The hornets spoke in Russian accents. They constantly said, "I sting your face!"
They first appeared in "Sting Operation," in which they are threatening a group of school kids. The penguins try to find a way to get rid of them. At the end of the episode, the hornet nest was destroyed, and the hornets were forced to leave.
They returned in "Siege the Day" with a newly built nest, this time outside the Zoo walls. When they smell the food at Julien's Pool Party. They decide to invite themselves to the party to get the food. Upon arriving, they scare off all the animals and force Private, Eggy, and the Ducklings into hiding in the HQ, and refuse to leave unless they are given food. When Private finally opens up the hatch, the Hornets wind up facing the Rat King and his Sewer Rats (whom also want the food) and a massive fight breaks out. In the aftermath, the Hornets give up and leave after getting beaten up by the Rats.