First Varroa drop in my new apairy

I'd expected that the new bees would bring Varroa. Today I found a dead mite from the colony which I hived earlier this week. It was on the floor of their hive.

The feral colony from which both casts came has apparently been there for 3 years. Today I could see their entrance which had lots of activity. It would appear that they are very strong despite this infestation.