Help with rhodies

Asked July 8, 2014, 12:51 AM EDT

Can someone please help me identify what is going wrong with my rhodies? I have four plants that are suffering in various degrees from the issue shown in the photo below. I've thought perhaps it was because of a lack of water, but the soil has stayed moist below the bushes most of the summer thus far. They do receive a lot of sunlight from sunrise through mid afternoon.

It might be hard to see in the photo, but there is also a light white powder like substance on some of the leaves too.

Washington County Oregon insect issues horticulture

1 Response

Thank you for including an image with your inquiry as it makes it easy to diagnose what’s occurring. Your rhododendrons have been attacked by a recently arrived invasive insect, the azalea lace bug. Even though the shrubs are moderately heavily attacked, the lace bugs won’t kill them.

Among the keys to successfully limiting azalea 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. 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.
- Most insecticide sprays are contact materials; in other words, the spray must thoroughly cover the undersides of the leaves. (See the list below.)
- Repeated 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.
- Don’t spray 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
- Several options among systemic pesticides are acephate and imidacloprid.

If you decide to replace your azaleas, Encore azaleas have proved themselves to be among those tolerant of lace bugs in other regions. Their performance in the northwest remains to be seen. No similar tests have been made with rhododendrons. But, in general, those with heavy indumentum (a felt-like layer) on the undersides of the leaves escape damage.

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