Rhododendrons and azaleas, in general, grow best in landscape conditions that mimic natural mountain habitats: acidic soil (pH between 4.5 - 5.5), good drainage, ample soil moisture and organic matter, understory areas with light shade, cool soil temperature, and protection from strong winds. Some cultivated varieties (Southgate series Rhododendrons) are more tolerant of high heat and humidity.
In less than ideal growing conditions (i.e., full sun, drought), plants may become stressed and more susceptible to pest problems.
The most common pests of rhododendrons and azaleas in Maryland are lace bugs, azalea bark scale, rhododendron borer, and weevils. Rhododendron and azalea varieties vary in their susceptibility to some of these pests.
Lace bug damage is indicated by stippling (very tiny spots) on leaves of plants growing on dry sites and those with low plant diversity. Damage usually begins in early May on old leaves and later appears on new growth. Black fecal spots will be present on the lower sides of stippled leaves.
Heavy infestations of lace bug may cause leaves to turn yellow and then brown. Shrubs that are exposed to full sun and heavily infested may be killed. There are multiple generations of lace bug per year. Azalea, rhododendron, and andromeda each have their unique species of lace bug.
Adult lace bugs are flat, about 1/8 inch long, with transparent lace-like wings. Immature lace bugs (nymphs) are black and covered with spines. Eggs overwinter in leaves and may begin to hatch as early as late April.
There are as many of four generations of azalea lace bug during the growing season and into the fall.
The following azalea cultivars have shown resistance to azalea lace bug damage: ‘Autumn Amethyst’, ‘Autumn Twist’, ‘Autumn Royalty’, ‘Autumn Sangria’, ‘Autumn Cheer’, ‘Indica alba’, ‘Flame Creeper,’ and ‘Delaware Valley White’, among others.
Plant azaleas in partial sun (morning sun and afternoon shade). Mulch with chopped oak leaves, leaf mold, or pine needles and water deeply during drought periods. Stressed plants are more susceptible to lace bug feeding.
Natural enemies and predacious beetles feed on lace bugs. They can keep small populations of lace bugs under control. Do not spray if you find beneficial insects feeding on the lace bugs.
Look for nymphs and black fecal spots on lower leaf surfaces in early May to estimate potentially damaging populations.
When lace bug populations are high, sprays of horticultural oil (at a 2% summer rate) or insecticidal soap will control lace bugs if the lower surfaces of the leaves where lace bugs are active are thoroughly covered.
A currently registered systemic insecticide may be necessary where coverage of the undersides of leaves is difficult.
Azaleas are visited by pollinators. Be sure to observe all cautionary statements on labels to protect pollinators.
Azalea bark scale (Eriococcus azaleae)
Bark scales are a type of soft scale and an infestation is indicated by clear sticky honeydew with attendant sooty mold (black fungus) on leaves or stems, yellowing of leaves, and twig dieback. This scale is most obvious from May through June when white egg sacs may be found in twig forks. Heavy infestations over several seasons may kill plants.
Overwintering immature scales (nymphs) are less than ⅛-inch long, gray, and are usually found in twig forks. The females appear totally white when they produce their white, waxy egg sacs. This scale primarily attacks azalea and rhododendron, but also has been found on andromeda, maple, arborvitae, willow, poplar, and hackberry. There are two generations each year in Maryland.
Azaleas can tolerate low populations of this bark scale without significant damage. If there are no yellowing leaves, no treatment is necessary. Consider hand–removal by squishing the scales to prevent populations from increasing.
Beneficial predators and parasites will usually provide adequate control of light bark scale infestations. Examine egg sacs for holes which will indicate control by parasitoids, and look for predators such as ladybird beetles.
To control heavy infestations, spray dormant plants with a 4% rate of horticultural oil to kill developing nymphs on twigs. If necessary a 2% summer rate of horticultural oil may be applied in July after all of the eggs have hatched.
Systemic insecticides applied to the soil are also highly effective but follow cautionary warnings on the label to protect pollinators visiting azaleas and rhododendrons.
