>> I don't want to use pesticides near my bees, but would like to 
understand why ants would leave their nest with eggs to end up on top of 
a beehive.

You don't need poison. Just leave the escape hole open.

The space between the inner and outer cover is warm and dry. Perfect 
place for an ant gallery. So the ants take advantage of an opportunity.

If these are the same large black ants we get in Vermont, they use the 
space for something else. A butchery.  I usually find one when there are 
ant galleries of this species, under the outer covers.
Mike

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