Bed Bug Evidence Field Guide

Why this matters

A bed bug call carries more weight than most residential calls. A positive finding can trigger a landlord's legal duties, move a family out of their bedroom, and start a treatment that runs several visits. A false positive does all of that for nothing; a false negative sends people home to keep being bitten while the infestation spreads into the next unit. The finding has to rest on evidence the tech can point to, photograph and, when needed, hand to an entomologist. This card owns what counts as bed bug evidence and what only looks like it. The inspection sequence is in How to Inspect a Bedroom for Bed Bugs.

The field card

BED BUG EVIDENCE CARD
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FECAL SPOTS
  dark brown to black dots, pinpoint size
  on fabric: wick into fibers like a marker dot
  on wood or plastic: slightly raised
  damp swab test: smears rust-brown (digested blood)
  where: seams, piping, box spring corners, joints

CAST SKINS (molted nymph shells)
  pale amber, translucent, hollow, bug-shaped
  size range: about 1 mm to about 5 mm
  several sizes together = breeding population

EGGS AND EGGSHELLS
  about 1 mm, pearly white, cemented in crevices
  older eggs: dark eye spot visible
  hatched: empty shell, cap open at one end

LIVE BUGS
  nymphs: pale when unfed, red after a meal
  adults: flat, oval, mahogany, apple-seed size,
          long and plump after feeding

LOOK-ALIKES
  bat bug: near identical, longer hairs on the
           plate behind the head, linked to bats
  carpet beetle larva: hairy, banded, bristly skins
  spider beetle: round, humped, long legs
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Reading each piece of evidence

Fecal spots. The most common sign and the most commonly mistaken. Bed bug feces is digested blood, and on fabric it soaks into the weave so each dot looks like the tip of a felt marker touched the cloth. On hard surfaces the spots sit up slightly. The damp swab test separates them from mold, ink, dirt and stains: wipe one spot with a swab moistened with water, and bed bug spotting smears a rusty brown. Spots cluster where bugs rest, not randomly: along the mattress seam and piping near the head of the bed, at box spring corners and under the dust cover, in headboard joints and screw holes. A single spot in the middle of a sheet, far from any harborage, is weak evidence on its own.

Cast skins. Bed bugs pass through five nymphal stages, molting between each, and each molt leaves an empty, translucent amber shell in the shape of the bug. Skins range from about 1 mm for the earliest stages to about 5 mm near adult size. One skin proves a bug molted there. Several skins of different sizes prove bugs have been developing through several stages in that spot, which means a population that has been feeding and growing, not a hitchhiker.

Eggs and eggshells. About 1 mm, pearly white, glued into crevices: seams, cracks in a headboard, the back of a picture frame, along the edge of a box spring's wood. A magnifier helps. Older eggs show a dark eye spot through the shell; hatched shells are empty with the cap open at one end. Eggs mean an adult female has been there, and she lays in or near the harborage where she rests between meals.

Live bugs. The strongest evidence. Unfed nymphs are pale and hard to see; a recently fed nymph is bright red. Adults are flat, oval and mahogany, roughly the size of an apple seed unfed, longer and plumper after feeding. Capture live specimens for the record rather than crushing them; a crushed bug is a smear, not an identification.

Blood smears on sheets. Rust-colored streaks on bedding can come from a crushed, fed bug. They suggest, they do not confirm, because a scratched bite or a nosebleed leaves the same mark.

Look-alikes that drive the wrong treatment

Bat bugs. Close relatives of bed bugs and nearly identical to the naked eye. Under magnification, bat bugs have longer fringe hairs on the plate behind the head. They live with bats in attics and walls and bite people mainly when bats leave or are excluded. Treating a bat bug problem as bed bugs without dealing with the bats fails, because the source stays in the attic. Swallow bugs, linked to bird nests, are a similar case.

Carpet beetle larvae. Small, hairy, often banded caterpillar-like larvae, and their shed skins are bristly, unlike the smooth, glassy skins of bed bugs. Customers bring them in bags as "bed bugs" often. They feed on wool, feathers, pet hair and lint, and do not bite, though their hairs can irritate skin.

