What are the little worms eating leaves on my flowering cherry tree - they also hang from the branches?
Hardiness Zone: 6a
They are probably canker worms ( Paleocrita vernata) or a similar species that defoliates trees. Apple and cherry trees are their favorites. The population peaks, then levels off to almost being unnoticeable for a few years before building back up again. Spraying (malathion, Sevin, etc.) will give partial control, but the ones that are missed or survive the spraying may live on to reproduce more resistant worms in future years.
Be careful spraying with pressurized insecticides which could cause physical damage to the plants. Schultz sells Fungicide 3, which uses a natural product from the Neem tree called Neem oil, which kills all kinds of chewing insects, controls many fungal diseases, and also acts as a miticide. It should be available in most stores that sell garden products. Rather than being pressurized, it is in a pump container that will be more safe for your cherry tree.
We get a horrible critter (quite beautiful! green with a blue back), tent caterpillars, that defoliate all kinds of trees. They tend to come in cycles, getting worse and worse year after year, until they eat themselves out of habitat, and then you don't have them for years. Your worm may also be this. They eat the leaves, then lower themselves down on silken threads, and move to the next tree or if they are mature, make a cocoon. Spraying them with an insecticide works if you have a slight infestation in your region. If there are billions of them, it just isn't worth it. Let nature take its course.
In the cities here, they put sticky bands on some of the trees that are prone to canker worms. It doesn't stop them when they are alread in the tree, but will prevent infestations in the next season. That is certainly worth looking into.
You probably have your answer by now but, it sounds like you have "Pear and Cherry Slug" Most fruit tree insecticides will work. They pupate in the soil and climb up the tree SO it is recommended to spray the base of your tree with a surface spray. Pear slugs can be controlled by spraying soapy water onto leaves then wood ash. They are the larvae of a sawfly, a tiny wasp.