Borers

• Borers typically affect plants grown east of the Rocky Mountains, according to the Montana State University Extension Service. Lilac borers are the larvae of a moth that has the appearance of a wasp. The pests lay their eggs on the bark of the shrubs, and the larvae eat their way into the wood, causing wilt and the death of branches. The presence of borers can decrease the number of blooms on lilac shrubs. Growers should look for swollen branches and borer holes marked by the presence of sawdust. Insecticidal treatment includes lindane spray. Good maintenance practices help fight borers; cut away and burn infested wood. Growers can renew lilacs and help the plants recover from scale and borer damage by removing the oldest canes, according to the Colorado State University Cooperative Extension.
  
  

Scales

• Lilacs may be affected by oyster shell scale or San Jose scale. Like tiny vampires, scale insects feed on the fluids of lilacs. Scales are covered in a shell that protects the insect beneath. The shells camouflage the insects on the bark of lilac shrubs. Scales lay their eggs on the lilac bark. Infestations lead to yellowed leaves, leaf drop and failure to bloom. Growers should prune out affected branches to treat heavy infestations. Application of an insecticidal soap may be used only during a specific stage of infestation, but chemical controls are available, including carbaryl, diazinon and acephate, commonly sold under the brand Orthene.
  
  

Leaf Miners

• Leaf miners feed on lilac plants, and the insects use lilac leaves as homes for their larvae. Entire plants may be affected, with heavy infestations causing shrubs to have a burned appearance. Growers should look for leaves with a rolled or blotchy appearance that signals the presence of leaf miner larvae. Insecticidal treatments include diazinon, Orthene and neem, according to the Montana State University Extension Service.
  
  

Disease

• Insects prove problematic not only because of the damage they cause but also because they can carry disease-causing organisms, such as the bacteria that lead to bacterial blight. Wounds caused by feeding insects create an opening for fungal and bacterial diseases to enter the plants' tissues. Insect damage can also weaken plants, rendering them more susceptible to the negative impacts of infections. Growers should do their best to maintain healthy plants, as such plants are better able fight off infection, and should plant cultivars less susceptible to disease when possible.