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Mystery Bites and Itches – Arthropod and Non-Arthropod Sources in Colorado   arrow

Guidelines for County Extension Offices

Quick Facts

There are some insects, mites, and spiders that bite humans and/or can cause skin irritations or dermatitis.

Often sources of skin irritation or those that produce “bite-like” reactions include environmental allergens such as chemicals or irritant fibers.  When handling a client with a concern about arthropod bites/itches, an open mind must be kept in regard to the possible identification of a source that may be the cause of the client’s concern.

Skin irritation resulting from bites of arthropods varies greatly between individuals, in large part due to individual immune responses.

Identification of an insect, mite or spider as the cause of a suspected “bug bite” is usually not possible based on just symptoms alone.

Very few arthropods can actually infest human skin (topically or subdermally) and reproduce. Scabies mites are a rare exception. Often clients that will insist otherwise.

Acknowledgment:  The development of this fact sheet, adapted to Colorado, was largely derived from work that was developed for the University of Kentucky Extension fact sheet by Dr. Michael Potter:

Everyone has experienced at some time various skin bumps, sores, or persistent itching that have no obvious source.  Often these are suspected – or self-diagnosed – as being Abug bites@ of some sort.  Sometimes this is a correct diagnosis of what cause the itch/sore/bite, as there are a few insects, mites and spiders that do bite humans.  At least as often self-diagnosed “bug bites” that occur in Colorado have a different origin.

This confusion is made worse as bites by insects or arachnids (e.g., mites, spiders, ticks) can rarely be diagnosed by symptoms alone.  Similar symptoms can be, and often are, produced by other causes that include:

-Environmental conditions (chemical or physical agents contacted or dispersed in air, abrupt changes in humidity)

-Reaction to chemical agents (personal care products, cleaning agents, inks, etc.)

-Underlying health conditions (bacterial infections, diabetes, drug reactions, etc.)

Properly diagnosing a “bug bite” is very difficult and often does not have a satisfactory end result.  A suggested protocol for attempting to determine the cause follows

Table 1.  Suggested protocol for county Extension offices in Colorado when handling queries regarding “invisible itches” or “bites” of unknown origin.

  1. Never attempt to diagnose the cause of a “bite” based on symptoms. Observation of the affected area by anyone other than appropriately trained medical personnel will not assist in the diagnosis. Similarly, skin scrapings cannot be evaluated except by medical personnel.
  1. Samples of specific arthropods or suspected arthropods can often be evaluated through CSU Extension. If county Extension offices are unable to make the diagnosis these can be forwarded to Extension entomology on campus. Samples that need identification should be preserved as well as possible to prevent fragmentation.  Samples collected on tape applied to the skin surface or from surfaces where a suspected insect/mite is present also can usually be examined successfully.
  1. Samples as vacuumed bulk samples, bundled clothing, or are similarly poorly collected are unusable for examination and will usually not be considered for further evaluation. Samples of that contain skin scabs, dried fluids, feces and other medically hazardous material will not be examined. Such samples will either be discarded or returned, unexamined.
  1. Review possible sources of cryptic arthropod bites and itches that can occur in Colorado. These are discussed in the following section and consist of a fairly short list of options. If conditions exist that may be a source of bites/itches (e.g., mites moving off an infested pet, bat bugs migrating from bat roost in attic) then suggest to the client that they remove or treat these sources.
  1. If conditions of the bite/itch occur in the absence of conditions suggesting an arthropod or if they persist after treatment efforts have been made, the source of the bites is almost certainly of some other origin. Review recent changes that have occurred in a client’s habits.  These might include recent travel or outdoor activity.  Reaction to new products used in and about the home or work area may be a cause.  Make changes that eliminate contact with these products.  Review the list (Tables 4, 5) of some possible environmental causes that produce symptoms mimicking “bug bites”.
  1. If symptoms persist, an underlying medical condition is should be strongly considered as the cause. You may wish to review the list of some possible medical conditions that can produce the sensation of bug bites or that can produce bumps and lesion mimicking ”bug bites”.  However, resolution of these cases often will require attention from medical professionals.

Biting Arthropods That Occur in Colorado

Several arthropods occur in the state that can bite humans (Table 2).  These should be first considered as the possible cause of a “bug bite”.

