rhododendron/azalea disease

Asked October 2, 2014, 3:34 PM EDT

My plants are mottled with tiny flicks of yellowing, they are not a healthy green. Underneath the leaves it appears to be a bug infestation. Adult bugs are the size of a fruit fly with reflective wing areas, while the young are shiny black. What is needed to save my garden? Any recommended solutions will be greatly appreciated. Thank you, Evelyn

Clackamas County Oregon trees and shrubs insect issues horticulture

1 Response

Thank you for the excellent description of azalea lace bug. Since the azalea lace bug arrived in our region, it has been the scourge of azaleas and rhododendrons. It’s worth knowing that even though the shrubs look nasty, most will live to bloom year after year.

Among the keys to successfully battling lace bugs are these:

- Stressed plants appear to be the most common victims. To relieve stress, thin out individual plants to increase aeration, and ease of applying sprays; irrigate every two weeks through our dry months for good health; and decide if the plants are in excess sun – if so, consider rigging temporary shade or moving the shrub in the fall.

- Start spraying the undersides of the leaves when the first generation hatches, usually mid-May because killing the new hatchlings is the best bet to decreasing the population. Monitor to determine the appropriate timing. To do so, check the undersides of the leaves every several days as you look for the small dark colored nymphs (youngsters). The most effective time to spray is right after the hatch, when the nymphs are in a rather tight cluster on the undersides of the leaves. Even something as simple as a harsh water spray to the undersides of the leaves will be very helpful at this early stage.

- Insecticide sprays must thoroughly cover the undersides of the leaves. (See the list below.)

- Repeat sprays of these materials are required during the growing season because the lace bugs have multiple generations. You have the best chance to decrease the population – and damage – when new generations hatch in mid-May and again in June, July, August, with perhaps another in September. Again, a harsh water spray can be very effective.

- Don’t spray any pesticide if the temperature is, or will be, above 80F.

The following contact insecticides will temporarily control lace bugs if the product thoroughly covers the underside of leaves where lace bugs live and feed. Repeat the spray according to label directions:

- Azadirachtin

- Insecticidal soap

- Narrow-range horticultural oil. Use in the fall to coat the undersides of the leaves where the eggs are laid alongside the midrib.

- Neem oil

- Spinosad

You also have several options among systemic pesticides: acephate and imidacloprid.

You might like to review the recently published “Azalea Lace Bug” http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/40424/em9066.pdf

If you decide to replace your azaleas, Encore azaleas have proved themselves to be among those tolerant of lace bugs in other regions. Their value in the northwest has not yet been evaluated.