I can't seem to keep the robbers out of my nucs. Iv'e been keeping them closed except for night and early morning. But they have virgin queens and they need to mate. I only put a little sugar syrup in them at night but if I leave them open they jump on them. I've lost several queens as a result of this killed them I suppose. I have tried vapor rub around the entrance but they like it too. HELP.... Rube