Which of the following beetles infests only hardwoods?

The Lyctid beetles are specifically known for their association with hardwoods. They primarily infest and feed on the wood of hardwood trees, particularly species such as oak, maple, and ash. This specialization makes them unique among wood-infesting beetles, as many other types of beetles can infest a broader range of wood types, including softwoods.

In contrast, the Old House Borer, Anobiid, and Bostrichid beetles are known to infest both hardwoods and hardwood products, and some have a more diverse wood preference that includes softwoods. The Old House Borer, for example, can infest both hardwoods and softwoods, while Anobiid and Bostrichid beetles have a similar lack of specificity regarding wood type.

Thus, the distinctive characteristic of Lyctid beetles is their exclusive relationship with hardwoods, making this the correct answer. Understanding the specific dietary habits of these beetles helps in pest management strategies and in protecting wooden structures made of hardwood from damage.