We would like to have more information. From your description and photos the white bumps on the foliage looks like it may be a type of scale insect (a sucking insect).  Let us know how old the tree is, site conditions, how you care for it, etc. 

Also, please send us a photo of the whole tree, around the base of the tree, and clear photos of the insect (white bumps) on the foliage and or branches so we can see what you are dealing with.

Marian

The acer palmatum Kamagata ( sorry but auto correction happened in my first email that I failed to see an dit happened again now ) was purchased and planted in Sept 2009 so it is 11 year. old  It was slightly shorter, perhaps 2-3 feet but  not as full. Since then, it has grown and became fuller. 

It is planted on the open next to  a huge expanse of cobblestone plaza which surround two marble slabs that has 2 bronze sculptures by Martin  Eichinger . A few feet away are some microphyllia compacta Kingsville dwarf  boxwood. Behind it, is a micro clover lawn.

The tree is surrounded by thick slabs of granite ( I believe 11 inches thick as they had to be installed with a crane) and  I do not mulch the tree but as usual, I often have to pull weeds such as a few days ago, Most of the weeds are chickweeds and some evening primrose that pops up everywhere. Since I was pruning  my laceleaf maple that day, I thought I would thow the leafs rom the maple ( no disease in them, just that I usually prune to give them the oriental Japanese look and so I pruned the ones that are crossed and pointing downwards) . This is the first time I did so . Will that harm the tree? It was. them when I noticed the scale insects. 

Enclosed are 2 pictures of the tree. 

Thank you for prompt response.

The acer palmatum Kamagata ( sorry but auto correction happened in my first email that I failed to see an dit happened again now ) was purchased and planted in Sept 2009 so it is 11 year. old  It was slightly shorter, perhaps 2-3 feet but  not as full. Since then, it has grown and became fuller. 

It is planted on the open next to  a huge expanse of cobblestone plaza which surround two marble slabs that has 2 bronze sculptures by Martin  Eichinger . A few feet away are some microphyllia compacta Kingsville dwarf  boxwood. Behind it, is a micro clover lawn.

The tree is surrounded by thick slabs of granite ( I believe 11 inches thick as they had to be installed with a crane) and  I do not mulch the tree but as usual, I often have to pull weeds such as a few days ago, Most of the weeds are chickweeds and some evening primrose that pops up everywhere. Since I was pruning  my laceleaf maple that day, I thought I would thow the leafs rom the maple ( no disease in them, just that I usually prune to give them the oriental Japanese look and so I pruned the ones that are crossed and pointing downwards) . This is the first time I did so . Will that harm the tree? It was. them when I noticed the scale insects. 

Enclosed are 2 pictures of the tree. 

Thank you for prompt response.

Thank you for the additional photos and the landscape looks nice. Overall, the maple tree looks okay from a distance. However, we would like to see photos of the insect that you are referring to. The photos need to be close, in focus, in good lighting, include a ruler or coin to show size. Please send photos of the insect on the foliage or the bark or stems so we can see what you are dealing with.

Marian

In your first photos that you submitted, the white bumps (marshmallow consistency) on the leaves were out of focus and we could not identify them.   The photos were out of focus.  

We think the white bumps look like a type of soft scale insect.  In general, scale insects suck sap from the foliage and exude a honeydew as they feed. That is the sticky residue that you mentioned. 

There are many types of scale insects. In order to control them  they should be identified first. Underneath the bumps are eggs which turn into small crawlers. The crawler stage is when they are best controlled.  

 Go out and look on the tops and undersides of the leaves and stems/branches of the maple tree to see if you notice the white bumps. Look for the sticky residue on the leaves.  

If you see them, send us clear photos of the white bumps on the leaves, stems, or branches so we can give you more information.

Here is our link on scale insects https://extension.umd.edu/resource/soft-scales-trees-and-shrubs

Marian

Here are some pictures which will help identify the scales ( I think they are soft scales) and perhaps you can suggest treatment  when they become crawlers. 

Yes, we can see them more clearly now, thank you. They are indeed soft scales - in particular, either Cottony Maple Scale or Cottony Camellia Scale. The insect itself is the flattened brownish oval; the egg sacs are what's covered in the cottony material. Females are dying off this time of year, and can fall off the plant when they do, so egg sacs may be present without an attached scale. Despite their names, either species can feed on maple and a wide range of other trees/shrubs; they look nearly identical. Fortunately, their treatment approaches are the same.

This is looks like a light infestation, so control tactics are not necessary; the plant's overall condition looks very good. If you wish to treat anyway, the most practical method would be to hire a professional pesticide applicator to use a systemic product. These are absorbed into plant tissues and not subject to weather, plus tend to be long-lasting. They should have negligible impact on pollinators since the maple will not be in bloom again until next spring.

Otherwise, horticultural oil would need to be applied very thoroughly on all upper and lower leaf surfaces, either in June for the crawlers and/or in winter for the older immature stages. The former will be a lighter dilution, called a "summer rate," while the latter would be a "winter rate," a.k.a. dormant oil, applied to bark. Multiple treatments may be needed to provide good control, though see the product label for specific instructions about dosing and timing of repeat sprays. No topical spray should be applied during temperatures above 85 degrees or they risk damaging the foliage.

If you opt to take no action for now, simply monitor the plants for the development of higher populations as we move through the year. Both species only have one generation per year, so the easier-to-see older juveniles and adults will not be present until late this year or early next spring. It's possible their numbers will be kept low without your intervention by the action of beneficial insects that find them on their own.

To keep the tree in general good health and to minimize its stress, consider mulching its root area so roots face less competition from weeds and any lawn (clover, evening primrose, or otherwise) nearby. The reduction in suitable root area from the patio stones, plus the full-sun exposure and reflected warmth from those stones, may make the tree more prone to drought stress. Mulch helps moderate soil temperatures, cooling roots and slowing evaporation. It will also slowly add organic matter to the soil, which also improves conditions for roots. Since our recent weather has put us behind on average rainfall, check the soil in the maple's root zone periodically for watering needs, and soak well when it gets too dry.
https://extension.umd.edu/resource/watering-trees-and-shrubs

https://extension.umd.edu/resource/mulching-trees-and-shrubs

Pruning to remove crossing branches and those growing at awkward angles is fine. According to Virginia Tech data, the ideal times to prune maples are May through July with a second window of opportunity November to December. Trimming in late winter or early spring, on the other hand, risks lots of sap ooze from wounds that don't seal as quickly.

Miri

You're welcome.