Control of Borers in Planted Trees in the Prairie Provinces


Wood borers are important pests of trees and occur in such major groups of insects as beetles, moths, and horntails. Some species make burrows in the bark and wood, which become filled with fine, compacted frass. Most of this type infest only over-mature or otherwise weakened trees, and no effective applied control is known for them. Other species, however, make burrows in the bark and wood, which they keep open; since they are capable of attacking healthy trees, many are important pests in forests and in farm and urban plantings.

In the Prairie Provinces the ash borer Podosesia syringas fraxini (Lug.) and the carpenterworm Prionoxystus robiniae Peck, which have open burrows, are serious pests of several kinds of trees planted for shade, ornamental and windbreak purposes. Since they prefer trees with sunny exposures, open-growing plantings are more seriously affected than denser, shelterbelt plantings. For this reason, shade and ornamental trees in parks, on boulevards and on private and public properties are especially vulnerable.

Host Trees and Damage

The ash borer attacks green ash and mountain ash. A closely related form infests lilac. The carpenterworm, which also attacks green ash and mountain ash, occurs abundantly in poplars (including balsam poplar in natural stands) and readily infests American and Siberian elms. In other geographical regions oaks, black locust, maple, willow, cottonwood, boxelder and fruit trees are recorded hosts.

The borers devour portions of the inner bark and the outer sapwood; this produces dead, sunken areas on the trunks and branches, and frequently results in dead "stagged" tops. The aesthetic values of shade and ornamental trees thus affected are lost or greatly reduced. The tunnels in the wood, made by the borers, weaken the trees structurally and increase the hazard of breakage from winds. The burrows also permit moisture and decay to enter and cause further deterioration. Up to 50% of the green ash trees in some urban centres in the prairie region have suffered damage of this nature in varying degrees.

Signs of Borer Attack

Trees infested with the ash borer or the carpenterworm show the following symptoms:

  1. Holes 1/8 to one inch in diameter extending into the trunks or branches; the holes may be few and scattered or they may occur in groups, frequently at the base of the tree or at the lower branch levels. If the holes occur in groups they are often associated with dead and sunken areas on the stems and branches.
  2. Small to large masses of frass borings clinging to the trunks or branches, or occurring on the ground at the base of the trees. The frass is ejected by the borers in order to keep the burrows open and is most noticeable from May to July.
  3. Swollen bands or ridges on the stems and branches of small trees; especially noticeable on green ash trees with thin, smooth bark.
  4. Rough, enlarged areas on the trunks frequently covered with dead, loose bark which conceals the burrow openings and masses of dark, wet borings; chiefly noticeable on poplars which may also have dead branches and dying tops and on elms.

Borer Development and Description of Stages

The ash borer and the carpenterworm have four developmental stages - adult, egg, larva, pupa. The adults of both species are typical moths but differ greatly in size and appearance. The larvae are true caterpillars and bear several pairs of feet on the abdominal region of the body.

Ash Borer

The adults are wasp-like moths with clear or translucent wings. The males and females are alike. Their bodies are slender, dark brown or almost black with yellowish bands on the abdomen. The wings are narrow with a spread of approximately 1 1/4 inches. The legs are long and orange coloured. At rest, the moths frequently hold the end of the abdomen in a raised position. The eggs of the ash borer are elliptical in shape, gray or black and very small - 30 placed end to end would span only one inch. Ash borer larvae are tiny, white caterpillars with dark heads when newly hatched. The full-grown caterpillars are creamy white, about one inch long, with reddish-brown heads. The pupae are about 3/4 inch long and reddish-brown with small backward-projecting spines.


The adults of the carpenterworm, unlike those of the ash borer, are large moths; the females and males differ greatly in appearance. The female is grayish and stoutbodied with a wing-spread of three inches. The front wings are mottled gray and black; the hind wings are smoky. The overall coloration of the female blends remarkably with the colour of rough bark and makes her difficult to detect when at rest. The male is less Stout than the female with a shorter body and a wing-spread of two inches. The front wings are also mottled gray and black but the hind wings are yellowish-orange with black base, and margins. Carpenterworm eggs are oval-shaped, 1/16 inch long and light to dark brown in colour. The shell is hard with a netlike pattern of ridges.

