The cold weather of recent days has probably slowed the expansion of all bee colonies in general. The package bees that were installed on April 10th should be having emerging brood and the colonies populations will be expanding.
With the increase in population the bees will be drawing out the foundation quicker. When the bees have drawn out 8 of the 10 frames it will be time to add a second box.
When adding a second box, take a frame that the bees are working on and move it from the bottom box to the second box. The bottom box will have 9 frames and the top box will have 10. Continue feeding syrup if foundation is still needing to be drawn out.
If the bees were installed on drawn comb they may be ready for another box by this weekend.
