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Which Tree Insects Should You Watch Out For?

Beware of creepy-crawlies in your trees that can damage foliage and tree structure and cause life-threatening diseases. Uncontrolled infestations of tree insects may require treatment by a professional. Left unchecked, these pests can cause the death of weakened trees, requiring removal and replacement.

There are three types of tree insects to watch for in the Northwest—sapsuckers, defoliating insects, and wood-boring insects. All inflict their damage by feeding on different (tasty-to-them) parts of the tree.

Sap- and leaf-feeding insects weaken the structure of trees, making them prone to more damaging diseases, such as fungus. Wood-boring insects infest the inner bark and tissue that transports nutrients and water from the roots throughout the structure, clogging the delivery system, in effect, starving the tree.

Signs of tree insects differ according to each pest but are easily recognizable even in the early stages. Treating your trees and ridding them of pests early is fairly simple and affordable.


Aphids are tiny, soft, pear-shaped or oval-looking insects. They can be almost any color. Infestations of these tree insects can be spotted in clusters on the undersides of leaves, usually nearer the stem. Tender new shoots and leaf buds are also a favorite feeding spot for aphids.

Aphids feed on the sap of the tree by sucking it out with a needle-like proboscis. They excrete a sticky substance known as honeydew. A tree with a serious infestation of aphids will literally drip honeydew from its leaves, often attracting a secondary infestation of ants. Other visible signs include leaves or needles turning yellow, curling leaves, and leaves dropping out of season.

Aphids are common in the Pacific Northwest and may be seen on dogwood, lilac, maple, ash, linden, and tulip trees, as well as pines, spruce, and firs. Aphids also spread fungal disease, a much more serious threat to trees than the aphid infestation itself.

If you have ants farming honeydew from aphid activity, eliminate the ants first. Ants protect the tree insect from many of its natural predators, such as ladybugs and green lacewings, which eat as many as 60 aphids a day. Ant traps work well. Just be sure to use them with caution if children and pets can reach them.

Spraying aphids off with a strong blast of water from a hose is also an effective treatment. Commercial applications, such as neem oil or insecticidal soap, work well if the tree is saturated until the insecticide is dripping from the leaves. It usually takes several applications to rid the tree of all the aphids. Stronger insecticides are recommended only as a last resort.


Elm leaf beetles are about a half-inch in length and olive green in color, with yellow margins. An infestation of these leaf-munching tree insects won’t kill your tree directly, but it can weaken the structure, making the tree more susceptible to diseases or damage from other tree insects.

An infestation of elm leaf beetle is easily identified by brown, lacy-looking or skeletonized leaves that drop prematurely. Close inspection to find the feeding larvae, by a professional service like Mr. Tree, will confirm a beetle infestation. Elm leaf beetles can be controlled with oils, soaps, and applications of systemic insecticides.

Bronze Birch Borers

This tree insect is categorized as both a beetle and a wood-boring insect. It lays eggs in cracks in the tree bark, beneath the bark, and in any wounds from damage or careless pruning. The hatched larvae chew deeper into the phloem and xylem tissue, compromising the tree’s water and nutrient–delivery system.

Adult bronze birch borers are slender and dark olive to bronze in color with a metallic iridescence. At about 1/2-inch in length, the female is slightly larger than the male. Light brown larvae are approximately 3/4-inch long.

This tree insect attacks many varieties of birch, though European and Asian birch are more susceptible. Damage to vulnerable species can be severe, often resulting in the death of the tree. Native species with non-white bark are somewhat more resistant to the beetles.

Early signs of infestation include crown thinning and dieback. If not checked, the dieback will progress to the main branches and eventually the trunk, killing the entire tree. Examine trees for D-shaped exit wounds in the bark, made when mature beetles emerge in late spring or early summer.

Emerald Ash Borer


Another wood-boring beetle, this tree insect gets its name from the bright, metallic-green color of the adult insects. Like the bronze birch borer, the feeding larvae damage inner tree tissue, disrupting the delivery of water and nutrients.

While the emerald ash borer has not yet made its way to the West Coast, there’s a growing concern over the possibility. Awareness of the emerald ash borer and strict adherence of non-transportation of cut wood will help protect our ash populations from the devastation seen on the East Coast and in the Midwest.

Early signs of infestation are crown thinning and die-off that progresses, if untreated, resulting in tree death.

Worms and Caterpillars

There are numerous species of worms or caterpillars that create webs or “tents” in the branches of trees. These tree insects feed on the foliage and can strip acres of their favorite trees in a matter of days. They rarely cause severe damage unless affected trees are otherwise stressed; foliage will grow back naturally.

You may have heard of these tree insects as tent worms or tent caterpillars, webworms, or armyworms because they advance through an area with a precision attack, leaving skeletal trees of select varieties, while other species go unscathed.

Favorite trees of these marauding munchers include ash, aspen or poplar, cottonwoods, and some fruit trees. Infestations are unmistakable for the same characteristics that give the tree worm its colorful names—large webs woven in the V’s of branches and armies of ants crawling up and down the tree trunks and over the ground. In areas of severe infestation, roadways can become slick with the crushed worms.

Worm and caterpillar infestations are controlled by pruning out the webs of larvae and then burning or otherwise destroying them. However, in heavily infested areas, pruning can cause more damage than simply letting the worms work their way through. Insecticide treatment of young colonies is an option. Contact the professionals at Mr. Tree for more information.