Why do bees swarm

Beekeepers are always wondering why their bees swarm. If we look at the causes it becomes easier to minimize the swarming behavior.
Reasons:

• Old queen. A queen that has been through one winter is looked as old to a colony of bees. Swarming is nature's way of replacing the old queen with a new queen. The odds of a colony making it through the next winter is greatly enhanced if there is a new young queen in the hive. The odds of a swarm making it through the winter is usually not very good in MN. Requeening an overwintered colony lessens this swarming impulse.
• Heat/Overcrowding. An over wintered colony is always subject to overcrowding. Dividing a colony helps keep the numbers of bees more manageable. A strong colony is the ticket to a big honey crop. So swarm control management practices need to be employed. Keeping grass down in front of hives so the bees can cool their hives easier. Removing entrance reducers for better air flow. Giving bees more room. Boxes with foundation is not considered room. Bees usually will not occupy foundation boxes in large numbers unless they are being fed or a nectar flow is on. Drawn comb is considered room. Overcrowding can happen in any colony of bees if it is not being managed properly.
• No nectar flow. Large colonies with no nectar flow can swarm at anytime. A bad nectar year kicks up swarming to a higher level. It is like the bees think they will not survive and leave for possible a better chance of survival.
• Mites/Absconding. High level of Varroa causes absconding of a colony. This usually happens in late fall but can happen on an overwintered colony with a high mite count. Example, A colony inspected in early October may look great but a return inspection in mid October may reveal an empty hive with not a single bee in the hive. Treating for Varroa in mid August will usually prevent this from happening.

 Management practices of looking for swarm cell in colonies once a week through the month of June will help prevent swarming. Cutting out swarm cells before they are capped is proper management. Once a swarm cell is capped the colony usually swarms. Removing capped swarm cells will eliminate any queens from coming back into the hive. A new queen would need to be purchased to get a queen into the hive. Buying a new queen is usually a better fix than letting a hive make their own queen. A purchased queen will give a colony eggs in 10 to 14 days.
 The swarm cell queen emerges from the swarm cell 6 days after the hive has swarmed, 7 days until the queen can fly, 7 days to get mated, another 7 days before she starts laying.  So that puts new eggs in the hive around 30 days after the colony has swarmed.
  Colonies that have swarm cells can easily be fixed by cutting out all the swarm cells, then switch the colony's location with a weaker colony. This removes the large field force from the strong hive and gives them to a weaker colony. This removes the swarming impulse from the strong hive.
Example: Hive A is very strong and is making swarm cells. Hive B is a weaker hive or a new package of bees. Move the entire colony, put Hive A where Hive B is and put Hive B where hive A is. The field bees fly out and then return to the hive where the were before. Hive A gets weaker now with a smaller field force and loses the desire to swarm. Hive B gets stronger with the larger amount of field bees. Hive B now may make more honey than Hive A and it may have an increased risk of swarming.
 Swarming is always a challenging time of year but employing good management practices will keep the bees at home instead of in a tree somewhere.
Switching hives for swarm control