Table 2.  Possible sources of “bug bites” occurring within a home in Colorado that may be due to insects or other arthropods.

Bed bugs Swallow bugs
Bat bugs Mites originating from a bird nest
Mites originating from a pet Mites originating from grain storage
Scabies mites Yellow-legged sac spider
Fleas Masked hunter
Head lice Mosquitoes

a Other biting flies (biting midges, black flies, no-see-ums)
b Chiggers
c Thrips

a Other than an occasional mosquito, no biting flies – or any other flying insect – are associated with human bites that occur indoors.

b Chiggers occur only occur outdoors, most often in grassy areas near water.  However, the skin reaction to their “bite” typically develops a few days after exposure.

c Thrips that are plant feeders, develop only on outdoor plants.  However, occasionally some may be transferred indoors on clothing and produce transient episodes of indoor biting.

Situations where insects or mites that bite human will occur often are very restricted. Many require that some animal host (household pet, nesting wildlife) be present as a source of the biting arthropod. Some occur only outdoors and some occur only during certain times of the year. Also, the response of individuals to these bites can greatly vary, largely due to differences in immune response.

The insects and mites that may bite humans in Colorado and their diagnoses are reviewed below.


Flea bites of humans are rare in Colorado.  Arid conditions in the state largely prevent development indoors of the flea species that cause serious problems in the more humid areas of the US (e.g., cat flea).  Almost all human cases involve fleas (usually Pulex irritans) that originate from the dens of wild animals, notably skunks and foxes; fleas that may incidentally bite humans also can originate from squirrels and rabbits.  Occurrence of these fleas in homes occurs when they then attach to pets or disperse from dens under or in close proximity to the home. Fleas are covered in more detail in Fact Sheet 5.600

Fleas, although small (ca 1/8-inch), are relatively easily detected, very distinctive in their appearance and can be active insects capable of jumping.  Bites appear as small reddish spots that may itch and often are concentrated on the lower leg.  Some questions to ask to determine if fleas may be the source of a Abite@:

Have there been skunks or fox nesting in or in/under the home?

Fleas developing on these hosts will scatter when the nests are abandoned.  The human flea (Pulex irritans) is associated with skunks and fox and is the most common flea found biting humans in Colorado.

Is there a dog in the home?

The cat flea, Ctenocephalides felis, is the common species associated with dogs in the US.  It survives poorly in Colorado and rarely persists in a home, but infestations do occur.  If this is the source of fleas, myriad treatments can be effectively used; these are covered in the Fact Sheet 5.600.

Does your dog go outdoors to sites where foxes, skunks, rabbits or squirrels are present?

In these sites fleas dispersing from denning animals may be picked up by the dog.  The fleas are then moved indoors by the dogs, which then may cause an episode of biting on humans.  The most common flea that dogs will bring indoors is Pulex irritans, associated with dens of fox and skunks.  Other fleas that may be brought indoors by dogs include certain Orchopeas species that occur on squirrels and Hoplopsyllus glacialis affinis, that develops on rabbits

Cheyletiella Mites

Cheyletiella mites are small parasites of mammals that can infest the skin of dogs and, less commonly, cats.  They may produce itching in the animal or irritated skin.  However, often the animal may support a population of these mites but is asymptomatic.  The mites can be detected by brushing the pet over paper or some other surface that can dislodge the mites.  A common name given to the mites brushed off a pet is “walking dandruff”.

Effective treatments exist that can control Cheyletiella mites on a pet, which will prevent further human biting.  Treatment course is best handled in consultation with a veterinarian.

Cheyletiella mites may also incidentally bite humans that are in close contact with an infested pet.  Usually bites occur if the pet often lies in the lap or similarly has prolonged close contact.  In these cases bites are often concentrated in parts of the body where the animal rests.

Some questions to ask to determine if Cheyletiella mites may be the source of a “bite”:

Do you own a dog or cat?

Bites from Cheyletiella mites only occur via incidental transfer from infested pets. The mites do not reproduce on humans and problems will quickly end in the absence of an infested pet

Do you let the dog/cat lie in your lap or have similar periods of prolonged close contact?

