Western subterranean termite colonies are usually located in the ground below the frost line but above the water table and rock formations. They are typically detected by the presence of the mud tubes they construct, or when large numbers of winged termites "swarm" or leave the colony to search out mates. Swarms occur in the daytime, and in California, they occur on warm, sunny days during the fall, winter, or early spring. In the northern sections of the termites' range, spring swarms commonly occur in the absence of rainfall.