Hello Olga,

Based on what I can see, I think you're dealing with European Elm Scale, a type of insect that sucks plant juices from branches and leaves.  

Because the tree has not yet broken into leaf, you could still be successful treating them with horticultural oils (also known as "dormant oils").  Good coverage of the tree and insects is key; the oil smothers them.  

Another option for small trees is to hand-scrub or power-wash them from branches.

Insecticides are also available that will kill these insects.  Many active ingredients are available, including pyrethroids and carbaryl.  Always read the label of any insecticide product before purchasing and applying, and always apply in strict observance of the label instructions.  The most effective insecticide active ingredients for European Elm Scale are available only to licensed pesticide applicators, should you choose to hire someone to treat the trees they will have the most chemical options possible.

More information about this insect and treatment options can be found here.

I hope this is helpful!

Cordially,

John