Infested pets that rest on one’s body are most likely to allow transfer of mites to humans, which may then bite.  A pattern of bites around the point of the body where the animal rests (e.g., lap) are most likely to show

Bird Mites

Mites that infest nesting birds (Ornithyssus spp., Dermanyssus spp.) may sometimes bite humans.  These situations arise most often when a wild bird (e.g., robin, pigeon, purple finch, grackle) makes a nest that is attached to a building.   Bites of humans from these bird feeding species usually then occurs after the nest is abandoned, at which time the mites abandon the nest disperse and may incidentally bite humans.

These bird mites cannot sustain themselves (cannot reproduce) without their wild bird hosts.  As such problems are of short duration, as the mites die out within a few weeks once the bird hosts are absent.  This is particularly true of Ornithonyssus species, such as the northern fowl mite (O. sylviarum) that can survive only a few days off the bird host.  Dermanyssus species, such as the American bird mite (D. americanus) and chicken mite (D. gallinae), can survive quite a bit longer, but cannot reproduce in the absence of their bird host.

Such situations can be prevented in the future by taking measures that prevent birds nesting on the home.  (Note: All birds that have established a nest with young cannot be disturbed in any way, under regulations – with substantial penalties – associated with the Migratory Bird Act.)  Caulking and other methods that seal all openings into the living area of a building can also exclude entry of mites that do develop on birds nesting on buildings.

Insecticide treatments and desiccating dusts that dry out insects/mites on contact, can be applied in the vicinity of the abandoned nest to kill mites that otherwise may migrate into living areas.  Mites that are crawling on windowsills and walls near entry points into buildings can be picked up by wet cloths, mops, and sticky tape (e.g., lint rollers).

Some questions to ask to determine if bird mites may be the source of a “bite”:

Do you have a bird nest on the side of your house or behind the wall of your house?

Bird mites will only be found in a home if there has been a nesting bird attached to the building or behind the wall.  They do not reproduce on humans and problems dissipate a few weeks after the birds have abandoned these nests.

Did the nesting birds recently abandon the nest?

Biting by bird mites usually takes place for a couple of weeks after nesting birds have abandoned the nest attached to the home.

Bird mites that are present on the skin may be able to be identified from samples on adhesive tape placed on an infested area of body.  However, they are more commonly discovered near entry points into the building, such as around window wells.


Chiggers are the minute, first stage larvae of mites that occur outdoors and feed on the skin of various animals.  They are not commonly encountered in Colorado, but do occur in localized natural areas during the warm months.  Chiggers are almost always found in areas of lush grass, usually near waterways.

Technically speaking, chiggers do not bite, but feed by regurgitating digestive saliva that liquefies a small area of the skin. The area where this feeding occurs produces a bite-like reaction in response to proteins introduced by the saliva.  This reaction can develop within a day or so after exposure, but may take 2 or 3 days to become noticeable.

The reaction to chigger feeding results in an inflamed spot that is often very itchy.  Chiggers are very rarely noticed in the course of infesting humans since they are not only extremely small but spend only a short period of time on the body and are removed by changing clothes and showering.  By the time the reaction to chigger saliva occurs the chiggers that produced it will no longer be found, having dropped off and moved to develop to their next stage.

Later stage chiggers do not feed on humans and adult chiggers are not associated with humans.  Chiggers do not reproduce on/in human skin or in homes/buildings.  All bites will occur from accidentally acquiring them when moving through infested grasslands.

Chigger “bites” can largely be prevented by use of insect repellents containing DEET or other repellents effective against mites and ticks. Various skin salves that reduce inflammation and itching, and, sometimes, antihistamines are used to reduce itching from chigger “bites”.

Some questions to ask to determine if chiggers may be the source of a “bite”:

Have you recently (within the past 3 days) been walking through a grassy area, particularly near a waterway?

Such natural sites are the only place where one would acquire chiggers, as they do not reproduce within homes and rarely, if ever, occur in yards in Colorado.  Reaction to the “bite” usually takes a couple of days to develop.

Is the area where itching is present concentrated along at the sock-line, belt-line or other areas where clothing is in close contact with the skins?

Chigger feeding is typically concentrated around the ankles, less commonly at the waistline, where clothing is constricted next to the skin.


Scabies mites (Sarcoptes scabei) develop in tiny burrows made under the skin.  This typically produces a pimple-like rash that itches, particularly at night.  Scabies occur worldwide and are very common in many parts of the world.  It is uncommon in Colorado, but does occur and can be acquired when in prolonged close physical contact with another person infested with scabies mites.  However, symptoms are slow to develop, usually taking two to three months or more after initial infestation before itching symptoms are noticeable.