Rhododendron borer (Synanthedon rhododendri)
Rhododendron borer is the larvae (caterpillar) of a clear wing moth. The borer causes wilting of leaves and twig dieback. Prune the suspected branches and split them open longitudinally to see if larvae are present. Mature larvae are about 1/2 inch long, white with brown heads. The adult moths, about ¼” long, resemble wasps, have mostly clear wings, and black bodies with 3 gold bands on the abdomen. Boring larvae may cause branches to crack. Heavy infestations cause wilting and eventual branch dieback. This borer prefers rhododendron but occasionally attacks deciduous azalea and mountain laurel. There is one generation a year and larvae overwinter in tunnels in branches.
There are no conventional insecticides that will kill borer larvae once they are inside the branches. The best control option for residents with only a few plants is to prune out (cut below active larvae) and destroy wilting branches in late summer or early spring before adults emerge.
Beneficial nematodes, available under several trade names, are a control option. They may be injected into the active borer tunnels. See the package for specific instructions.
Azalea caterpillar (Datana major)
These caterpillars are black with rows of white or pale yellow spots, reddish brown legs, head, and neck area, and are 2 1/2 inches when mature. Preferred host plants are azaleas, but they may also attack witch hazel, sumac, apple, red oak, and andromeda. The caterpillars feed in groups when young and disperse as they mature. Branches or entire plants may be defoliated. Damage occurs in late summer and fall.
Look for caterpillars when chewing damage occurs. If only a few caterpillars are present, pick them off by hand.
If hand removal is not feasible, spray shrubs with Bacillus thuringiensis (Bt), a microbial insecticide, that is specific for young caterpillars and is sold under various trade names. Insecticidal soap may also be used. Apply either spray when caterpillars are numerous and young (less than 3/4 of an inch long). Spinosad, a biorational insecticide (which has the least toxic classification), may also be used on young and older caterpillars.
Azalea Leafminer (Caloptilia azaleella)
Damage is indicated by the presence of blotch mines in leaves of azalea beginning in May. Mines are initially formed near the midrib and caused by caterpillars feeding between the upper and lower leaf surface. As larvae mature they curl the tips of the leaves with silk and feed inside the curl. Large populations cause leaves to brown and drop prematurely. Curled leaf tips in June indicate the completion of the first generation. The second generation mines begin in July. Adult moths are present in late June and August. The moths are 3/8 inch long and yellowish brown. Mature larvae are 1/2 inch long and yellowish brown. Azalea leafminer overwinters as pupae in the leaf mines.
Rake and destroy fallen leaves in the fall to remove overwintering pupae.
Azalea whitefly (Pealius azaleae)
This whitefly is usually limited to azalea varieties that have hairy leaves. Whitefly adults and nymphs feed on the underside of the foliage. Heavy infestations cause the margins of terminal leaves to cup. These infested leaves will eventually turn yellowish and appear wilted. The leaves become covered with honeydew, followed by sooty mold (a black coating). To check for the presence of whiteflies, shake the branches of symptomatic azaleas to flush out adult whiteflies which look like tiny white moths. Examine the lower surfaces of leaves for the presence of nymphs, which are flat, yellowish green, and resemble scale insects. All stages occur on the undersides of leaves.
If the infestation is light, little or no plant symptoms are evident, and if beneficial insects are present, spray the undersides of leaves with insecticidal soap or horticultural oil at the 2% summer rate. Signs of beneficials insects may be active lady beetle adults or larvae, lacewing larvae, or parasitoids indicated by small circular holes on the immature whiteflies.
Rhododendron Stem Borer (Oberea myops)
Damage caused by the stem borer, which includes wilting terminals and dieback, is similar to the Rhododendron borer. This borer, however, is a longhorned beetle. Adult beetles feed on the underside of leaves on the midvein, causing the leaves to curl. The larvae bore down the center of twigs causing individual branches to wilt, and eventually die back to the ground. The adult beetles are about 5/8 inch long, have long antennae, and are pale yellow with two black spots on the thorax and on the margins of the wing covers. Adults are present in June and July. The larvae are whitish and have no visible head. This beetle prefers rhododendron, but will also attack azaleas and mountain laurel. The larvae overwinter in branches the first year and in roots the second year.
