Friday, May 25, 2007

Boring Insect Damaging My Lilac

I have a lilac bush with a white flower that seems to have some kind of insect boring into it. I see small sporadic holes towards the base of the trunk and wood shavings on the ground. I applied horticultural oil in the late winter. Can you tell me what it is and what I can do?
Insects often prefer specific host plants from which to feed so it helps a lot to know this is happening to a lilac. Sure enough there is a boring insect that likes to target lilacs, called the ash borer (Podosesia syringae). As you have noticed most damage is done to the bottom 12” of the host plant. For lilacs in particular ash borers prefer feeding on older wood. Let me describe the life cycle because that helps us to figure out the best time of year to go after these damaging insects.

Partially grown larvae overwinter under the bark of your lilac. Eventually the larvae mature into adults and emerge. The adults are brown and black and resemble paper wasps both in size and appearance. In the northeast adults emerge from under the bark in April or May when the weather has warmed over 60 degrees. Usually your best chance of seeing them is in the early morning. In some cases you may even see the pupal skin remaining in the bored hole where the adult emerged. Adult females lay their eggs on the bark of the lilac 4-6 weeks after they emerge. The eggs hatch about one-and-a-half weeks later, and the larvae eventually bore into the bark to begin the whole process over again. Ash borers only have one generation a year.

To best combat them, you want to target ash borers when the eggs and hatched larvae are most vulnerable. Spray the lower portion of the bark with an insecticidal spray in early June and then again four weeks later. This will give you the best chance of killing the exposed eggs and larvae before they get a chance to bore and hide under the bark again. As far as the rest of the year, continue to try and maintain as healthy a plant as possible. Mulch your lilac to help regulate fluctuation in soil temperature. If the lilac is older consider supplemental water and fertilizer to help reduce the amount of stress on the plant.

(for this and many other insect questions I turn to Garden Insect of North America by Whitney Cranshaw, a tremendous reference book. This book is available to view and read any time at our horticultural library located at 148 W. 37th between Broadway and 7th Avenue.)

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