The newly-hatched caterpillars are dark brown, slightly hairy with a large black head. They quickly attain a length of 1/8 inch. The full-grown caterpillars are two to three inches long, pinkish-white with brown heads. The pupae are brown and 1 1/4 to 2 inches long. They are equipped with circular bands of backward-pointing dark spines.

Life Cycles And Habits

Ash borer

The ash borer requires a minimum of two years in which to complete its life cycle. The moths begin to emerge from the host trees in late May and are present till late July in the prairie region. The earliest recorded date for Saskatchewan, obtained from field studies, is May 31; the latest is July 18. The eggs are laid in crevices in the rough bark on the trunks and branches, and in wounds in the bark. Injuries to trees from cultivation, pruning, hail, frost or snow attract infestation, and old burrow openings in the bark encourage re-infestation. All sizes of trees may become infested but smaller, younger trees appear to be the most susceptible. The parts of the tree most frequently attacked are the lower portions of the trunk to ground level and the area where the lower branches arise.

The larval period of the ash borer extends from July of the year when the eggs are laid to early May of the third year following oviposition. Most of the caterpillars start their burrows in cracks and crevices in the bark. Bits of fine frass and webbing indicate where the entries occur. Some also enter old burrows. At the end of the first season, when activity is discontinued, some of the borers will have penetrated into the wood; others may still be in the bark. During the second season the caterpillars enlarge and lengthen their burrows greatly. In large trees they bore into the sapwood at a slight incline for one or more inches, then turn upwards and outwards to terminate the burrows at the end of the season in or near the bark, a short distance above the level of the burrow openings. In small trees and in branches of large ones the burrows penetrate to the centres and then rise for several inches before turning outward again, or they may pass through the trunk or branch to the bark on the opposite side. Masses of fresh moist frass ejected from the burrows are much in evidence in June and early July. During May of the third season boring is resumed until only a thin layer of bark or wood tissue remains to be penetrated. The caterpillars then pupate in the burrows behind this thin layer. Shortly before the moths are to appear, the pupae, with the aid of the backward-pointing spines on their hodies, wriggle forward, breaking the thin layer of bark or wood, and push themselves partly out of the burrows. The moths emerge and escape directly to the outside, leaving their pupal skins protruding from the burrow exits.


The carpenterworm has a three-year or longer life cycle. The moths begin to emerge in early June from infested ash trees, and 10 to 14 days later from infested poplars, mountain ash and American and Siberian elm. The moths may be present till early August. The females are inactive and poor fliers because of their stout heavy bodies, whereas the males are strong fliers and very active. Mating and commencement of egg-laying takes place soon after the moths emerge, and each female may lay 300 eggs or more. The eggs are stuck firmly to the surface, usually in small groups, in cracks,crevices and wounds in the bark, or in or near old burrow openings.

The attraction of wounded or previously infested trees for egg-laying along with the poor ability of the females to fly results in the development of "brood" trees from which numerous moths emerge year after year. More than 150 have been taken from the trunk of a green ash "brood" tree in a single season. Before ovipositing, the female probes the bark surface with the tip of her extended abdomen to locate suitable places for her eggs. The eggs incubate in 10 to 16 days and produce the small, dark, somewhat hairy caterpillars or larvae. The larval period extends from July of the year in which the eggs are laid to May of the following fourth or fifth year. The young caterpillars crawl about freely on the host trees soon after they are hatched and often cover long distances on the trunks before they begin to burrow. Small shelters, made of particles of frass and excrement held together by bits of webbing, are constructed by the young caterpillars to cover themselves and their burrow entrances. By August, frass ejected from the new burrows becomes noticeable.

In the second and following seasons the burrows are greatly extended and enlarged, usually forming a maze of criss-cross tunnels in the wood of heavily-infested trees. During this period the caterpillars eject a great deal of coarse frass from the burrows which may cling ribbon-like on the outside of the trunk or accumulate like sawdust around the base of the trees. The caterpillars also return at intervals to the outer sapwood to feed. Often they emerge from the burrows to crawl about on the bark, and then re-enter the burrows. in May of the fourth or fifth year, depending on the length of time required for larval development, the caterpillars retreat to the upper parts of the burrows where, in specially prepared chambers, they transform into pupae. The period of pupal transtormation is relatively short, as the adults begin to appear in June. Before changing to the adult form, the pupae propel themselves downward by means of the backward-projecting spines on their bodies to the burrow openings so that the moths upon emerging from their pupal skins escape directly to the outside.


