Photograph courtesy of the University of Nebraska Adult after emerging from the tree. Pupal case on the left.

Typical view of borer-infested trunk and lower branches of European Ash. Note general marred appearance, exit holes and swelling Photo by Curtis Swift

The ash borer Podosesia syringae Harris (classified as Podosesia syringae fraxini Lugger by some authors) is a serious pest of the European ash (Fraxinus excelsior) planted throughout Western Colorado. Emerging in spring, the adults mate, lay eggs and the resulting larvae bore into and 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 April 22 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.

The 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 one millimeter long 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 Canada.

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 indicating the severity of the problem.

Sprays need to be applied before the eggs hatch and the larvae move into the safe environment under the bark. Solomon reports that the adults emerge as early as mid-December in southern Florida, February in northern Florida, mid-March to mid-July in western central Mississippi and May through June in the Canadian Prairie Provinces. Due to the variation in emergence of the adults, the timing of sprays is best determined by the use of pheromone traps.

Gene Nelson, former entomologist with the Colorado State University Extension Tri River Area Grand Junction office, with assistance from Alan Lundy, Grand Junction Forestry Department, trapped this insect for a number of years to determine its emergence date. Once that date is determined, sprays can be effectively applied. Pheromone traps are use to attract the male of the species and set the respective spray date each year. The decision on when to spray is only a guess when based on calendar days (i.e. the 1st of May every year) and can be significantly off from the actual biological event if local weather conditions are much cooler or much warmer than normal. The use of pheromone traps helps pinpoint this date.

Information on control measures from from the Dakotas indicate that pheromone traps should be set out on May 15. Dix, et. al. (1978), indicate that in North Dakota, emergence of the adult varies from mid-May to early June and sprays need to be applied from the latter part of May to the first week of June. If these recommendations were followed in the Tri River Area, sprays would be too late to control infestations of this insect pest.

The literature also 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.

Products containing the insecticide permethrin have been found to provide excellent control if applied at the proper time.

Dix, M.E., A.D. Tagestad, D.G. Nielsen, and F.F. Purrington. 1978. Biology and Control of Lilac Borer in the Northern Great Plains. Research Note RM-358, USDA - Rocky Mountain Forest and Range Experiment Station.
Dix, M., A.D. Tagestad, J.D. Stein, and M.E. McKnight. 1979. Insecticidal Reduction of Carpenterworm and Lilac Borer Infestations in Green Ash in North and South Dakota Shelterbelts. Farm Research, Vol. 36, No. 5, pp. 26-29.
Johnson, W.T. and H.H. Lyon. 1976. Insects that feed on Trees and Shrubs: An illustrated Practical Guide. Cornell University Press.
Solomon, J.D. 1995. Guide to Insect Borers in North American Broadleaf Trees and Shrubs. USDA Forest Service Agriculture Handbook, AH-706.