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.
Azalea Lace Bug: Azalea lace bug (Stephanitis pyrioides) is a major pest of azaleas in South Carolina. Lace bug adults are about ⅛-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.
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 spray insecticides include acephate, bifenthrin, lambda cyhalothrin, malathion, and cyfluthrin. Acephate may give the best control, as this insecticide is a foliar systemic that will move through the leaves to kill the lace bugs on the undersides of the leaves. The other insecticides are contact insecticides, and sprays need to be directed onto the lower surface of the foliage to be effective. 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. 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 an alternative to spraying the azaleas, products containing either imidacloprid or dinotefuran may be used as a soil application around the base of the azaleas. These products are available as either granules or concentrates. The granular products are sprinkled around the base of the shrubs, and the concentrates are diluted with water and then poured around the base of the shrubs. The amount to apply is based upon the height of the shrubs. If granular products are used, the shrubs are then watered to enhance the uptake of the product. The advantage to these soil applied products is that they give season-long control of lace bugs. See Table 1 for products. 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.
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 and may 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, leaves will turn to a gray-green or bronze-green color. Also with heavy infestations, leaves may be covered with fine webbing.
The southern red mite is a “cool weather mite,” which means that it is active in spring and fall, but almost inactive during the summer and winter. However, most species of spider mites develop most rapidly in dry, warm (temperatures greater than 70 °F) weather, and their population peaks in midsummer.
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 or horticultural oil sprays. Insecticidal soaps and horticultural oils offer no residual activity and help to conserve beneficial insect species. Test insecticidal soap on azalea varieties before applying to all plants. It is best to spray insecticidal soap or horticultural oil when the temperatures are between 45 and 85 degrees, and spray in the evening to slow drying time and increase effectiveness. Thorough coverage of both the tops and bottoms of leaves is important for best control. Apply horticultural oil sprays at a 2% solution (5 tablespoons oil per gallon of water).
A recommended miticide for use on azaleas is tau-fluvalinate. It is best to alternate the miticides that you use to decrease the chance of mites developing resistance. An alternate insecticide with miticidal activity is bifenthrin.
If mites have been a problem on azaleas, do not use carbaryl sprays or imidacloprid soil drenches to control other pests. Their continued use may cause a subsequent spidermite outbreak. See Table 1 for products. As with all pesticides, read and follow all label directions and precautions.
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 time before they settle and start feeding. Both adults and crawlers suck plant sap.
Control: A light infestation of scale can be scraped off the plant and discarded, or heavily infested branches may be pruned out. A horticultural oil spray applied in early spring before new foliar growth begins will kill many over-wintering adults and eggs by smothering them. Apply horticultural oil sprays at a 2% solution (5 tablespoons oil per gallon of water). Insecticidal soap sprays during the growing season will help control crawlers and adults of soft scales, like the azalea bark scale. Spray the plants thoroughly, so that the soap spray drips or “runs off” from the upper and under sides of leaves, twigs, and plant stems.
Contact insecticides are only effective against crawlers. Recommended insecticides that are available in homeowner size packaging include acephate, bifenthrin, lambda cyhalothrin, malathion, and cyfluthrin. Scale insects can also be controlled by a soil drench with a product containing dinotefuran. One treatment will last for a year. See Table 1 for products. 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 white broken longitudinal lines running along the body.
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 B.t., spinosad, bifenthrin, lambda cyhalothrin, permethrin, acephate, and cyfluthrin. Spinosad and B.t. will more effectively control younger caterpillars. If the caterpillars are large, choose one of the other contact insecticides. See Table 1 for products. As with all pesticides, read and follow all label instructions and precautions.
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 will protect trunks and limbs from the borer larvae. Treatments should take place in May and June. Thoroughly spray bark and repeat three times at 10 to 14 day intervals. See Table 1 for products. As with all pesticides, read and follow all label instructions and precautions.
