverings of sooty mold can cause trees to experience low vigor, due to a lack of sunlight.
Light infestations of Japanese beetles and bagworms are often controlled by hand-picking the insects from the tree. Japanese beetles are typically sluggish in the early morning, making this the best time to remove them. Remove bagworms by picking the brown, silky bag from the tree. The best time to remove bagworms is during the fall, winter or early spring before bagworm eggs hatch. Place Japanese beetles and bagworms in a bucket of soapy water after picking them from the tree. Spider mites and aphids are often controlled by spraying trees with a hard stream of water with a garden hose. This process knocks the insects from the tree leaves and should be repeated twice each week for maximum control. Moderate insect infestations may require washing the tree with liquid dish soap for control. Combine 1 quart of water with 1 tbsp. of liquid dish soap in a sprayer and wash your tree. Repeat this process once each week while insects are a problem.
Insecticides are available for the elimination of Japanese beetles and bagworms at your local garden center. Chemical control products work best when applied when pests are still small, making them more vulnerable to insecticides. Spider mites and aphids often require horticultural oil applications for insect control. Horticultural oils coat the insects, causing them to suffocate and die. These products must thoroughly coat the tree leaves for best results. Concentrate the application of horticultural oils to the undersides of tree leaves, as this is where sucking insects congregate. Heavy infestations of spider mites can require the use of miticides.