Visual identification is the first step in determining if an arthropod associated with a woody plant is harmless, beneficial, or a serious pest. While trees host thousands of insect species, only a few distinct types cause recognizable damage that warrants concern. Learning to identify the appearance of the insect itself, or the unique visual evidence it leaves behind, helps distinguish between a minor annoyance and a threat to the tree’s overall health. This guide focuses on the most common categories of tree-dwelling insects that leave clear signs of their presence.
Bugs That Tunnel Beneath the Bark
Insects that bore into a tree’s internal structure are often difficult to see, so damage signs are the primary visual evidence. These pests, including certain beetles and moths, spend most of their life cycle as larvae feeding on the inner bark or wood. Adult beetles are typically small, dark, and cylindrical, sometimes possessing a metallic sheen. Adult clearwing moths often mimic small wasps, having long, slender bodies and clear wings.
The most telling sign of borers is the exit hole they create upon emerging. Bark beetles leave numerous small, circular holes, giving the bark a “shothole” appearance. Metallic wood-boring beetles, such as the Emerald Ash Borer, produce a distinctive D-shaped exit hole. Larvae produce frass, a powdery mixture of wood particles and excrement, visible as fine sawdust in bark crevices or around the trunk base. Clearwing moth larvae push out a sawdust-like frass that mixes with sap, creating a gummy, sticky exudate near the tunnel entrance. The larvae themselves, if exposed by peeling back bark, are typically pale, legless grubs with a hardened head capsule.
Small, Clustered Sap Feeders
Sap feeders are typically small and found in dense clusters on stems, leaves, or new growth. Their feeding results in honeydew, a sticky, sugary waste product that coats plant surfaces below them. This honeydew often supports the growth of sooty mold, a black, velvety fungus that serves as a secondary visual indicator of infestation.
Aphids are pear-shaped, soft-bodied insects, usually green, yellow, or black, that cluster on the undersides of leaves or new shoots. Their limited mobility makes them easily recognizable. Scale insects, in contrast, look more like small, motionless bumps permanently attached to the bark or leaves. Armored scales are the smallest, protected by a hard, waxy cover, and they do not produce honeydew.
Soft scales are slightly larger and dome-shaped, possessing a waxy coating that is part of their body, and they excrete significant amounts of honeydew. A third group, mealybugs, are easily identified by their oval, segmented bodies covered in a white, cottony or powdery wax, often found clustered in leaf axils or stem joints.
Large Visible Defoliators
Defoliators are insects that chew and consume leaf tissue, and they are typically larger and more mobile than sap-feeding pests. The most destructive stage is often the caterpillar, the larva of a moth or butterfly. Caterpillars have segmented bodies with three pairs of true legs near the head and fleshy prolegs on the abdomen, identifiable by their color patterns, stripes, or the presence of dense hairs or spines.
Larvae leave behind large, solid droppings (frass) that accumulate on leaves or the ground below the feeding area. Their feeding patterns range from completely consuming the leaf to skeletonizing it, which involves eating the tissue between the veins and leaving a lace-like remnant. Adult defoliating beetles, such as the Japanese beetle, are often conspicuously colored, with a metallic green body and coppery-brown wing covers, making them easily visible as they feed in groups.
Essential Features for Identification
To classify any unknown insect found on a tree, the first step is a basic anatomical check of its adult form. A true insect has a body divided into three distinct regions: the head, the thorax, and the abdomen. The head contains the antennae and mouthparts, and the abdomen is the largest region, housing the digestive and reproductive organs.
The thorax is the central segment where appendages are attached. Insects possess three pairs of jointed legs, totaling six, fixed to this region. Most adult insects also have one or two pairs of wings attached to the thorax, although some groups, like certain scale insects, lack wings entirely. By observing these three body parts, the number of legs, and the presence or absence of wings, one can quickly narrow down the identity of the organism.