Spider beetles. Round, humpbacked, long-legged, reddish-brown beetles found in pantries and stored goods. Too round and too leggy to be bed bugs at a glance, but customers who have read about bed bugs see one everywhere.

Also commonly confused: young cockroach nymphs (longer legs and antennae, fast), ticks (eight legs), fleas (laterally flattened, jump), and booklice (tiny, pale, soft, found on damp paper and starchy surfaces).

Why a bite is not evidence

People react to bites differently, and some do not react at all, so two people in the same infested bed can show very different welts. Mosquitoes, fleas, mites, other biting insects and a long list of skin conditions produce welts a tech cannot tell apart by looking. The tech does not diagnose a bite; a physician does, and even a physician cannot name the biter from the welt. A bite report is a reason to inspect, never the finding. When bites continue and the inspection finds nothing, follow Bed Bug Bites Continue But No Evidence Found Decision Tree rather than treating on the strength of the bites.

When to send a specimen for ID

Send a specimen to an entomologist, typically your state's land-grant university extension diagnostic lab, whenever:

• the insect could be a bat bug or swallow bug (top-floor unit, bats or birds known in the structure, bug found near a wall or ceiling rather than the bed);
• the finding will be disputed (landlord and tenant disagree, a hotel complaint, anything likely to involve an attorney);
• you have a single specimen and no other evidence;
• you cannot confirm the identification yourself to the level your report will state.

Put a live or dead specimen in a small vial of rubbing alcohol, or press it onto clear tape on an index card. Label it with the date, address, unit and the exact spot, photograph it, and note on your record that a specimen was submitted and to whom.

Worked example: two units, one building

Unit 3B, tenant reports bites for two weeks. The tech inspects the bed and finds: a line of spots along the mattress seam near the head, which smear rust-brown on a damp swab; two cast skins in the same seam, one about 2 mm and one about 4 mm; no eggs found; and in a screw hole in the headboard, one live adult, captured into a vial.

The read: spots alone, at the right place and smear-positive, are evidence. Two skins of different sizes mean bugs developed through more than one stage there. The live adult confirms. Evidence of an established infestation: positive. The record lists each item with its location, the photos, and the captured adult.

Unit 5A, top floor, the next day. The tenant reports bites and has saved an insect in a sandwich bag. The tech finds no spots, skins or eggs on or around the bed, but the bagged insect looks like a bed bug to the eye. The unit is on the top floor, and the building manager mentions bats in the attic last summer, recently excluded.

The read: one specimen, no harborage evidence on the bed, and a known bat history above. The tech does not call it a bed bug on sight. He submits the specimen for identification and writes the record as "suspect specimen collected, no harborage evidence found, specimen submitted for ID". The lab returns it as a bat bug. The recommendation changes from a bed bug treatment to referral for the bat situation: bat exclusion is regulated in many states, including seasonal limits during the bat maternity season, so it goes to a wildlife control operator, licensed where your state licenses that work, with the bat bug treatment coordinated after the bats are dealt with.

Why the two calls differ. In 3B the evidence was multiple, at the harborage, and included a confirmed live bug in context; nothing in the setting pointed elsewhere. In 5A the only evidence was one bug out of context, and the setting pointed to a look-alike whose treatment is different. Calling 5A on sight would have started a bed bug program that could not have worked.

What would change 3B's read. If the spots had not smeared and there were no skins or live bugs, 3B would have had no confirmed evidence, and the bite report alone would not change that.

Before you leave the unit

Bag captured specimens and anything you collected before you leave the room, and check your own clothing and kit. Keep your kit off the bed and upholstered furniture; set it on a hard floor or in a plastic bin, and inspect it before it goes back in the vehicle so you do not carry bugs to the next unit. Encasements, if recommended, go on after the inspection is complete; see How to Install Mattress and Box Spring Encasements.

References

• See related: How to Inspect a Bedroom for Bed Bugs
• See related: Bed Bug Bites Continue But No Evidence Found Decision Tree
• See related: How to Install Mattress and Box Spring Encasements
• See related: Pest Activity Signs Reference
• Your state's land-grant university extension diagnostic lab for specimen identification