There are several distinctive features in the pattern of scabies:

  1. Itching occurs at night.
  2. Infestation occurs in thinner areas of the skin. Webbing between fingers, elbows, the buttocks, anterior of the thigh and the genitalia are the most common areas where scabies infestations occur.  Scabies does not involve the face.
  3. Scabies mites produce a minute linear burrow, about 1/8-1/4 inch long. However, it is difficult to see unless trained, appearing as a fine scale.

Positive diagnosis of scabies is done with the assistance of a physician using skin scrapings that expose the mite.  These tests cannot be done by Extension personnel.  Effective medical treatments for scabies exist that can eliminate the mites.   The web page developed by the Centers for Disease Control on scabies is at:

Some questions to ask to determine if scabies may be the source of a “bite”:

Does the itching occur at night?

Nocturnal itching is a distinctive reaction to infestation with the human variant of the scabies mite.

Have you had close physical contact with someone who has scabies?

Scabies is not transmitted readily by casual contact, such as hand shaking.  Closer, prolonged contact with a person harboring scabies can allow transmission.

Where does the itching occur?

Scabies burrow into thinner areas of the skin.  Areas between the fingers, back of the hand, elbows, buttocks, anterior thigh, and genitalia are usually where scabies mites are found.  Thicker skin and the face are not affected.

Related species of mites cause a skin condition known as mange in other animals, including dogs, fox, cats, cattle, sheep, goats and pigs.  Very rarely these mites can transfer to humans if they are in very close contact with the animal.  A transitory irritation may be produced by these mites, arising shortly after contact with the animal and often will produce itching on the face.   However, these mange producing mites cannot survive on humans and die quickly in the absence of their animal host.  The scabies mite that is associated with dogs is apparently the only species known to have transferred to humans in Colorado, and this has occurred only very rarely.

Mites Associated with Stored Products

Stored foods are sometimes infested with various species of mites.  However, this is relatively uncommon in Colorado since low humidity reduces the growth of molds on which mites of stored grains develop,

If molding grain materials are present, these can support mites that feed on the mold.   These mold-feeding species do not bit humans.  However, they may, in turn, support predators that feed on them, notably the straw itch mite (Pyemotes tritici).  Straw itch mite can bite humans and produce some skin irritation.

Some questions to ask to determine if straw itch mites associated with stored foods may be the source of a ”bite”:

Do you have a substantial storage of stored grain, seeds, or cereal products in your home? 

Mites associated with stored foods, and the straw itch mite, develop in such sites.

Is the grain stored in an area that is humid and allows growth of mold?

High humidity conditions are required for development of most stored grain mites, which often feed on associated molds.  Died products stored and maintained at the normal low humidity common to Colorado will not be infested by stored grain mites.  Straw itch mites will not be present if mold-feeding mites are not present as prey.


No source of “bites” is so over-diagnosed as are spider bites.  Unfortunately, this situation is often aggravated not only by self-diagnosis but also by misdiagnosis by medical personnel.

Spider bites, when they do occur, are of defensive nature.  Biting occurs when the spider is confined or threatened; they do not attack humans.  Most often spider bites occur when a web is gently disturbed, when a spider has retreated for shelter in shoes/clothing, or during other accidental contacts.

Spider bites often are immediately felt as a point of sharp pain.  However, this is not always the case, and sometimes bite reactions are so mild that they are not noticed.  Regardless, a single defensive bite is typical of spiders.  The occurrence of multiple “bites” indicates some other cause; spiders would not produce multiple bites over time.

The venom of spiders is the primary medical concern.  Fortunately very few spiders have venom that produces anything more than a transitory, mild irritation to humans.  Those that most often are discussed as possibly of medical importance include widow spiders, brown recluse, the “hobo spider”, and yellow sac spiders.

Widow spiders, specifically the western widow (Latrodectus hesperus), are quite common in many areas of the state.  However, bites are very rare.  The venom that widow spiders use has neurotoxic effects which include pain in various parts of the body, a general sense of malaise, and other distinct symptoms.  Widow bites do not produce persistent, noticeable irritation at the bite site nor any secondary lesions.  This spider is further discussed in the Fact Sheet

The Brown recluse or the Violin Spider (Loxosceles  reclusa) is a common household spider found within homes of some midwestern and southern U.S. states, but is extremely rare in Colorado, mostly confined to southeastern Colorado.  Very few (about a half dozen) confirmed cases of brown recluse being found in the state are on record and many have involved a recent transfer of furniture or other items from areas (e.g., eastern Oklahoma, Kansas, Missouri, Texas) where this spider occurs much more common.

