How do I know if I need to re-queen?
There are many different indicators that you need to kill the old queen and install a new one. Two of the biggest reasons are that your hive has become less productive or your queen has a poor breeding pattern, producing spotty brood or even laying lots of drone brood and producing few worker broods. Most queens will run out of semen within two seasons of their initial mating, so beekeepers often re-queen as a preventative measure every spring, or at least every other spring (drones result from unfertilized eggs being laid). So, if your queen is getting older, it's probably time to re-queen.
Another reason people re-queen is they have a "hot" hive. If your bees are angry, aggressive, or have become a problem for you and your neighbors, most likely they have a queen that is producing defensive bees. Replacing your queen is the best way to fix the problem.