Bees become more aggressive for a number of different reasons. Consider the following possibilities, and see whether any apply to your situation with your beehives:
A newly established colony almost always starts out gentle. As the colony grows in size and the season progresses, the bees become more protective of their honey stores. Likewise, a growing colony means many more bees for you to deal with. But if the colony is handled with care, this is seldom a problem. Be gentle as you work with your colony.
Incorrect use (or lack of use) of the smoker can result in irritable colonies.
Do you launder your bee clothes and veil? Previous stings on gloves and clothing can leave behind an alarm pheromone that can stimulate defensive behavior when you revisit the hive. Be sure to keep your garments clean. You can also smoke the area of the sting to disguise any alarm pheromone that may linger on clothing or on your skin.
When colonies are raided at night by skunks or other pirates, they can become cross and difficult to deal with.
Do you still have your original queen? Are you sure? If you had a marked queen, you’d know for certain whether the queen now heading your colony is your original queen. (See if she’s marked!)
A colony that supersedes the queen sometimes can result in more aggressive bees. That’s because you have no guarantee of the new genetics. The new queen mated with drones from goodness knows where. Her offspring may not be as nice as the carefully engineered genetics provided by your bee supplier. When this happens, order a marked and mated queen from your supplier to replace the queen that is now in your hive.
When you purchase a marked queen from a supplier, the marking stays on for the full life of the queen. It’s like spilling nail polish on the living room carpet. It never wears off!