He attended our Beekeeper's meeting this month (Middlesex, Elgin & Oxford Beekeepers' Association) and he talked about some of the studies and surveys carried out over the last two years and gave a demonstration on how to create splits to make up for winter losses.
In Ontario, it is agreed that hives need to have their fall treatment done in August. Waiting to Sept/Oct is too late and a couple beekeepers who reported losses of 80% had done their treatments after Aug.
Note that these notes apply to beekeeping in Ontario. April and May is too early to create splits because the bees have not built up their population yet. The methods to divide a colony vary considerably, depending on the circumstances.
One or two days later you can introduce a local queen. He did not recommend letting a weak colony raise it's own queen, stating that a weak colony won't raise a good queen.