its one of the scale bugs. If there are not too many squish them, or wipe them off with cotton woo soaked in meths. if there are lots then a systemic insecticide is your best option.

Looks like moth larvae - you should be able to wash them off with soapy water.

Thank you both very much indeed for the prompt response to my question. It is very much appreciated.

One of my customers had this bug on one of her hydrangeas. I sprayed it with all sorts of bug killers to no avail and regrettably it died.

If anyone can recommend a specific systemic insecticide for this infestation I'd be pleased to hear it.

Provado will work, but it is pretty slow. The best thing that I have used is a combination of a non-ionic surfactant, and carbaryl, mixed together.

When I first saw this I thought it was bird muck, I find wiping it off works for me.

Anchorman - Hydrangeas are extremely sensitive to a vast number of pesticide sprays - it may be that you killed it off yourself by treating it, lol! This is hydrangea scale - may cause die back of some stems if infestation is heavy, but doesn't usually kill the plant. Malathion used to be the treatment for it, but its been withdrawn, so an insecticidal soap solution is the next best thing, sprayed when the nymphs are young. Tugbrethil, what do you mean by a 'non ionic surfactant' precisely?

Well, they are frequently sold as wetting agents, or "spray helper", or as "spreader-sticker". They help the chemical involved get through the waxy shell and semi-waterproof fuzz of the scale. They don't break down into potassium and fatty acid ions, like soap does, so they are much less likely to degrade the performance of the insecticide (or weedkiller).

I'm just wondering how you get hold of such stuff, perhaps its only available to the horticultural trade here. I've certainly seen the term 'non ionic surfactant' on cleaning products, but hesitate to use those for this purpose.

I wouldn't either! In the States, surfactants are commonly found at full up plant nurseries, and some of the bigger hardware stores. The brand that I sell at work is ferti-lome Spreader Sticker. Some stores with a big selection of organic sprays will also sell yucca extract, which does the same thing. I am still investigating it's effect on chemical stability, since it is primarily intended for helping with foliar feeding!

Something for me to Google later, 'surfactant'...