Looks like aphids and mealy bugs... nice combination. These critters like the new tender shoots; and especially if the tree has been well, or too-well fertilized. You can just wash them off with a water spray, if you only have a few; but you have to do it when you first see them; in a few days they can do a lot of damage. If you have a lot of trees or a larger infestation, I would treat with Sevin; pretty harmless and it is a good general garden sanitizer. A note of caution; over using insecticides, even Sevin, can eliminate beneficials; and sometimes lead to outbreaks of other insects. This time of year, it is not too hot, so you might add a little soap to your cleaning spray to make it work better; don't do that when it is hot, as it will burn your leaves.

Sevin is absolutely famous for causing a rousing population of pests such as spider mites, scale, aphids, and mealies.

The big black ones are 'aphid mummies', which are the swollen shells of parasitized aphids. The parasitoid is a tiny wasp...which would, of course, be wiped out with Sevin.

How many trees do you have and how large are they?

spray your trees with soapy water, use a strong spray. check your trees every few days, spray as needed.