OK, your hives(s) have made it through winter. Things were looking good, the hive was building upnicely for the honey flow. Then bam, activity at the entrance is off sharply; the population is way down. You look inside and the hive appears queenless. What happened? Well it could be several things, all of which are out of your control.
First the queen has an awesome job in the spring. She must eat her weight in food every day and then pump out more the 1000 eggs each and every day for about 5 months in a row. First to increase the hive population and then to maintain it until the honey flow is complete in mid-summer. For some queens, especially the older ones, this is too demanding and they fail. It could be that they run out of stored semen or just old age. With old age comes a decrease in their pheromone output which can trigger their ejection from the hive. It seems logical that the queen would fail at this time of maximum effort. Then it takes the hive roughly 4 weeks to make a replacement providing they recognized the symptoms.
A second reason the hive may go through a decline is that the old queen was too successful and built up the hive to the point of swarming. Off she goes with the swarm. Most beekeepers are not aware their hive has swarmed because they have other things to do; like working. After the swarm leaves the hive must wait while the replacement queen emerges, matures, mates and finally begins laying. This takes roughly 3 weeks.
In both of these queen replacement scenarios success occurs about 70% of the time. The other 30% the hive ends up queenless. In this situation you as a beekeeper can do something.
New packages can also go queenless. It can be for: 1) the queen not being accepted by the hive, 2) poor mating of the queen, 3) or just queen failure.
So now what is a beekeeper to do? First and foremost you need to conduct regular inspections of your hives so you can realize something is amiss. Mainly it is to verify the hive is queenright (eggs and brood present) on a regular basis. But is also lets you see there are swarm or supercedure queen cells. These cells inform you something is afoot. Another good strategy is to mark your queens. This makes them easier to find and if you see an unmarked queen you know some type of supercedure has occurred. You can then also schedule a special inspection to verify that the new queen successfully mated and began laying.
It is important to realize your hive is queenless. Then you can look for signs that a replacement queen is being reared or is already present. The danger is that if a hive remains queenless for more than about 1 month some of the supposedly non-fertile workers will begin laying drone eggs. Once the laying workers are in control of the hive it becomes near impossible to requeen the hive. The laying workers gang up on any introduced queen and either outright kill her or eject her from the hive. This hive is then lost for the season. There are numerous schemes such as putting in frames of eggs and brood or shaking out all the bees 100 feet from the hive but these usually fail, because the only thing that suppresses laying workers is a strong queen pheromone.