Yesterday I managed to video a Cerceris rybyensis that had ejected four paralysed halictine bees from a burrow. I saw no other female, but I am intrigued by this behaviour. Is this likely to be an instance of one female usurping another from a burrow? I have surmised that the new occupant of the burrow removed all the previous prey items, in case the competing female has already layed eggs on the cache of prey. The new occupant only wants her progeny to survive.
The video is here: http://www.youtube.com/watch?v=vMNLu_lidZo You can see the four paralysed bees that have been ejected from the burrow and the wasp in the background. A Myrmosa atra also puts in an appearance.
It was also intriguing that the burrow had two entrances. The wasp entered through one hole and came out via another, then flew down to the first entry and re-entered. So there must have been interconnected burrows within the bank.
Very interesting video. Did she put the bees back, or was she just stealing the burrow? I have seen (and filmed) an Ammophila sabulosa remove a caterpillar from the nest of a different female, remove the egg from the caterpillar, replace the caterpillar and lay another egg on it inside the burrow. (I know she laid another egg on it, as another Ammophila female appeared and did the same thing. I checked all the shots closely, and there are definitely at least two different females).
Spent about a month late last summer with a group of Ectemnius wasps nesting in a fallen beech very close to the sea and was lucky enough to catch the moment one female usurped the nest of another. Pics 1-4 show usurping female entering the hole and then chucking out a hoverfly prey. Picture 5 shows usurped female wasp returning. She was soundly rebuffed by the usurper and kept returning to her rightful nest to see if she could get back in. After about an hour, she gave up.
I have read that it's the destruction of the rival's egg that motivates the removal of existing prey, as noted in previous reply. Quite savvy behaviour!