Table 1. Insecticides for Azalea and Rhododendron Insect Pest Control in the Home Landscape.
|Pesticide Active Ingredient||Brand Names & Products|
|Acephate||Bonide Systemic Insect Control Concentrate|
|Bifenthrin||Bifen I/T Concentrate
Ferti-lome Broad Spectrum Insecticide Concentrate
Hi-Yield Bug Blaster Bifenthrin 2.4 Concentrate
Ortho Bug-B-Gon Insect Killer for Lawns & Gardens Conc.; & RTS1
Talstar P Concentrate
Up-Star Gold Insecticide Concentrate
|B.t.(Bacillus thuringiensis)||Bonide Thuricide Bt Concentrate
Natural Guard Caterpillar Killer Spray with Bt Conc.
Safer Caterpillar Killer with Bt Concentrate
Southern Ag Thuricide Bt Caterpillar Control Concentrate
Tiger Brand Worm Killer Concentrate
Garden Safe Bt Worm & Caterpillar Killer
|Cyfluthrin||Bayer BioAdvanced Vegetable & Garden Insect Spray Concentrate; & RTS1|
|Dinotefuran||Gordon’s Zylam Liquid Systemic Insecticide (drench3)
Gordon’s Zylam 20SG Systemic Turf Insecticide (drench3)
Valent Brand Safari 2G Insecticide (2% granules)
Valent Brand Safari 20SG Insecticide (drench3)
Ortho Tree & Shrub Insect Control Ready To Use Granules (2%)
|Horticultural oil4||Ferti-lome Horticultural Oil Spray Concentrate
Bonide All Seasons Spray Oil Concentrate
Southern Ag ParaFine Horticultural Oil Concentrate
Espoma Earth-tone Horticultural Oil Concentrate; & RTS1
Monterey Horticultural Oil Concentrate
Summit Year Round Spray Oil Concentrate
|Imidacloprid||Bayer BioAdvanced Garden 12 month Tree & Shrub Insect Control Landscape Formula Concentrate (drench3)
Bonide Annual Tree & Shrub Insect Control w/ Systemaxx (drench3)
Ferti-lome Tree & Shrub Systemic Insect Drench3
Hi-Yield Systemic Insect Spray (drench3)
Monterey Once A Year Insect Control II (drench3)
Martin’s Dominion Tree & Shrub Insecticide (drench3)
|Insecticidal soap5||Bonide Insecticidal Soap Concentrate
Espoma Earth-tone Insecticidal Soap Concentrate
Natural Guard Insecticidal Soap Concentrate
Safer Brand Insect Killing Soap Concentrate
Garden Safe Insecticidal Soap Insect Killer Concentrate
|Lambda or gamma cyhalothrin||Spectracide Triazicide Insect Killer – Lawns & Landscapes Conc.; & RTS1
Martin’s Cyonara Lawn & Garden Concentrate
|Malathion||Spectracide Malathion Insect Spray Concentrate
Southern Ag Malathion 50% EC
Hi-Yield 55% Malathion Insect Spray Concentrate
Ortho Max Malathion Insect Spray Concentrate
Tiger Brand 50% Malathion Concentrate
Gordon’s Malathion 50% Spray Concentrate
Bonide Malathion Insect Control 50% Concentrate
Martin’s Malathion 50% Concentrate
|Permethrin||Bonide Eight Insect Control Vegetable, Fruit & Flower Concentrate
Bonide Total Pest Control Outdoor Concentrate
Hi-Yield Indoor/Outdoor Broad Use Insecticide Concentrate
Bonide Eight Yard & Garden RTS1
Tiger Brand Super 10 Concentrate
|Spinosad||Southern Ag Conserve Naturalyte Insect Control Concentrate
Bonide Colorado Potato Beetle Beater Concentrate
Bonide Captain Jack’s Dead Bug Brew Concentrate; & RTS1
Ferti-lome Borer, Bagworm & Leafminer Spray Concentrate
Monterey Garden Insect Spray Concentrate
Natural Guard Spinosad Bagworm, Tent Caterpillar, & Chewing
Insect Control Concentrate; & RTS1
Dow Conserve SC Turf & Ornamental Concentrate
Ortho Insect Killer Tree & Shrub Concentrate
|Tau-fluvalinate||Bayer BioAdvanced 3-in-1 Insect, Disease & Mite Control Conc.; & RTS1|