Answer: The yellowjackets are finding a source of food. Cherry trees exude a sticky sap which is sweet and that's probably why the yellowjackets are swarming around your tree. About the only thing you can do is discourage them by hosing off your tree periodically. Water from a sprinkler will drive them away and it will also dilute or rinse away the sap. There's not much else you can do to discourage them. Wish I had more solutions!
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