It's a disease, not an insect - Coryneum blight, aka shothole fungus. It is extremely common on members of the Prunus family (and on other stone fruits) and tends to be most prevalent in warm, wet, humid springs and early summers, like much of the country has experienced.

Treating with a fungicide in fall can help to eliminate overwintering fungal spores but you will also need to spray in spring as well.

Here is a link that might be useful: shothole fungus

spraying will do absolutely nothing to cure what is already there...

insure full fall cleanup of all dropped leaves ....

and following spraying instructions EXACTLY .. should you go that route...

all that said.. ITS AUGUST.. everything in my garden looks a little beat up..i say.. so what ... rarely do things die from bugs.. or disease ... though i do admit.. i never diagnosed this one.. i would have said JBeetles.. lol ...

i simply dont waste the time.. nor money ... to deal with it ... this time of year ....

and if you never saw it before.. there is probably an argument.. that you dont need to spray next year ... it being one of the vagaries of ma nature ...

ken