Strangers on my tree.

We have a dwarf cherry tree that hosted some strange parasites last
year. They were small, black, flat, shiny, slow-moving worms that
looked just like the flatworms that sometimes threatened Muldur on the
X-Files. They ate lace patterns on the leaves. Rosebush dust put
them to rout last year.

Now the buds on the tree are opening and I am concerned about the
infestation returning. I have Googled around but have not been able
to isolate their identity or find a specific for their prevention.
Does anyone here have a clue? TIA.
.