Azalea & Rhododendron Insect Pests

Prepared by Clyde S. Gorsuch, Extension Entomologist, and J. McLeod Scott, HGIC Horticulture Extension Agent, Clemson University.  Revised by Joey Williamson, HGIC Horticulture Extension Agent. (New 10/99. Revised 05/04. Images added 01/10.)

HGIC 2051

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While many people think of azaleas and rhododendrons as being completely different kinds of plants, they actually belong to the same genus, Rhododendron. In terms of the pests that attack these plants, some occur on both azaleas and rhododendrons, but some attack only one or the other. Azaleas and rhododendrons should be inspected regularly for insects and mites. If detected early, before pest numbers reach high levels, homeowners often can physically remove the pests. In addition, when pests are immature and few in number, the less toxic pesticides such as insecticidal soaps and horticultural oils are often effective at controlling them.

Some common insect and related pests of azaleas and rhododendrons in South Carolina are described below.

Lace Bugs

Azalea lace bugs mottled wings provide natural camouflage.
Forest & Kim Starr, U.S. Geological Survey,
www.forestryimages.org

Azalea Lace Bug: Azalea lace bug (Stephanitis pyrioides) is a major pest of azaleas in South Carolina. Lace bug adults are about 1/8 inch long. The wings are highly sculptured, giving them a lacy appearance. The adults also have some dark markings on the back and wings. Their markings make them difficult to see when on the leaf. The immature forms, called nymphs, are colorless initially but turn black over time. The nymphs have a number of spines on their back.

Both adults and nymphs have needle-like mouthparts that they use to suck plant sap from the leaf’s underside. As a result of the feeding damage, leaves develop pale colored speckling (stippling) on their upper surfaces, giving leaves a grayish cast. When damage is severe enough, the whole leaf appears white and drops early. This early leaf drop can make the azalea susceptible to some of the dieback diseases. As a result of their feeding on the underside of the leaf, most people do not see lace bugs until damage is visible. Black shiny bits of insect waste and cast off skins from immature forms also can be found on the undersides of leaves.

Old lace bug injury with healthy new growth
Whitney Cranshaw, Colorado State University.
www.forestryimages.org

Lace bugs overwinter (survive the winter) as eggs. Adult female lace bugs insert their eggs into the leaf tissue and then cover them with a dark splotch of a varnish-like material to seal the egg into the leaf. This, along with their droppings, gives the underside of the leaves a "fly-specked" appearance. There are usually three or more generations of this pest in South Carolina each year.

Control: Control of this pest on azalea begins with the planting of resistant varieties. The following azalea cultivars have resistance to azalea lace bug: ‘Dawn,’ ‘Pink Star,’ ‘Ereka,’ ‘Cavalier,’ ‘Pink Fancy,’ ‘Dram,’ ‘Seigei,’ ‘Macrantha,’ ‘Salmon Pink,’ ‘Elsie Lee,’ ‘Red Wing,’ Sunglow’ and ‘Marilee.’

Lace bugs have several natural enemies that feed on them. These include lacewings, assassin bugs, spiders and predaceous mites. However, when lace bug populations get out of hand, using chemical controls is necessary. Insecticidal soaps may give some control of young lace bugs, and complete coverage of all leaf surfaces is essential. For adult lace bugs, recommended insecticides include malathion 5EC, carbaryl (Sevin 50WP), cyfluthrin (Bayer Power Force Multi-Insect Killer) and imidacloprid (Bayer Advanced Garden Tree & Shrub Insect Control). Azaleas should be sprayed when the first lace bugs appear. A second application in seven to ten days may be needed to control newly hatched lace bugs. The imidacloprid is mixed with water and used as a soil drench around the plants.

Good control of the first generation in March to May will greatly reduce problems later in the season. Control of second (July to August) and later generations (September to October) may be necessary. As with all pesticides, read and follow all label directions and precautions.

Rhododendron Lace Bug: Rhododendron lace bug (Stephanitis rhododendri) adults are slightly larger than azalea lace bugs, and they are yellow. Their nymphs are similar in color, but are slightly larger than azalea lace bug nymphs.

Feeding behavior by the rhododendron lace bugs is the same as with azalea lace bugs, and the symptoms that are produced are similar.

Control: Rhododendron lace bugs should be treated when they appear in early May. Repeat sprays as needed. Recommended insecticides to control the Rhododendron lace bug are the same as for the azalea lace bug. As with all pesticides, read and follow all label directions and precautions.

Mites

Spider mites are not insects but are more closely related to spiders. Mites, such as azalea mite (Eotetranychus clitus) and southern red mite (Oligonychus ilicis) are a common problem on azaleas, but can also attack rhododendrons. They are very small and barely visible to the naked eye. Spider mites are typically found on the undersides of leaves although with heavy infestations they will feed on the upper surface also. Mites suck plant sap causing leaves to change from their normal green color to dull green and with a heavy infestation to a gray-green or bronze-green color. With heavy infestations, leaves may also be covered with fine webbing. The southern red mite is a "cool weather mite," meaning it is active in spring and fall and almost inactive during the summer and winter. However, some species of spider mites develop most rapidly in dry, warm (temperatures greater than 70° F) weather, and their populations peak in midsummer.

