whiteflies on rhodies
I have a bed of large, mature rhodies - leaves have become mottled and pale, undersides spotted with rusty looking residue that can be wiped off. Local garden shop identified as whitefly, the rusty spots as eggs. What is the best treatment at this stage? Plants are loaded with buds and have been beautiful and trouble free for years. Will work to save them!
Clackamas County Oregon
Thank you for your question to Ask the Expert.
Based on your description of mottled leaves and dark spots, it is possible that you have lace bugs, which are en emerging pest in the Pacific Northwest that is affecting azaleas, rhododendron, and related shrubs. Here is a discription of the pest: http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7428.html
Does this diagnosis match what you are seeing? White flies typically do not damage rhododendrons as you've described. Here is a description of whitefly damage for comparison: http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7401.html
If you suspect the lace bug and you are willing to do the care needed to protect them, then below are your management options: cultural, biological, and chemical.
Management—cultural control Maintain plant health by providing proper water and nutrition. Stressed plants are more susceptible to insect damage. Grow azaleas in shady areas to minimize damage. Remove leaves with brown patches of eggs along the midrib. Hosing plants with a strong stream of water directed at the underside of leaves will help to remove them, and wingless nymphs will not return. Encore Azalea resistant cultivars include: ‘Autumn Amethyst’, ‘Autumn Twist’, ‘Autumn Royalty’, ‘Autumn Sangria’, ‘Autumn Cheer’, and ‘Autumn Rouge’ He also showed that some cultivars were moderately resistant: ‘Autumn Embers’, ‘Autumn Bravo’, ‘Autumn Starlite’, ‘Autumn Ruby’, and ‘Autumn Princess’. Management—biological Release of lacewing larvae have proven successful in one experimental test. Select insecticides to preserve populations of beneficial predators which will help control lace bugs. Management—chemical: HOME USE
2. azadirachtin (neem oil)
4. horticultural oils
6. insecticidal soap
7. spinosad A & D
Please let me know if you have further questions.
Thank you for your inquiry.