Lilac borer and ash borer
Click for larger image Sap flow from a borer hole (Lepidoptera) in trunk of a summit ash (Fraxinus pennsylvanica 'Summit')

The lilac borer, Podosesia syringae, can be a serious pest of lilacs, privet, and other members of the olive family. It also attacks ash trees, Fraxinus spp., where it is then known as the ash borer. Another similar appearing borer is the banded ash clearwing, P. aureocincta. It only attacks ash plants. Plants that have been wounded or are under stress are more susceptible since the eggs are almost always laid in or near wounds that provide easy entry into the plant.

A heavy infestation may kill a plant quickly while lesser, but continual, infestations may weaken and stress the tree, causing dieback and death over a longer period of time. Infestations of borers also provide entry sites for the fungus, Polyporus versicolor, which destroys wood.

Symptoms and Diagnosis

One of the first symptoms of borer attack is a slight sap flow mixed with frass at the entrance site of the larvae. Later, "sawdust" accumulates at the entrance and on the ground as the larvae clean out their tunnels. Leaves often wilt and turn reddish-brown, and branches die back.

Entrance holes are irregularly shaped at the bottom of the gallery and are often associated with sunken and cankered areas on stems and branches. The exit holes at the top are round and often have empty pupal skins protruding from them. Completed galleries are about 3 inches long and 1/4 inch in diameter.

Infested branches may also have swollen areas with cracked bark that has broken away from the wood. This may be more noticeable in the spring of the year.

Life Cycle

The adult lilac borer is a day-flying moth that looks very much like a wasp. Its color may vary from black and yellow to orange and brown with clear wings.

The moths begin to emerge from the tunnels in the bark in March and April with peak emergence in May or June. The female moths live for one week and lay eggs during daylight on the bark, in or near wounds. The eggs are flattened, oval, and less than 1/16 inch long. They are deposited singly or in clusters. The young larvae chew into the bark, attacking the sapwood during the summer. The mature larva is creamy white with a brown head and is about 1 inch long. It overwinters as a mature larva and pupates in the tunnels under the bark. Usually one generation is produced per year.

Integrated Pest Management Strategies

1. Prune. Prune out and destroy infected branches if infestations are small. Renewal pruning to remove the largest branches at ground level is effective in keeping lilacs healthier and less susceptible to attack. Avoid pruning when the moths are active.

2. Remove larvae. Dig out larvae with a sharp knife or crush by carefully inserting a wire into the tunnels.

3. Avoid wounding plants. Wounds provide good entry points for hatching larvae. Prune properly to promote rapid wound closure.

4. Use biological controls. Control larvae by spraying gallery entrances with the nematode, Steinernema carpocapsae.

5. Use chemicals if necessary. Treatment should begin in mid to late April when moth flight begins. Pheromone traps are useful in monitoring moth activity. Spray or paint trunks and main branches with endosulfan (Thiodan) or pyrethrins. The application should be repeated 2-3 times at ten day intervals during moth activity. Be sure to allow the spray solution to run all the way down to the ground.

Organic Strategies

Strategies 1, 2, 3, and 4 are strictly organic approaches. For an organic approach to Strategy 5, consult the Organic Materials Review Institute (OMRI™) for appropriate pyrethrin products.

More images:

Click for larger image
Close-up of borer hole (Lepidoptera) in trunk of a summit ash (Fraxinus pennsylvanica 'Summit'); note mixture of frass and sap