One of those signs is their droppings. Bed bug poop is composed of partially digested blood and usually appears as very dark rust colored or black dots. These spots of excrement are only about the size of a dot from a marker and may also bleed into sheets and bedding, much like a marker. Signs of bed bugs, including black droppings by louento
Bed bugs form clusters around the edge of your bed. Their droppings take the form of undigested blood which ceases to be red once it dries. Their droppings are black in color, measuring micrometers in diameter.
The bed bugs poop the partially digested blood on their way to their hideouts, leaving behind clusters of tiny spots on your sheets or mattress and this is a very common sign of bed bug infestation. The feces appear a red colour when wet but dry quickly into a rust-coloured, dark or black speck.
What Does Bed Bug Poop Look Like? Bed bugs themselves are very small insects; the size of a small tick or a roach hatchling. Their feces, on the other hand, will be even smaller. However, identifying bed bug feces may be the only indication you get that you have an infestation.
When a bed bug first hatches it is nearly colorless, and mostly transparent. As it develops it goes from pale, to tan, to light brown, to eventually rust-colored. A newly hatched bed bug is only about the size of the tip of a pen. You see black feces - Bed bugs leave their feces everywhere. They leave streaks on pillowcases, sheets, and bedding.
If you are being bitten by bed bugs, you will also see fecal stains. Fecal stains on sheets look like the marks of a felt tip pen and tend to bleed into the fabric. The picture below is a great example of what bed bug fecal stains look like. Note the live bed bugs in the photo and how flat they are.