Termites typically start swarming when temperatures get warmer outside. In most parts of the world, this would be in the middle of the spring season. Subterranean termites in the ground will want to create new colonies of their own once they sense there are warm temperatures above the ground. The swarming will usually increase after it rains outside because termites like to inhabit environments with a lot of moisture in them.
The age of a colony is not a sole determining factor for when termites will swarm. But you can expect a termite colony to be at least 3-years-old before they start swarming. This gives the colony enough time to fully mature. After that, swarming termites will emerge from the colony with wings and then go looking for a mate. Once the swarmers find mates for themselves, they will search for a location where they can form a new colony. The mating of the two usually starts after their wings drop off. It is just an endless cycle of mating, starting new colonies and swarming. In between all that, they feast on cellulose-based materials and provide for their existing colonies.
Every colony has a female member that is made to be the “queen” of that colony. The queen termite lays her eggs and gives birth to “workers” for that colony. Many of these termites become soldiers for the queen and defend the colony along with all the other termites. One queen termite can lay as many as one million eggs within her lifetime. What’s worse is there are secondary reproductive termites that exist within a colony which can also lay eggs and create new termites. If the queen termite dies then one of these secondary termites will likely take her place as the new queen. Either way, the more termites that get created the more swarmers you will have coming from a colony. It is very similar to how an ant colony works, which is why so many people mistake ants and termites because they act very similar.
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Pest control companies will tell you to take precautions in the spring time in order to reduce the chances of termite colonies from being created around your home. This means reducing moisture in the rooms of your home, keeping wood away from the exterior siding of your house and having regular inspections every month by a pest control technician to look for termites. If a colony can be killed while it is still new then this will prevent termites from swarming and forming new colonies in other places.