Hi Steve, After making up a nucleus colony and placing a ripe Queen cell in it there comes a perfect time to do an oxalic acid dribble. Because the brood that was used to make up the nucleus colony will be all emerged and the new queens larva will not yet be capped, So the varroa mites have no place to hide. The timing for me is about 20-21 days after making the nuc and about 16-17 days after queen emergence.