Hello
I recently noticed that my Summersweet shrub has some sort of infestation. There are white sticky clumps on the underside of the leaves with some dark clumps (shown in photo below). The shrub is located on the east side of the garage in a partly shady/partly sunny location. I’d appreciate your advice on the probable cause of the infestation and what I can do about it.
Summersweet (Clethra alnifolia) is a lovely shrub and typically has few pests. I believe you have found a kind of soft scale insect.
Steven Frank’s publication at North Carolina State Extension describe soft scale as such:
“Soft scales often produce ovisacs (egg cases) that are very convex or cottony and look different than other life stages. Crawlers and second instars are small and inconspicuous and may feed on the leaves, branches, or move between the two depending on scale species and time of year. Many soft scales have nymphs that feed on leaves in the summer but move back to branches before leaf fall. Soft scales damage plants by extracting plant fluids from the phloem. This can reduce plant growth and cause leaf drop or branch dieback. The most common symptom of soft scale infestation is accumulation of honeydew and sooty mold on or beneath a plant.”
A succinct response from the Home & Garden Information Center’s Maryland Certified Professional Horticulturists sums up management:
“They are insects that feed on the plant sap, which can lead to yellowing and gradual decline of the shrub if there is a severe infestation left unmanaged. If you see just a few leaves with these, the easiest thing to do it to wear garden gloves and pick them off by hand — and pick off any you see along the stems. There are some beneficial insects that can help to control these pests naturally. Once you remove these scales, continue to monitor your plant regularly during the growing season. If further treatment is necessary, you can spray the shrub with a dormant rate of horticultural oil during the dormant season (when leaves have fallen off deciduous plants) to control overwintering scales.”
The Missouri Botanical Garden also has good information on both armoured and soft scale pests here.
This article from the Piedmont Master Gardeners has information on care and maintenance to keep your Summersweet shrub healthy which is the best way to support it when stressed by a pest. Something to note is that Clethra alnifolia prefers moist, well-draining, slightly acidic soil.
June 26, 2024