Southern red mites, eggs and cast off skins.
John A. Weidhass, Virginia Polytechnic Institute and StateUniversity,
www.forestyimages.org

An easy way to detect spider mites is to take a white sheet of paper and wipe the undersides of several leaves. If mites are present, there will be red streaks on the paper.

Control: Natural enemies of mites, such as ladybird beetles (ladybugs), thrips and predaceous mites usually keep mite populations reduced. While pesticides are available, their misuse often makes the problem worse by killing off the mites’ natural enemies. If the population level is low, early season mite infestations can often be controlled with insecticidal soap (such as Safer brand). Insecticidal soaps offer no residual activity, and thorough coverage of both the tops and bottoms of leaves is important. Test insecticidal soap on azalea varieties before applying to all plants. Similarly, a spray of horticultural oil (such as Sunspray Ultra-Fine Spray Oil) can help reduce infestation levels. A recommended miticide for use on azaleas is fenbutatin oxide (Ortho Systemic Insect Killer. It is best to alternate the miticides that you use to decrease the chance of mites developing resistance. As with all pesticides, read and follow all label directions and precautions.

Scales

Various scales attack azaleas and rhododendrons, but one of the most common is azalea bark scale (Eriococcus azaleae). Scales are unusual insects. They vary in appearance depending on age, sex, and species. The adult female azalea bark scale is dark red with a long sucking mouthpart. It is hidden from view by the matted waxy threads of the egg sac that covers it. The egg sac is about 1/8 inch in length. Adult females are immobile and can usually be found feeding in the forks of branches and on twigs. The nymphs (immature forms called crawlers) move around for a short before they settle and start feeding. Both adults and crawlers suck plant sap.

Adult azalea bark scales feeding on small branch.
Clemson University - USDA Cooperative Extension Slide Series,
www.forestyimages.org

Control: A light infestation of scale can be scraped off the plant and discarded. Spraying with a dormant oil in early spring before new growth begins will kill some adults and eggs by smothering them. Insecticides are only effective against crawlers. Recommended insecticides that are available in homeowner size packaging include malathion 5EC, carbaryl (Sevin 50WP) and cyfluthrin (such as Bayer Advanced Garden Multi-Insect Killer). As with all pesticides, read and follow all label directions and precautions.

Red-Headed Azalea Caterpillar

The red-headed azalea caterpillar (Datana major) is the larva (immature form) of a moth. At about 3/8 inch long, the caterpillar is reddish- to brownish-black with white and yellow stripes. When mature, the caterpillar reaches 2 inches in length and is almost black with a red head and legs and with white broken longitudinal lines running along the body.

A group of red-headed azalea caterpillars devouring a leaf.
Chris Evans, River ti River CWMA, www.forestyimages.org

Red-headed azalea caterpillars feed in groups and may defoliate much of an azalea before they are detected. When disturbed, they raise their heads and tails. Most of the damage is done in late summer.

Control: The caterpillars can be removed by hand as they are harmless to humans. For chemical control, treat when caterpillars are first noticed. Recommended insecticides that are available in homeowner size packaging include carbaryl (Sevin 50WP) and cyfluthrin (Bayer Advanced Garden Multi-Insect Killer). As with all pesticides, read and follow all label instructions and precautions.

Rhododendron Borer

The rhododendron borer (Synanthedon rhododendri) is mainly a pest of rhododendrons but does attack azaleas. The borer is the immature form (caterpillar) of the rhododendron borer moth. The adult is a clearwing moth that somewhat resembles a wasp. The adult female lays eggs on the bark of the plant. The borer is pale yellow with a dark head and about ½ inch long. It chews a hole to the inner bark and forms long tunnels in the branches. By late fall, it enters the sapwood where it survives the winter.

The rhododendron borer prefers twigs and small branches, but may infest main stems and branches as well. Leaves on infested branches are often off-color and wilted. Early symptoms are similar to those resulting from drought stress. In addition, infested branches tend to be somewhat stunted compared to healthy branches. Heavily infested branches turn brown and die. Damage from this pest is most evident in the fall.

To detect this borer, prune wilted branches and cut them open to see if the borers are present. There are also pheromone (insect chemical attractants) traps that are available. These traps attract only the adult males. The presence of male adults indicates the need for control measures to be taken.

Control: Nonchemical control is limited to pruning and destroying wilted branches. Chemical control with permethrin is directed at the adults. Treatment should take place in May and June. Thoroughly spray bark and repeat three times at 10 to 14 day intervals. Permethrin is available in products such as Bonide Borer/Miner Killer; Ferti-Lome Kill-A-Bug II; Spectracide Eight, Vegetable Fruit & Flower Concentrate; and Ferti-Loam Indoor/ Outdoor Insect Spray. As with all pesticides, read and follow all label instructions and precautions.

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