Understanding the lace bug invasion
Have the glossy green leaves of your azalea acquired green or white speckles? Living in the Pacific Northwest, this unasked-for leaf variegation may likely be due to the azalea lace bug (Stephanitis pyrioides)—a tiny insect with distinctive lacy wings and a passion for sucking the sap from the undersides of leaves. But don’t get out the sprays just yet. As with many sap-sucking insects, lace bugs often show up in gardens when an azalea is stressed and has a lowered immune system, whether it is not getting enough water, is located in too much sun or does not have the food it needs. Adopting a “Right Plant, Right Place” mentality and working from there can discourage a lace bug invasion and the use of pesticides that might be harmful to beneficial insects.
Where did these buggers come from?
Native to Japan, lace bugs are world travelers that catch rides to new countries on their host species. They were most likely introduced to the East Coast by hitching a ride on a ship importing plants from overseas, and then traveled west across the United States via transportation of plant nursery stock. So far, lace bugs seem to have escaped their original natural predators by traveling to new homes across the ocean. While there are not yet any known predators specific to lace bugs, they are consumed by many generalist insects such as spiders, green lacewings, soldier beetles, assassin bugs, pirate bugs and mites. Green lacewing larvae in particular have been observed to enjoy munching on lace bugs so far.
Lace bugs: A closer look
All Lace bugs are host-specific, meaning that species generally evolve in close concert with their host plant. Azaleas solely host the azalea lace bug (Stephanitis pyrioides). Only four species of Stephanitis occur on plants in the heather or Ericaceae family, which includes azaleas, rhododendrons, blueberries, cranberries and more. These lace bugs have two stages of growth—adult and nymph—each with piercing/sucking mouth parts. As they feed on the underside of the leaves, lace bugs they extract chlorophyll from the leaves which leaves a stippled appearance to the tops of the leaves and black speckles (feces) on the undersides. With several generations every year, lace bug adults lay their eggs inside the leaf cells of the host plant and cover them with excrement, making treatment with any pesticides difficult. Lace bug has become a problem in the Pacific Northwest because the mild climate means cold weather rarely kills off lace bug in any of its life stages.
Other plants in the garden are also hosts to different species of lace bug, all easily recognizable by their lacy wings and by the characteristic stippling on tops of leaves and black excrement on the underside. A few other lace bugs include rhododendron lacebug (Stephanitis rhododendri), andromeda lace bug (Stephanitis takeayi), hawthorn lace bug (Corythucha cydoniae), grass lace bug (Leptodictya plana) and avocado lace bug (Pseudacysta perseae). The greater world of lace bugs enjoys feeding on an even wider range of hosts: alder, ash, coyote brush, birch, ceanothus, photinia, poplar, sycamore, and willow.
Take action
The number one way to control azalea lace bug is to plant resistant varieties in the first place or as a replacement for overinfested plants. Of course, it can be heartbreaking to remove a plant for which you have strong affection, but there is a certain threshold of lace bug damage an azalea can handle until it is too weak to revive itself. Per Dr. Peter Schultz’s Virginia Beach Experiment, the following azalea cultivars have resistance to lace bugs and are listed in order of decreasing resistance:
Indica alba - white
Flame Creeper- white
Delaware Valley White - white
Rosebud* - pink
Cooperman - red
Hahn’s Red - red
Boldface - lavender
Mrs. G.G. Gerbing - white
Dream - white
Salmon Beauty - pink
Hinocrimson* - red
* indicates cultivars that may be easier to find in the Seattle area.
If unable to plant a new variety, start by attracting beneficial insects. While azalea lace bug has no specific predators, attracting green lacewings and other general beneficial insects such as spiders, pirate bugs and lady bugs will help lower lace bug numbers. Green lacewings specifically can be attracted into gardens by planting habitats of ornamental grasses and flowering plants like dill, dandelion, fennel, cosmos and yarrow. Maintain a wild area in your garden for a natural habitat that will protect and grow populations of beneficial insects.
Another method that has been successful for landscaping professionals is to spray the undersides of the leaves with products that contain potassium salts of fatty acids. These products are certified organic by OMRI, the Organic Materials Review Institute, which means they are not as toxic to humans, water quality, birds, mammals and other life than other pesticide. Still, any pesticide will kill insects, so be sure to follow label instructions closely and try to avoid spraying or minimally spray if you see beneficial insects on the azalea. Because lace bugs lay eggs inside the leaf tissue, it is difficult to kill the eggs, but spraying the underside of leaves will eliminate a number of hiding adults and nymphs. A second application within two weeks should capture any adults or recently-hatched nymphs that were not killed in the first application.The timing of year for spraying is crucial to capture these adults. In places like Georgia or California where the weather is warmer, lace bugs live year-round and may have multiple generations per year. Here in the Pacific Northwest, lace bugs begin to emerge from hibernation when the temperatures begin to rise into the 50s at night. This may happen between March and May and can be active all the way until late September to November. It is important to note that these sprays will not restore damaged leaves, so anything that has been infected by lace bugs will continue to look stippled or white through the next year until new leaves and emerge. Find products with potassium salts of fatty acids that are clearly labeled for lace bug control at www.growsmartgrowsafe.org.
Applying sea kelp to damaged plants can also help heal wounded tissue and increase the plants’ immunity, ultimately helping them to fight off secondary diseases that may arise thanks to their weakened state.
For more information on lace bugs or attracting beneficial insects, please contact the Garden Hotline at 206-633-0224 or www.gardenhotline.org. You can also find us on Facebook, Twitter, Pinterest and YouTube.
Other references:
National Pesticide Information Center's factsheet on Potassium Salts of Fatty Acids
Oregon State University's Lacebug factsheet
University of Georgia Extension's Factsheet on Lacebug Control