Q: We have cherry laurel shrubs that were growing very well for a few years. Yesterday, I noticed some of the leaves on one side of a laurel were brown. When I looked closer, I found that all of the branches inside were covered with a white substance. What is it? What can I do about it?
Answer: The white substance on the branches is an infestation of white prunicola scale. Scales are very tiny insects that feed on plant sap. Their feeding leads to leaf yellowing, browning, and eventual dieback of branches.
Scale insects are challenging to manage. The waxy white substance they produce provides a form of protection from desiccation and predators. They produce three generations each year, so if a population is not controlled all at once, they can continue to reproduce and be a persistent problem.
When there is a heavy infestation of white prunicola scale and dieback is severe, it may be best to remove the shrub altogether rather than try to treat it. Depending on where the infestation is located, you may be able to prune out branches selectively and discard them.
Fertilized female scales overwinter on the bark of the branches, so another step in management is to apply a dormant rate of horticultural oil during the dormant season (when deciduous plants have lost their leaves). You can first use a soft-bristled brush to scrub off the scale patches gently. Then apply the horticultural oil.
If female scales remain on the shrub, juveniles (called “crawlers”) will emerge in the spring. Crawler periods vary from year to year depending on temperatures. For this species, new generations may be out in May, mid-July to mid-August, and September.
The tiny juvenile crawlers are salmon-colored. You can place a simple trap to detect their activity; wrap a piece of double-sided tape around a few branches. When you see crawlers stuck to the tape, that is the ideal time to apply horticultural oil. Oil spray is the least harmful to beneficial insects such as ladybird beetles, which you want to keep in your landscape because they help tamp down on other pests.
White prunicola scale is a fairly common problem on cherry laurels. Residents who have these shrubs should check them periodically for leaf yellowing and white spots on lower branches. Scales are easier to manage if you catch them early.
By Christa K. Carignan, Maryland Certified Professional Horticulturist, Coordinator, University of Maryland Extension Home & Garden Information Center
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