Which type of termites typically begin infesting exposed wood, such as window and door frames?

The correct answer is Western Drywood termites. These termites are known for their tendency to infest dry wood, including structural elements like window and door frames, without the need for contact with soil moisture. They can establish colonies in the wood itself, often going unnoticed until damage becomes significant.

Western Drywood termites have a more localized presence, typically found in infested wood, and can survive in areas with lower humidity compared to other types of termites. This characteristic is what primarily distinguishes them from subterranean termites, which require soil contact for moisture and usually infest buildings from the ground up. Formosan termites are a highly aggressive variant of subterranean termites, often causing extensive structural damage but do not directly start their invasion in exposed wood like Western Drywood termites. Meanwhile, Eastern Drywood termites share similarities with Western Drywood termites but are primarily found in the eastern regions of the United States, and the question specifies Western Drywood termites as the best answer due to their specific behavior and habitat.