Over the past few days, I've noticed some strange honeybees hanging around the entrance to one of my hives, the other on the opposite side of the property has not had these visitors. I have Italians, and these guys have substantially larger bodies and have much bigger eyes (see picture below). I saw some of mine fighting with one of them a day or two ago, and today there seem to be more around my reduced entrance... and they are lifting their rears like they might be marking. Is this an invasion waiting to happen? I think the hive is in reasonably good shape - anything that I can do other than keep the entrance reduced and hope for the best? Thanks for your advice!