Are any of your rhododendrons looking pale? You may be bugged. If you see pale speckles all over rhododendron or azalea leaves, turn over an affected leaf. If there are smutty-looking, blackish spots on the underside, especially along the midrib, then you may be looking at the larvae and poop of azalea lace bugs. You may see adults, too. An adult lace bug is a small, slow-moving, fly-like creature about one eighth of an inch long, its transparent wings marked with a black and white pattern. Last year, Hendricks Park staff noticed damaged plants in the rhododendron garden. Both rhodies and azaleas were afflicted. I’ve since learned that other gardens in the Hendricks Hill area have this pest, too.
The azalea lace bug (Stephanitis pyrioides) is an Asian immigrant that has been present in the Eastern U.S. for decades but was not reported in Oregon until 2009. Adult lace bugs insert eggs along the midrib on the underside of leaves and cover them with excrement (nice!). Eggs overwinter there, and hatch out in spring — mid-May in our area. Hatchlings go through five immature stages before adulthood. Young larvae are transparent at first, becoming dark and spiny as they mature.
All stages of the lace bug have sucking mouth parts that penetrate the lower surface of the leaf and suck out cell contents, including the chloroplasts, the green power houses that turn light into food. Badly infested leaves may turn almost white, and eventually shrivel and die. If it happens repeatedly to enough leaves on one plant, that bush may be severely damaged. At best it will look awful.
Unlike the relatively benign rhododendron lace bug, an indigenous species that goes through one generation a season, the azalea lace bug may have three or more generations a season. This is probably one reason it appears to be more damaging. And in the Pacific Northwest, unfortunately, azalea lace bug attacks a number of native plants in addition to native and non-native rhododendrons and azaleas.
According to Jim Labonte, a reseacher with the Oregon Department of Agriculture, damage has been detected on four species of vaccinium (the genus that includes huckleberry) as well as kalmiopsis, heathers and salal. This insect could become a serious problem for Northwest ecosystems.
As far as gardeners are concerned, there is some good news. Infestations seem to be worse on plants that are already stressed — by drought, for instance — and on ones growing in the shade. And not all rhododendrons and azaleas are equally susceptible. The difference seems to lie in the texture of the leaf’s underside. Natural waxes may play a role, and rhodies with heavy indumentum (the furry coating on the underside of leaves) appear to be resistant. If you are planting new azaleas and rhododendrons, it would make sense to research resistant varieties.
What about affected plants already in your garden? Systemic synthetic insecticides are effective, needless to say, but you don’t want to use those, do you? The good news is, there are alternatives, such as insecticidal soap. A crucial factor for success with any treatment is timing. Another is coverage. And you need to be persistent. Effective treatment requires hitting the underside of every leaf and repeated spraying, perhaps every two to four weeks throughout the growing season. Appearance of the first hatchlings in May is the signal to start a spray schedule.
This summer, the Hendricks Park gardener implemented a test project using four treatments. A contractor was hired to spray affected plants with either Safer insecticidal soap or with EcoTech, a blend of rosemary and peppermint oils. Other plants were subjected to heavy pruning and manual defoliation, with the infected material removed from the park. In another area, beneficial insects were released. The most effective treatment appeared to insecticidal soap.
Treatments are continuing this year on the selected plants, and will be expanded to all affected plants next year, if funding can be found. In addition, a landscape service has been spraying Azatrol, a neem derivative, on affected plants for private clients in the Hendricks Hill area and has reported a substantial degree of control. Azatrol and insecticidal soap both meet organic standards.
The company hired by the city of Eugene to spray at Hendricks Park is Glass Tree Service, which recently achieved some notoriety for killing bees at a Eugene apartment complex by spraying a neonicotinoid product on flowering linden trees. That inexcusable event occurred after the first spraying at Hendricks Park for azalea lace bugs, and was an embarrassment for both park staff and for the Friends of Hendricks Park, who helped fund the spraying. The Friends attempted to find a different contractor, but Glass appears to have a monopoly on spraying pesticides of any kind on trees and shrubs in our area. It would be really nice to see a different contractor, preferably using organic methods only, license and equip themselves for this kind of job.