Species: Bed bug (Cimex lectularius); Bat bug (Cimex pilosellus); Swallow bug (Oeciacus vicarius); Booklice (family Liposcelididae); Carpet beetle larvae and German cockroach nymphs, the other frequent lookalikes
Side by side
| Bed bug | Bat bug | Booklouse | |
|---|---|---|---|
| Adult length | About 3/8 inch, 9.5 mm when full grown | About the same. Colorado State says size does not separate the cimicids | 1 to 2 mm, about 0.04 to 0.08 inch |
| Head | Narrower than the thorax and body | Short and broad, broadly attached to the prothorax | Wide, nearly as wide as the body, with a large clypeus like a nose; NC State calls it a bulging face |
| Color | Reddish brown to grayish brown | The same range. Colorado State separates the species on hair patterns and wing-pad structure | Light tan; Penn State adds grey, dark brown or nearly clear |
| Host | The only cimicid in Colorado adapted to living entirely with humans | Develops in colonies of roosting bats, often in attics or wall voids | Feeds on mold and fungi in humid, damp places |
| Does it bite | Yes. Feeds at night; Colorado State says the bite itself is painless | Incidentally bites people, particularly when bats migrate or are removed | Penn State says booklice do not bite and do not vector disease |
| What it means indoors | Colorado State says all infested sites must be treated at the same time | Cannot sustain or reproduce without the bat hosts | An indicator of high humidity or a moisture problem |
What to look for
- Length. Colorado State gives bed bugs and their cimicid relatives as about 3/8 inch (9.5 mm) when full grown, reddish brown to grayish brown, oval and wingless with only small wing pads on the back, flat when unfed and swelling rapidly with a blood meal. Penn State gives booklice as 1 to 2 mm (about 0.04 to 0.08 inch).
- Head width against the body. A booklouse has a wide head, nearly as wide as the body and wider than the thorax, plus a large clypeus that reads as a big nose. A bed bug's head is narrower than its thorax and body.
- Hairiness and shape. NC State separates carpet beetle larvae from bed bugs by the larvae being much hairier and lacking the flat bed bug shape, and says adult carpet beetles differ in shape and color yet are still regularly mistaken for bed bugs.
- Leg count. NC State gives the quickest separation from a tick: ticks have eight legs, bed bugs have six.
- The close relatives. Colorado State says bed bugs, bat bugs, swallow bugs and poultry bugs are all reddish brown to grayish brown, oval, wingless with small wing pads on the back, and about 3/8 inch (9.5 mm) when full grown, and that telling them apart relies on patterns of hairs and wing-pad structure.
- Where it was found, which is the real evidence. Colorado State says bat bugs develop in colonies of roosting bats in attics or wall voids, and swallow bugs in cliff swallow nests attached to a building.
Where and when
Colorado State says bat bugs develop with roosting bats in attics or behind walls and may move into living areas when bats migrate or are eliminated; swallow bug bites tend to occur in late winter and spring after swallows nested the previous summer. Penn State puts booklice in humid, damp, generally dark places where mold or fungi grow, including paper and stored grains.
Does it matter?
Colorado State is blunt about the limit here: there is nothing unique about a bed bug bite that can be used for positive diagnosis, and the insect itself has to be found. Reactions are highly variable. Penn State says booklice do not bite, do not carry disease and do not damage paper or fabric, though their feces can contaminate stored food; its remedy is fixing the underlying moisture.
Related guides
- Water bug vs cockroach: Water bug is a nickname, not a species.
- Carpet beetle: Carpet beetles are small oval beetles, 1/16 to 1/8 inch long, that Kentucky extension describes as black or mottled with white, brown, yellow and orange.
- Silverfish: A silverfish is a wingless, flat, teardrop-shaped insect covered in silvery scales, with long thread-like antennae and three tail-like appendages at the rear.