The Ash Borer


Ash Borer AdultThe ash or lilac borer, Podosesia syringae is a serious pest of ash trees planted throughout Western Colorado. Larvae of the borers feed under the bark and into the wood of infested trees, weakening and often killing them. Urban trees are often attacked, and trees that are not treated often die.

The moths emerge during the spring, mate and lay eggs, before the larvae bore into the trunks of ash trees. They construct galleries in wood beneath the bark causing severe damage to the infested trees. Ugly scars, accompanied by enlarged or swollen areas are associated with repeated infestations. The adult ash borer emerges as early as the third week of April in the Grand Junction, Colorado area and residents need to have sprays on their ash trees 10 to 14 days after emergence to control this pest.

Description of Pest

Ash Tree TrunkThe adult Ash Borer is best known as a clearwing moth because the greater part of its wings are without scales and thus transparent. With their very long yellow and black hind legs they closely mimic wasps in their appearance and flight. This day-flying moth is from three-quarters of inch to an inch and one-half long.

Eggs are laid on the surface of the bark, usually within ten to fourteen days of the emergence of the adults. In Western Colorado the smoother bark European Ash (incorrectly called the Blue Ash) is the preferred host, but the other ash, privet (Ligustrum spp.), lilac (Syringa spp.) and mountain ash (Sorbus spp.) are also attacked.

The newly hatched larvae are white with amber-colored heads. These worm-like pests tunnel beneath the bark where they are safe from insecticide treatments. During the summer, larvae continue to feed eventually attaining a length of about 1 inch. This borer has one generation per year in the Tri River Area but a 2-year life cycle has been reported in northern areas.

The feeding injury of the larvae causes serious damage to the wood and bark tissue of the tree. Some trees in this area have contained 50 or more borers. The presence of many borers leads to stress on trees, and ultimately mortality.


Sprays need to be applied before the eggs hatch and the larvae move into the safe environment under the bark. The timing of egg hatch varies with local conditions. It hatches earlier in southern regions and lower elevations, and in years with warmer temperatures in the springtime.

Pheromone traps are used to monitor moth flights. Traps are available from several suppliers on the internet, and use a pheromone that attracts many species of clear wing moths. Homeowners who have only a few ash trees are safe applying trunk protective sprays in early May. City and pear employees who have hundreds or thousands of trees to protect should have their sprays completed before egg hatch begins, about three weeks after the initial moth captures.

Sprays are applied to the trunk of the trees as high as the sprayer can reach. It is essential that an insecticide that provides residual control is used. These would include materials with the active ingredients carbaryl or permethrin. Check to find insecticides labeled for use. Complete coverage is very important. The amount of residual control is dependent on the amount of active ingredient applied. Always read and follow label directions for use. Apply the product at the higher end of the label rate if there is a range of rates specified.

Older literature on ash borer recommends that sprays be applied to the trunk and branches to a height of 10 feet. We have found, however, that sprays applied to the lower ten feet of the trunk causes the adult insects to move higher in the tree possibly due to the repellent activity of the spray. Trees should be sprayed from top to bottom to avoid this problem.

This page was updated on April 26, 2014