Over the Garden Gate: Lace bugs on rhododendrons and azaleas - The Times
In nature, rhododendrons and azaleas grow in filtered shade beneath taller trees in mountainous areas with rich organic soil and ideal moisture. When planted in home landscapes lacking these conditions they can become stressed and susceptible to insect attack.
A common pest that attacks rhododendrons and azaleas are lace bugs. These insects are true bugs and feed exclusively on plants, but only a few are important pests on broad-leaf shrubs. The rhododendron lace bug (Stephantis pyriodes) is a native species and closely related to the azalea lace bug (S. pyrioides), an accidental import from Japan. These insects feed on specific host plants and are found on the undersides of leaves. Lace bugs are easy to overlook until the damage is done.
The first indication of an infestation is a light-colored speckling (stippling) on the top surface of the leaves. When examining the underside of the leaves, adult or immature forms called nymphs are visible along with shiny black bits of waste. Adults are flattened, approximately 1/8 inch long, whitish tan, with, lacy-looking wings. Dark markings on the transparent wings make them difficult to see on the leaf underside. Nymphs are black and covered with spines.
Both adults and nymphs use needle-like mouthparts to suck sap and chlorophyll from the underside of the leaves. With severe infestations, the entire leaf appears bleached and can die and drop early. This can decrease the plants' vigor and even kill small shrubs.
Since rhododendron and azaleas are evergreens, the damage will persist after the lace bugs are controlled.
Lace bugs that feed on azaleas and rhododendrons spend the winter as eggs laid in leaf veins on the underside of leaves. Adults cover the eggs with varnish-like material to attach them to the leaf. This along with droppings and cast-off skins of immature forms gives the lower surface of the leaves a speckled appearance. Eggs laid in fall will hatch from late April through May and can be found in various developmental stages until August or September. Two or three generations are produced in Pennsylvania each year, depending on the length of the growing season.
Controlling lace bugs on azaleas begins with choosing resistant varieties. Examples include: "Autumn Amethyst," "Autumn Twist," "Dawn," "Pink Star" and "Red Wing," among others. When choosing plants from a nursery or catalog, be sure to read the descriptions or ask about resistance.
Plant azaleas and rhododendrons in filtered shade or expose them to morning sun and afternoon shade to prevent stress. Water deeply during drought and provide good quality soil with ample drainage. Use a soil test to determine nutrients needed and to control pH. Monitor plants in the spring and use horticultural oil or insecticidal soap to thoroughly cover the lower surface of leaves when populations of young lace bugs are high. For adults, there are some recommended insecticides that you would need to repeat in 10 to 14 days to control for newly hatched insects. Read and follow all label directions and precautions. However, be sure to protect their natural enemies that act as predators by minimizing the use of insecticides. Assassin bugs, lacewings, parasitic wasps, spiders and mites are a part of long-term integrated pest management and can keep small infestations under control.
Plants grow best and thrive in their natural environment. Consider the effort needed to adapt the plant to your landscape before selecting those that require extensive maintenance. Make choices that fit the conditions you have to ensure success. If you have questions about planting azaleas and rhododendrons or control of pests, contact the Garden Hotline at beavermg@psu.edu. Include a photo or two to help Master Gardeners further analyze and respond to your inquiry.
Mary V. Clark is a Master Gardener with Penn State Extension, Beaver County.
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