Identifying Insect Damage on Ash Trees

Ash trees are one of the most widely planted trees in Colorado.  Ashes are tolerant of difficult soils and tough weather conditions.  Ash availability, successful planting, and moderate growth rate encouraged the extensive use in the landscape.  Many were planted as replacement trees when American elms fell to Dutch elm disease in the early 1970’s.

In Colorado, many insects are associated with ash causing both aesthetic and health harm.  With the discovery of emerald ash borer in Colorado, ashes are being closely scrutinized for insect infestations.  Emerald ash borer is

  1. The most injurious insect of ash in North America;
  2. Thus far has only been identified in Boulder; and
  3. Signs of this insect are the most difficult to detect of any of the current pests.

Learn more about our Ash Borer Beetle Treatments.

Aphids and Scales

Aphids and scales are known as the piercing and sucking insects.  Their mouthparts pierce plant cells and suck the sap from the tree.

Aphids are a nuisance and can be a serious pest.  In ash, the aphids can be found by uncurling the leaves revealing a sticky mass of 1/8 inch long pale colored, pear shaped insects.  When abundant, aphids remove large quantities of sap, reducing the growth and vigor of the plant. The aphids cause the leaf to curl and distort.  Heavy infestations will result in leaf drop, sparse canopy, and also a sticky honeydew secretion from the leaves.

Scales include oyster shell and common falsepit.  Scales are unusual looking and many people do not at first recognize them as insects. Oyster shell is aptly named as the scale resembles an elongated ¼ inch brown shell.  Common falsepit scales are about 1/6 inch in diameter, rounded and cream colored.  Scale insects attach themselves to the bark of twigs and branches causing injury by sucking plant juices, sometimes killing a branch or entire stem.

Major Wood Borers of Ash

The wood borers are difficult to distinguish from each other as they develop inside the tree, insects are not often seen outside the tree and the tree damage each insect causes is similar.  Different species of wood boring insects may attack the same tree.

Wood borers of ash may be categorized by the holes they leave when they exit the tree. Emerald ash borer and the flatheaded appletree borer both produce a very distinct “D” shaped insect exit hole.  The larvae of these insects have a distinct flat head.  Lilac ash, red headed, and the banded ash borer all produce a ¼ inch roundish exit hole.  The ash bark beetle forms a very small 1/16 inch round exit hole.

 Emerald ash borer.   It is highly unlikely that you will see the signs or symptoms of emerald ash borer in Denver before it infests an individual tree. Where emerald ash borer is common in the Midwest and Eastern United States, by the time you see the “D” shaped exit holes at eye level, the tree is very infested and often cannot be saved with insecticide applications.

In the spring of the year, the adult emerald ash borer emerges from the tree briefly feeding on leaves.  The feeding as well as the insect is difficult to detect.  Insects mate and females lay eggs on the bark surface of ash trees. Initially, most egg laying is concentrated in the upper canopy of the tree and near branch crotches areas.  With continued insect attacks, egg laying occurs lower in the canopy. Eggs hatch and the larvae bore into the tree.   Larvae of emerald ash borer are cream colored, have a small head with pronounced dark jaws, slightly flattened area behind the head, and have a very elongate and somewhat flattened body.  Overwintering emerald ash borer bores deep into the wood. Young larvae may take two growing seasons to mature.  In the spring of the year, the insect matures into an adult emerging through a characteristic “D” shaped exit hole completing the life cycle.

Symptoms of emerald ash borer infestations begin as a thinning of the ash canopy.  Dead branches are a symptom of advanced infestations. Trunk and twig sprouts form.  The bark begins to crack revealing the insect galleries.   Also present are D-shaped exit holes in trunk and branches.   Woodpeckers also visit the tree in search of the young emerald ash borers as a food source.  Sawdust is not expelled in the exit holes.

 Lilac/ash borer.   Without question, the lilac/ash borer is the most commonly encountered wood boring insect in ash.  Adults of this insect emerge from trees during warm days in mid spring.  The adult borer mimics paper wasps and may be seen flying about the tree.  Adult lilac/ash borer is a moth and does not feed on the ash foliage.  When adults emerge from the tree the pupal skin is often pulled out and will remain for some time partially extruded from the trunk. The hole through which the adults emerge is generally round and somewhat irregular. Lilac/ash borer is the only ash wood boring insect on ash that produces sawdust visible on the outside of the trunk.  The adult lays eggs in bark crevasses.  Eggs hatch and the insect bores into the tree. Larvae of lilac/ash borer are cream colored with a dark head.   The larvae overwinter in the ash tree emerging the following year completing the life cycle.

Lilac/ash borer will produce more generalized riddling of the trunk and limbs than do the other borers associated with ash. External symptoms of lilac/ash borer injury often include some areas of swelling on the trunk and some sprouts. Branches are killed.  Repeated injuries weaken branches making them susceptible to breakage.

 Minor Wood Borers of Ash

 Flatheaded appletree borer.   The flatheaded appletree is associated with several trees in Colorado including oak, maple, ash, and apple.

Adults of this insect emerge from trees during May and June. They then move to the canopy of ash feeding on the foliage.   After mating, the females will lay eggs on the bark surface of ash trees. Egg laying is concentrated on limbs that are showing decline or injury.

In ash trees flathheaded appletree borer is almost entirely restricted to limbs that are previously injured or in decline and not a primary pest of ash.   Adults also produce D-shaped exit holes in branches which resemble those made by emerald ash borer.

Larvae of the flatheaded appletree borer are cream colored, have a small head with pronounced dark jaws, feature a broadly flattened area in the behind the head, and have a very elongate and somewhat flattened body.

The damage includes trunk injury, stem cankers and a general degradation of the tree health.

Redheaded ash borer.  Redheaded and banded ash borers target weakened and dying trees.

The adult is reddish brown with four narrow yellow bands and may be seen congregating on weakened trees in the spring.

Redheaded ash borer is almost entirely restricted to ash trees that are seriously injured or in advanced decline and it is not a primary pest of ash.

Bark Beetles

Ash bark beetle.  Adult bark beetles cut egg galleries under the bark and larvae tunnel perpendicular to the gallery.

Egg galleries run across the grain and often have two “arms” with a central chamber in the middle.   Also characteristic of these insects are that small “ventilation holes” perforate the bark above the egg galleries.   The tunnels are almost invariably colonized by fungi that stain the wood a rich brown color around the feeding sites.   Sap may ooze from wounds in twigs, staining the bark.

These injuries can girdle and sometimes kill branches.   Entire trees may be killed by bark beetles.  Injured limbs and heavily shaded branches in the interior of the tree are most commonly attacked. Ash bark beetles may infest almost the entire tree. After a year or two, the bark of infested branches may come loose revealing the insect galleries.