Several years ago in June while conducting a site evaluation, I saw an azalea with leaves that were snowy white instead of deep green. I asked the homeowner what he thought of his white-leaved azalea. He remarked, “Well, they are supposed to turn white in the summer aren’t they?” Well, not really. Azaleas, andromeda, and cotoneasters are some of the most commonly planted woody shrubs in residential landscapes in the greater Washington area. Almost all varieties of these beautiful shrubs are prone to attack by sucking insects called lace bugs. Lace bugs attacking azalea and andromeda are exotic pests. They arrived from Asia in 1916 and 1945, respectively, and have now spread throughout the eastern United States. Hawthorn lace bug is a native pest found on small trees such as serviceberry and hawthorn. However, firethorns and cotoneasters are also fine food for this pest and populations often erupt, especially on shrubs that are grouped in large, pure stands.
Lace bugs are usually found on the undersides of leaves where they use their beak, a straw-like appendage, to pierce plant cells and withdraw the contents. Chlorophyll gives leaves their green color. When lace bugs slurp the juice of cells, they remove chlorophyll, and the leaf tissue turns white. This damage, called stippling, first appears as small dots. Later, if feeding continues, entire leaves can turn white and appear strangely bleached. Prolonged bouts of lace bug attack can cause leaves, branches, and, sometimes, entire plants to turn brown, wither, and die. In addition to the stippled appearance of leaves, there is another sure sign of a lace bug infestation – tar spots. As lace bugs feed, they defecate, leaving behind tarry black spots of excrement on the undersurface of the leaf. This is a very good clue for diagnosing a lace bug problem.
Azalea and andromeda lace bugs spend the harsh winter months as eggs in leaf tissue. The mother lace bug inserts her eggs partially into the leaf surface. She then coats the exposed surface with a layer of excrement. Is this some strange, Freudian way of telling her children what she really thinks of them? Perhaps, but it is more likely that her fecal gift helps protect her babies by camouflaging them from lace bug killers such as predatory insects and parasitic wasps. When the warmth of spring returns, the eggs hatch, usually around the first week of May. The small, newly hatched lace bugs, called nymphs, also suck juice from plant cells and molt several times before becoming adults. Adults of our native hawthorn lace bug spend the winter hiding in the refuge of leaves and twigs beneath trees and shrubs. In spring, they too become active and move to young leaves of their host. Lace bugs develop rapidly in the warmth of the summer and may complete four or more generations in a growing season. A wide variety of natural enemies, including spiders, lacewing larvae, predatory bugs, and tiny wasps attack and kill lace bugs. These beneficial arthropods are usually able to hold lace bugs at bay when azaleas, andromeda, and cotoneasters are planted in somewhat shadier landscapes with diversity of plants rather than in sunny, exposed landscapes in which azaleas or cotoneasters are planted en masse. If you have planted azalea, andromeda, or cotoneaster in sunny locations, the lace bug battle may become a regular event. Consider moving these plants to shadier and more diverse spots. If you must resort to insecticides to manage this problem, consider ones that have a short period of residual activity, such as insecticidal soap or oil, or ones that are systemic and move into the plant through the soil or foliage. Sprays that are not systemic must be directed to the undersurface of the leaves where lace bugs are found. More than one application may be necessary if unhatched eggs are present in leaves at the time of the first application. Please be sure to follow directions on the insecticide label.