Several natural control factors affect the abundance of the ash borer and the carpenterworm in the Prairie Provinces. Woodpeckers dig out and feed on larvae in the wood. Moths are killed, newly-hatched caterpillars are eaten or lost before they can enter the trees and older caterpillars which emerge from the burrows and crawl about on the bark are destroyed. Parasites, disease, weather and the vigor of the host trees during the long period of caterpillar development also have controlling effects. These natural factors cause the fluctuations in borer abundance which occur in the absence of applied control. Despite the ameliorating effects of natural influences, applied control of the ash borer and the carpenterworm is usually desirable because of the importance of the trees they infest and the destructive capacities of the pests. The age, size and number of infested trees, the severity and extent of infestation, the value of the plantings and the availability of equipment and labour will determine the means used. The measures which may be employed involve prevention and applied control. A combination of both is usually desirable.


  1. Avoid wounding the trees - where wounds occur at the base from cultural practices or on the upper trunk from pruning, etc., cover the wound areas with a commercial tree dressing or a suitable water-base paint to eliminate them as desirable egg-laying sites.
  2. Remove and destroy "brood" trees before June. "Brood" trees may be green ash, mountain ash or elms that are damaged beyond recovery and still heavily infested; or old, unhealthy poplars as indicated by the presence of stagged' tops and burly roughened areas of dead,loose bark on the main stems, especially the lower portions. Except for the decadent poplars, trees which show severe damage to the trunks but which no longer appear to harbour an infestation may be saved to give many additional years of service. If the wound areas are cleaned out and treated to protect them, and good growth conditions are provided, new wood will gradually restore the vigour of the trees.

Applied Control

Applied control consists of physical means to destroy the pests and b) the application of chemical poisons.

Physical Means

  1. Trapping the moths - where only one or two infested trees are involved and re-infestation from outside sources is small. Tightly wrap the areas on the trunk and branches where borer openings occur, with burlap or cotton cloth, in mid-May to prevent the moths emerging from the burrows and laying eggs; remove the wrapping about mid-August. Repeat the treatment for at least three years to ensure that all adults developing from the caterpillars occurring in the trees have been destroyed.
  2. Killing the caterpillars - for small trees, particularly green ash, lightly infested with borers as shown by the appearance of moist frass at burrow openings in May and June. Carefully dig out the insects with a pointed knife or kill the caterpillars by probing the burrows with a wire. Repeat the probing operation at frequent intervals as long as new frass is being ejected. Treat the wound areas with commercial tree dressing or a water-base paint after the insects have been killed.

Application of Chemical Poisons

Information on the use of chemicals for control of borers on trees is limited and fragmentary. Special procedures are required for worthwhile results. The measures currently recommended for control of the ash borer and the carpenterworm are:

  1. injecting an effective poison into the larval burrows to destroy the caterpillars;
  2. spraying the trunks and main branches of the host trees with a suitable insecticide to kill the newly-hatched caterpillars before they form burrows, and the older caterpillars which leave the burrows during their developmental period to crawl about temporarily on the outside of the trees.

In addition to using the right chemicals and equipment, both treatments must be applied thoroughly and at the right time for efficient results. To ensure that the best choice of treatment is made and the most up-to-date information on insecticide application is utilized, advice on the chemical control of borers in planted trees should be requested from one of the sources given below. When seeking advice on a borer problem indicate the species of trees affected, the appearance, location and extent of the visible injury and the number of trees involved with approximate ages and heights. This information is essential for correct identification of the borer problem and for determining the best control measures for it.

Sources of Information

Extension Entomologist
Extension Services Branch
Manitoba Department of Agriculture
711 Norquay Building
Winnipeg, Manitoba

Pest Control Specialist
Plant Industry Division
Saskatchewan Department of Agriculture
Administrative Building
Regina, Saskatchewan

Entomology & Pesticides Section
Crop Protection and Pest Control Branch
Alberta Deprtament of Agriculture
Agricultural Building
Edmonton, Alberta