The venom of brown recluse spiders can cause cell damage at the bite site.  In a small fraction of individuals immune response to the venom can cause injury to progress, in extreme cases producing substantial tissue death and a slow healing wound.  However, no cases of confirmed brown recluse bites have ever been reported in Colorado.

Unfortunately, a large number of “brown recluse spider bites” have been diagnosed incorrectly in Colorado, all too often by inexperienced medical personnel.  In the absence of a specimen of the spider, all of these diagnoses must be considered highly suspect and an alternate diagnosis should be considered; there are many.  A list of medical conditions that mimic the slow healing lesions sometimes associated with brown recluse bites is listed below (Table 3).  Probably the largest single cause is infection of a wound with methicillin-resistant Staphlococcus aureus (MSRA), a bacterial infection sometimes known as “false spider bite diagnosis”.  Identification of brown spiders, such as a brown recluse is covered in the fact sheet

The yellow sac spider (Cheiracanthium mildei) is a nonnative the spider in North America that has probably been most commonly been associated with credible bites of humans.  This spider may be common in homes in Colorado.  At the point where bites occur there is usually an immediate stinging sensation and redness develops.  Two puncture wounds, produced by the two fangs, may be visible.  Sometimes sac spider bites can result in a small blister which, when broken, may produce a small sore.  However, this normally will dry and heal quickly.

With the probable exception of the widow spiders, which have a neurotoxin that produces characteristic effects, no spider bite can be diagnosed based on symptoms.  Whenever possible a spider that has bitten should be collected.  These spiders can usually be identified using Fact Sheet 5.512 (Spiders in the Home), which discusses the common species found in the state.  Spiders can be sent from Extension offices to campus for identification, although often only family-level identifications are possible.

Table 3.  Medical Conditions that Produce Symptoms that May Be Confused with Brown Recluse Spider Bites (from Vetter 2005)


Methicillin-resistant Staphylococcus aureus (MRSA or false spider bite diagnosis)
Other Staphylococcus infections
Streptococcus infection
Gonococcal arthritis dermatitis
Cutaneous anthrax

Reaction to drugs

Warfarin poisoning


Infected herpes simplex
Chronic herpes simplex
Varicella zoster (shingles)


Lyme disease
Rocky Mountain spotted fever
Ornithodoros coriaceus bite (soft tick)
Insect bites (flea, mite, biting fly)


Keratin cell mediated response to fungus


Poison ivy/poison oak
Chemical burn

Lymphoproliferative disorders

Lymphomatoid papulosis

Underlying disease states

Diabetic ulcer

Vascular disorders

Focal vasculitis
Purpura fulminans
Thromboembolic phenomena
Polyarteritis nodosa
Miscellaneous/Multiple causative agents
Pyoderma gangrenosum
Pressure ulcers
Stevens-Johnson syndrome
Erythema multiforme
Erythema nodosum
Toxic epidermal necrolysis
Lyell syndrome


Mosquitoes are present during the warm months if local conditions allow their breeding.  All mosquitoes develop in water but the different types of Colorado mosquitoes require different conditions.  The ones that most commonly bite are floodwater types (Aedes spp., Ocholeratus spp.) that lay their eggs at the edge of temporary ponds or other bodies of water.  Eggs hatch when a subsequent flooding event covers the eggs.  Other mosquitoes, including Culex spp. that can vector West Nile virus, lay their eggs as rafts on the surface still water and successfully breed in temporary pools where fish are not present.

Almost all mosquito bites occur outdoors.  The floodwater types bite during the day, particularly at dusk. Culex mosquitoes may feed at night and, if they manage to enter homes, may bite humans as they sleep.  Individual response to bites varies widely.  Itching and irritation rarely last more than a few days, although bites can become infected, particularly if they are scratched.

