Lilac Bushes & Insects

There are more than 20 species and thousands of cultivars of lilacs (Syringa spp.), which are long-lived, hardy, flowering plants that bloom in the spring. Lilacs have been known to live for centuries under the right conditions; however, there are a number of lilac pests that can shorten their life span. They include boring insects and sap-sucking critters that attack the leaves of a lilac bush if left unchecked.

One of the most serious pests that can be the cause of a dying lilac bush is the lilac borer, which is also known as an ash borer because it attacks ash trees as well. This insect is a type of moth that tunnels into the main stem of a lilac plant, leaving visible holes. You may also see frass, a type of debris produced by the insects, on affected plants. Infested lilac shrubs will wilt in the summer, and its branches will break off. The holes created by the lilac borer also create an entry point for the fungus Polyporus versicolor to enter the plant.

If a lilac borer infestation is minimal, removing the affected branches may be enough to solve the problem. It's also a good idea to prune lilac trees regularly to remove large branches close to the ground. This will make a lilac bush less vulnerable to infestation by the lilac borer.

Like the lilac borer, the lilac leaf miner is a moth that lays eggs on the underside of lilac leaves. It is the larvae of this insect that do damage, as they feed inside, creating discolored, shriveled blotches on the foliage. They cause the leaf to roll. Eventually, they move to a new leaf, where they overwinter as pupae before emerging as adults. These insects also attack other species of plants, including privets.

Although they can distort leaves, leaf miners generally do not cause severe harm to a lilac shrub, so control is not always necessary and may be counterproductive. The lilac leaf miner is also preyed upon by parasitoid wasps and some species of birds, which can help keep their numbers in check.

Among the pests that infest lilac plants is the oystershell scale, a type of small insect that survives by sucking the sap of leaves, which can contribute to a lilac bush dying. It gets its name from a waxy covering that makes it look like a miniature oyster shell. Proper fertilization and watering can help make a plant less vulnerable to oystershell scale. However, you should avoid overfertilizing, which can encourage scale activity. Once these insects have developed their waxy coating, spraying them with insecticides will be ineffective, and systemic treatments may be necessary.

Lilac shrubs and trees may also be attacked by the European hornet. These insects remove the bark from the stems of lilacs and other plants in order to access the sap underneath. If the infestation is severe, this can result in branch dieback. Note that these insects are known to sting if they feel threatened. Thrips and caterpillars may also feed on lilac plants and cause damage.