There are no conventional insecticides that will kill stem borer larvae once they are inside the branches.
The best management option for residents with only a few plants is to prune out and destroy wilting branches when beetle larvae are in the branches in early spring or late summer.
Beneficial nematodes are not effective on these borers.
Black Vine Weevil (Otiorhynchus sulcatus)
Adult feeding damage (notching on leaf margins) appears on azalea and rhododendron in May and June. Interior leaves usually show the most damage. Small plants when heavily infested may be defoliated. Adult weevils are about 3/8 inch long and black with faint yellowish flecks. They feed in the evening and at night.
The larvae live in the soil and feed on roots and may also girdle the plant at the root crown, causing the plant to wilt and die. The larvae are “C-shaped”, legless, and white with brown heads.
Rhododendron varieties with resistance to weevils are available and include species such as Rhododendron cuneatum, R. dauricum, R. fastigiatum, and others.
Inspect for adult weevils at night using a flashlight. Weevils can be hand-picked and dropped into a container of soapy water for disposal.
Beneficial nematodes may be effective in controlling larvae in the soil. Nematode species recommended for control are Heterorhabditis bacteriophora and Heterorhabditis megidis. Pull mulch back prior to application. Follow instructions on the label for timing and soil temperatures.
A registered foliar insecticide (e.g., azadirachtin) may be applied beginning in mid-June.
A registered systemic insecticide (e.g., Imidacloprid) may be applied by a certified pesticide applicator only (as per the Maryland Pollinator Protection Act). Insecticides applied to the soil can harm ground beetles, which are beneficial natural predators of a variety of pests.
Two-banded Japanese weevil (Pseudocneorhinus bifasciatus)
Adult feeding damage is very similar to that of black vine weevil but notches on leaf margins tend to be smaller. Adult feeding damage begins on lower leaves in mid-June. Small shrubs may be defoliated in heavy infestations. Larvae also feed on roots and crowns of hosts. Heavy larval populations may cause stunting, wilting, and may kill small shrubs and tree seedlings.
The adult weevils are about 3/16 inch long, brown, broadly oval, thick bodied, and brown to gray with two darker bands across the wing covers. Mature larvae are about 1/4 inch long, “C-shaped”, legless, and white with brown heads.
This weevil prefers azalea, rhododendron, privet, mountain laurel, forsythia, spirea, deutzia, lilac, and euonymus. There is one generation a year and all stages overwinter in the soil.
Adult beetles feed for two to three weeks before laying eggs. To check for adults, place a tray under damaged shrubs and shake the plants. Adult Japanese weevils feed during the day and will drop from the plants when disturbed.
If control is necessary, use a registered insecticide when damage begins in mid-June.
As with black vine weevil, soil drenches of entomopathogenic nematodes may reduce populations of grubs in the soil. Follow instructions on label for timing and soil temperatures.
Rhododendron gall midge (Clinodiplosis rhododendri)
This tiny fly attacks the new growth of rhododendrons. Gall midge larvae feed on the developing leaf tissue causing leaves to become distorted with puckered and curled margins. This deformation will last as long as leaves remain on the plant.
Rhododendron gall midge immatures spend the winter and early spring in the soil where they complete development and pupate. In spring, adults emerge from the soil and fly to the tips of branches where they lay eggs in new developing leaf tissue. Eggs hatch into tiny white maggots less than 1/16 inch in length. Their feeding causes leaf margins to curl and the maggots complete their development within the curled leaf tissue. Damage on new foliage will be evident in May and July. Fully developed larvae drop to the ground where they will pupate. Studies in Connecticut indicate as many as three to five generations annually depending on climatic conditions. Susceptible cultivars include those of Rhododendron catawbiensis and R. maximum.
Inspect plants for signs of early damage and remove curled leaves promptly. This will interrupt the life cycle and reduce subsequent populations. Diligent, yearly removal of infested leaves may dramatically reduce damage in a few years.
Originally written by Mary K. Malinoski, University of Maryland Extension Specialist. Revised and edited by Dr. Michael J. Raupp and Dr. Paula M. Shrewsbury, University of Maryland, April 2020.