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my mum asked me for some help with a rhododendron shes had for years. Shes noticed recently some bugs on the underside of the leaves - little yellow things less than 0.5mm in size that seem very flat and sleek in nature. Also, there is a very visible widespread black covering over the topside of the leaves. This however just seems to rub off the leaves if touched leaving a seemingly healthy leaf underneath.
She has told me that spraying it with some insecticide of sorts has not worked and she hasnt a clue what it is or how to get rid of it. Any ideas?
They're scale insect. They suck sap and excrete sugar, and this drips onto the top of the leaves and mould grows on it, hence the black leaves.
Spray with Provado Ultimate Bug Killer, making sure that you wet the underside of the leaves, and that will kill the scale insect. You can then wash the leaves after a couple of weeks to get rid of the mould.
If the insects appear again, spray again until they're gone.