Insects are a natural part of forest ecosystems, but population surges of certain species can significantly threaten tree health and survival. Pests cause damage differently based on their feeding habits and life cycles. The mechanism of attack—external chewing, internal tunneling, or fluid extraction—determines the severity of the damage and the tree’s ability to recover. Understanding these varied interactions is key to protecting the trees in a landscape.
Pests That Consume Leaves and Needles
Insects that consume a tree’s foliage, known as defoliators, impair the tree’s primary function of photosynthesis. This external damage is highly visible, appearing as chewing, skeletonizing, or mining the leaves. Healthy deciduous trees can often withstand a complete defoliation event, especially if it occurs late in the season, because they can regrow their leaves the following year.
Repeated defoliation over several consecutive years significantly depletes the tree’s stored energy reserves, making it vulnerable to other stressors and secondary pests. The most common defoliators are the larval stages of moths, butterflies, and sawflies, such as gypsy moth and tent caterpillars. Young gypsy moth caterpillars may skeletonize a leaf, but mature larvae consume the entire leaf blade.
Beetles like the Japanese beetle also skeletonize leaves, leaving a lacy, brown appearance that causes premature leaf drop. Defoliation of coniferous trees is often more severe than on hardwoods, as conifers do not typically refoliate, which can lead to branch dieback or total tree loss.
Pests That Damage the Vascular System
Insects that bore into the trunk and branches attack the tree’s vascular system, which includes the xylem and phloem. This internal damage is often fatal because it effectively girdles the tree, cutting off the flow of water and nutrients. Larvae of beetles and moths, known as borers, cause this damage by tunneling through the inner bark and sapwood.
The Emerald Ash Borer (EAB), an invasive metallic-green beetle, is responsible for the death of millions of ash trees across North America. EAB larvae create serpentine, S-shaped tunnels called galleries just beneath the bark, disrupting the phloem and xylem. This feeding prevents the tree from transporting sugars and water, leading to branch dieback that often starts in the upper canopy.
Bark beetles, such as the Mountain Pine Beetle, also target the inner bark and often introduce pathogenic fungi. These fungi, like the blue stain fungus, colonize the sapwood and actively block the water-conducting tissues of the xylem. The combination of tunneling damage and fungal infection overwhelms the tree’s defenses, leading to rapid decline and death.
Pests That Extract Tree Fluids
A third group of pests uses specialized piercing-sucking mouthparts to extract sap directly from the leaves, stems, or bark, weakening the tree over time. These insects access the nutrient-rich sap from the tree’s circulatory system. Common examples include:
- Soft scale insects
- Aphids
- Mealybugs
- Leafhoppers
These insects consume large volumes of sap to obtain protein and excrete the excess sugary liquid as “honeydew.” This honeydew creates a sticky, glossy residue on leaves, branches, and surfaces below the tree. While fluid extraction can cause leaf wilting and yellowing, the greater problem is the resulting fungus.
A black, crusty fungus known as sooty mold grows rapidly on the honeydew deposits. Although the mold does not directly infect the tree tissue, a heavy coating can shade the leaves, interfering with photosynthesis. Some pests can also transmit plant viruses as they feed, introducing disease risk.
Recognizing the Signs of Infestation
Identifying an insect problem early depends on recognizing the physical evidence left behind by the pests and the tree’s reaction to the attack. One common sign of internal tunneling pests is frass, the fine, sawdust-like droppings produced by larvae as they chew through the wood. Frass can often be found in bark crevices or accumulated at the base of the tree.
For borers, the shape of the exit holes in the bark is a specific clue to the pest type. Flat-headed borers, such as the Emerald Ash Borer, leave a distinctive D-shaped hole when the adult beetle emerges. Other borers, like round-headed borers, create perfectly circular exit holes.
Trees attempt to repel invading bark beetles by pushing them out with a flood of sap, which hardens into masses on the trunk called pitch tubes. Abnormal growths on the leaves, known as galls, indicate an insect (often a wasp or midge) has stimulated the plant tissue to create a protective home for its larvae. Canopy symptoms, such as flagging (sudden dieback of a single branch) or general leaf thinning, are often the first visible indicators of a deeper, internal problem.