Dark marks on a mattress, headboard or wall can be alarming. They might be bed bug droppings, but they might also be mould, dirt or ink. Because the shapes overlap, it pays to look carefully before drawing a conclusion.

This guide compares the two most common explanations. It covers colour, pattern, location and the other clues that help separate them.

What bed bug droppings look like

The NHS lists small brown spots on bedding or furniture as a sign of bedbugs. It describes them as bedbug poo. They are one of the more common signs people notice before they see a live bug.

University of California pest notes give more detail. According to UC IPM, droppings are reddish-brown to dark brown or black. They are roughly round or streaked.

Even so, a cluster of small dark marks in a seam is a typical place to find them. Our bed bug signs guide shows examples of how these marks tend to appear.

What mould is, and why it appears

Mould is a fungus that grows where things stay damp. The EPA explains that places often or always damp are hard to keep free of it. Bathrooms are a typical example.

It also damages the material it grows on. The EPA says that the longer mould grows, the more damage it can cause. As a result, damp patches on a wall, ceiling or window frame deserve attention in their own right.

Importantly, mould is a moisture problem and bed bugs are an insect problem. They can occur in the same room, but one does not cause the other. Treat each on its own terms.

Location is the biggest clue

Where a mark sits tells you more than how it looks. The CDC says bed bugs tend to live within about 8 feet of the sleeping area. The NHS lists bed frames, mattresses, clothing, furniture, behind pictures and under loose wallpaper as typical hiding places.

Droppings therefore turn up in mattress seams, along piping, on headboards and in joints of the bed frame. Mould, by contrast, follows moisture, so it turns up in damp places such as bathrooms, window sills and corners with poor airflow.

For example, a cluster of dark dots inside a mattress seam suggests bed bugs. A spreading dark patch in a damp corner suggests mould. For a full room tour, read where bed bugs hide in a bedroom.

Pattern and spread

Droppings tend to be small and repeated, while mould tends to be one growing patch. UC IPM describes them as roughly round or streaked, so you may see lines of dots or short smears along a fold. They sit in groups near the places where bugs rest.

Mould usually spreads across a surface rather than forming neat dots. Because it follows moisture, it often grows in a patch that gradually gets larger. That is a general pattern rather than a rule, however.

Mixed evidence is possible too, since a damp room can hold both. Therefore, treat each mark as a separate question and check them one at a time.

Other clues that point to bed bugs

Dark marks rarely settle the matter alone. The CDC lists shed exoskeletons, live bugs in mattress folds, rust-coloured blood spots and a sweet musty odour. The NHS adds bites on skin exposed while sleeping.

Look for several of these together, because a single clue can mislead. For instance, a dark speck plus a shed skin plus a bite pattern builds a stronger case than any single sign. Our bites guide explains why bites are only a hint.

You can also compare the marks with the stage of the insect. The bed bug life cycle explains how nymphs and adults leave different traces behind. Shed skins appear after every moult.

Other dark specks to rule out

Several other things look like droppings. Carpet beetle larvae leave fecal pellets about the size of a grain of salt, according to UC IPM. They also leave bristly shed skins. Our comparison of bed bugs and carpet beetle larvae covers the differences.

Flea dirt is another candidate, particularly near pets. UC IPM describes it as dried blood and excrement. If you have animals, check their bedding too.

Dirt, ink, pen marks and food stains can also mislead. Consequently, a dark spot is a reason to look more closely, not a conclusion.

Smell, stains and the wider picture

Smell can add a clue, although it is not reliable alone. The CDC mentions a sweet musty odour among bed bug signs. Mould, meanwhile, may add smells of its own, so odour alone cannot separate the two.

Stains on bedding add another layer. The NHS lists spots of blood on sheets, which can come from bites or from squashing a bug. While a small red or rust-coloured mark may point to bed bugs, it can also come from a scratched bite or a nosebleed.

For that reason, we suggest treating every mark as a lead to follow rather than proof. The more independent clues line up, the more confident you can be.

How to check without making things worse

Start by looking, not scrubbing, because cleaning removes evidence. Use a bright torch and a magnifier or phone camera. Photograph each mark with a coin beside it for scale, then note where it sits.

Our guide to how to photograph an insect for identification explains good technique. The same approach works for marks. Take one close image and one wider shot.

Then, while the marks are still in place, follow our step-by-step guide on how to inspect a mattress for bed bugs. It shows the order to work in, so you do not miss a seam.

If it turns out to be mould

Mould needs a different response. The EPA says that if the mouldy area is under about 10 square feet, you can usually handle the job yourself. It suggests detergent and water on hard surfaces, followed by thorough drying.

Porous materials such as carpet or ceiling tiles may need replacing. The EPA also advises seeking a professional if the area is larger, if there is significant water damage, or if you have health concerns. In rented homes, tell your landlord as well.

After travel and away from home

Marks in a hotel or holiday let need the same care. Our guide to checking a hotel room for bed bugs explains where to look on arrival. Headboards and mattress seams come first.

When you return, check your bags before unpacking. Our post on bed bugs after travel and luggage checks lists the steps. The CDC notes that bed bugs can live for several months without a blood meal. Our post on how long bed bugs live without feeding looks at that in more detail.

What to do next

Photograph the marks, note where they are and decide whether the area is damp. If the spots sit near the bed, check the seams and frame for other signs. If they sit in a damp corner, treat it as a moisture issue first.

Then run the five-question checker. It works in your browser, so nothing you enter is sent anywhere. If you find a live bug, read our found a bug guide for safe ways to keep it. The lookalikes guide covers other insects to rule out. You can also ask a pest professional to confirm what you have.

Questions and answers

What do bed bug droppings look like?

University pest notes describe them as reddish-brown to black, roughly round or streaked. The NHS lists small brown spots on bedding or furniture as a sign of bedbugs.

Can mould be mistaken for bed bug spots?

Yes. Both can appear as dark marks, but mould is tied to damp conditions while bed bug marks cluster near sleeping areas. Location and surrounding evidence help you decide.

Should I clean the marks before taking a photo?

No. Photograph them first, with something for scale, then clean. Cleaning removes the evidence you may need.

Sources

We checked the facts in this article against these sources. However, this article supports triage, not diagnosis, so keep any specimen for an expert.