CORVALLIS, Ore. – Gardeners and nurseries should be on the lookout this spring for a relatively new pest in Oregon that damages azaleas and rhododendrons, according to experts with the Oregon State University Extension Service.
The azalea lace bug was first confirmed in Oregon in 2009 by OSU researchers after it was found in Washington in 2008.
"On the East Coast, it's caused significant damage and since it's been here, certainly in the Portland area, I've seen a lot of damage," said Weston Miller, an OSU Extension horticulturist.
The pest, known as Stephanitis pyrioides, can survive the winter and breed multiple generations each year, making it particularly worrisome, said entomologist Gail Langellotto, the statewide coordinator of Extension's Master Gardener program.
It becomes active in mid- to late May and early June, when it starts laying eggs. So starting in mid-May, gardeners should keep an eye out for the eggs, which are partially embedded in the tissue underneath leaves, Langellotto said.
With its piercing-sucking mouthparts, the bug feeds on plants in the Ericaceae family, which includes the rhododendron, azalea and pieris. Adult and immature bugs eat the leaves, leaving a yellow dot-like pattern on the surface and black fecal spots underneath. Large populations can cause azalea leaves to turn white. On rhododendrons, severe damage may look like iron chlorosis with yellow leaves and green veins. Heavy feeding can kill plants.
Some azaleas are immune to the pest, including Indica alba, Flame Creeper and Delaware Valley White. But Langellotto cautioned that these and other resistant varieties have not been tested in Oregon's climate. Monitoring for the pest and dealing with it early in the season is a gardener's best defense, she said.
Extension educator and entomologist Robin Rosetta noted that azalea lace bugs could be managed using a combination of techniques.
"Hopefully, we will begin to see biological control over time," Rosetta said. "I would hate to see people removing plants that might one day have acceptable biological control."
Biological control is a strategy to reduce pest populations using techniques like introducing natural predators.
Azalea lace bugs are more likely to damage plants in hot, sunny locations or in drought-stressed conditions. So for new plants, choose a partially shaded spot in your yard. Water and fertilize the plant according to the instructions for it.
Natural predators such as tree crickets, earwigs, green lacewings, minute pirate bugs and spiders kill and eat azalea lace bugs. These "good bugs" are more abundant in areas with a variety of trees, shrubs and understory plants, so planting your rhododendrons or azaleas among them, Langellotto recommended.
You can also apply insecticidal soaps, horticultural oils and neem-based products regularly to the bottom of the affected leaves to reduce or prevent further damage, but they will not restore the plant to its untouched appearance.
Additionally, you can spray pesticides that kill insects on contact, such as pyrethroids or carbaryl. Coat the leaves well, including on the underside. Note that some insecticides can harm beneficial insects that eat pests or pollinate plants, Langellotto said. If using pesticides wear protective clothing and follow all label directions.
Rosetta has written a fact sheet for gardeners with information on how to control the pest and what varieties of azaleas resist it. She wrote a similar fact sheet for commercial growers that is available here.
Find her photos of the bug and links to other resources here.