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Possible insect pests for lilacs

By Melanie Mathieson
Gardening Guru

The last column helped you to determine whether it was a disease that was affecting your lilac shrub, but the descriptions didn’t seem to match to what you are seeing. If this is the case and you have followed the instructions for planting and care from previous columns, then you must have an insect pest problem. This column will assist you in determining which insect pest may be bothering your shrub.

Lilac Borers
These troublesome grubs are the larvae of Podosesia syringae var. syringae, a wasp like moth with translucent brownish wings. In late spring, it lays masses of eggs on stems of lilac and ash. The hatching larvae tunnel into the branches and feed on the wood. The first time you’ll likely notice the infestation is when all the leaves on an entire stem or branch turn yellow and wilt, usually in spring or early summer. Large branches may swell and crack. Look closely, and you’ll see small holes about the diameter of a pencil lead a foot or two (30-60 cm) above the ground. Directly underneath, you’ll see a small amount of sawdust. The small holes are exit wounds, so the perpetrators have already left, but chances are, more are at work. When you prune, you may see the borer tunnel penetrating the heartwood of older branches. Borers are most common on stressed and wounded plants and on large, older shoots, especially on the common lilac.
For control of this insect pest there are commercial pesticides labelled effective for lilac borer. As I always warn, follow the directions carefully. Good housekeeping is a must here as well. Prune away infected branches and clean up fallen leaves in the fall. Do not compost the waste – send it to the landfill or burn it. Borers can be a problem in our district because of the abundance of both lilacs and ash trees growing close to or in our yards.
Lilac Leaf Miners
The larvae of a small moth species, Caloptilia syringella, leaf miners tunnel between the layers of the leaves, making the leaves look blotched in early summer. They then roll the leaves and feed externally. Leaves eventually turn brown, so the entire plant may look burned. The common lilac is especially susceptible to leaf miners.

Once the damage is noticed, it is generally too late to take action. Remove affected leaves and keep the area under the shrubs clear of leaves in the fall to prevent reinfection. Again make sure you dispose of the infected leaves. The damage is more aesthetic than physiological.
Mites
The eriophyid mite (Aculus massalongoi) produces a silver or rust color on lilac leaves and may cause some leaf-rolling. Like leaf rollers there isn’t much that can be done to control or remove the pests. Again good housekeeping of the fallen can only help reduce next year’s infection.
Scales
Scales are odd insects; they look like flat, oval, lifeless bumps. What they do is extract fluid from the plant while they protect themselves under a waxy coating. They lay their eggs on the bark in fall or spring and the eggs hatch in late spring. The young scales, which are pale yellow or orange, are mobile and only about 0.1 inch (2 mm) long. Once they find a place to feed, their legs wither. What you may first notice, probably while pruning, are stems that look unusually gray and roughened. Look closely and you will see small bumps that can be scraped off with a fingernail or the blade of your pruning shears.
The type of scale that infests lilacs, Lepidosaphes ulmi, can be controlled by pruning out the most heavily infested branches and then applying a dormant-oil spray the next spring before the leaves have developed. This treatment should take place before bud break on a dry, sunny, mild morning. To prevent injury to the plant, do not apply within 48 hours of a frost or at a temperature lower than 39 degrees F (4°C). If possible, choose a calm morning when the temperature is above 60 degrees F (15°C) and frost is not forecast for that night. Scale can also be killed by a lime- HYPERLINK “http://www.herbs2000.com/minerals/sulfur.htm” sulphur solution painted directly on the branches or by direct contact with a spray of soapy water: a teaspoon (5 millilitres) of an additive-free liquid dish soap such as Sunlight to a quart (1 litre) of water. This soap spray is most effective in late spring and early summer, when scales are at the crawler stage. Spray with the soapy solution until the foliage drips.
Hopefully your lilacs are thriving and you will never have to use the advice in this column but just in case you may want to “clip and save” to have for future reference as you begin your luscious lilac grove.