Mosquito bites are easily diagnosed if mosquitoes are observed to feed.  However, since the bite itself is often not felt and reaction to bites only begins to occur hours later, they sometimes are not suspected.   Some questions to ask to determine if mosquitoes may be the source of a ”bite”:

Is it the time of year when mosquitoes bite?

Mosquitoes bite during warm periods from late May through late September.  Adults of some species survive winter in the adult stage but remain dormant and do not bite during the cool months.

Have you been outdoors in the past few days, particularly at dusk?

Almost all mosquito bites occur outdoors and dusk is the peak period of biting of the most common species that bite humans.

Are the bites on parts of the body that mosquitoes could reach to bite?

Mosquitoes will feed on parts of the body that are exposed and should not occur on areas where protective clothing has covered during periods of mosquito biting

Other Biting Flies

Other biting flies occur in Colorado.  Black flies are sometimes a problem in the vicinity of rivers, in particular during years of high run-off.  Biting midges in the genus Leptoconops occur in areas of western Colorado, developing in damp areas of ravines and other sheltered sites.  No-see-ums (Culicoides spp.) occasionally occur near areas of running water and deer flies may develop in muddy areas near ponds.  Stable fly, a.k.a. “the biting house fly”, is found around livestock where conditions of damp straw mixed with manure provide breeding sites.  These biting flies are discussed in more detail in Fact Sheet 5.582

None of these flies naturally occur in homes and die-out rapidly if they accidentally enter a home.  All biting occurs outdoors and none have any stages that develop within homes.  These flies are all visible and produce painful bites so their identity as a source of bites is always obvious.


Thrips are minute insects, about 1/16-inch, with an elongate body form.  They are extremely common outdoors where they feed on a wide variety of plants. During midsummer, particularly as small grains and weed hosts begin to dry, large flights occur during calm periods of the day.

Thrips develop on plants, which they feed on by use of minute mouthparts that puncture the surface of plants.   However, the thrips occasionally may land on skin when one is working outdoors and sometimes will attempt to bite.  Because of their small size thrips bites are often barely noticed, although they can cause temporary irritation and itching.

Problems with thrips biting may be associated with colored clothing.  Many thrips are attracted to bright yellows; others favor light blues.  Regardless, problems are temporary as they do not reproduce on humans nor develop indoors.  They are also easily washed off. Some questions to ask to determine if thrips may be the source of a ”bite”:

Is it the time of year when thrips are abundant?

Thrips are present during the growing season but rarely are abundant except during late spring and early summer.  Thrips would not be present as a problem during the cooler months.

Were you outdoors in the past few hours, during a relatively windless time of day?

Thrips fly during daylight hours.  Because of their tiny size and poor flying abilities they will usually only fly in large numbers during calm periods.

When outdoors were you wearing short-sleeved, bright colored clothing?

Thrips may be attracted to certain colors, such as yellows and light blues.  Bites are most likely to occur on a bare arm or leg that they incidentally land upon.

Bed Bugs

Bed bugs are one of the few biting insects found in Colorado that can develop entirely on a human host.  Although not abundantly found in homes, in recent years they are more commonly encountered and there has also been great increase in awareness about bed bugs in Colorado.

Bed bugs are active at night and usually hide during the day in the close vicinity of where people sleep.  Bites are painless, but may produce an itchy spot; individual reaction to bites ranges very widely.  Bed bug bites will primarily occur on parts of the body that are exposed during sleep, rather than covered by bedding.

Diagnosis of bed bugs requires a search of possible hiding sites in the vicinity of the sleeping area.  The insects are readily visible, with a distinct shape.  (This is shared by the closely related bat bugs and swallows bugs.)  Excreted fecal spotting following a blood meal is also a symptom used to identify areas where bed bugs hide during the day.  Information on bed bugs is covered in Fact Sheet 5.574, and in similar Extension publications produced by many states.

The primary question to ask to determine if bed bugs may be the source of a ”bite” is ”Have you examined the area where you sleep for the presence of bed bugs?

Bat Bugs

Along with bed bugs, the bat bugs (Cimex adjunctus, C. pilosellus) are the most common of the biting insects of the ”bed bug” family that occur in Colorado.  These are parasites of bats and develop within bat colonies, which often develop in attic areas or in cavities of the building.  Bites of humans are incidental and occur as the bat bugs disperse from the nesting bats.  Control of bat bugs involves elimination of nesting bats on or in a residence.  Along with this are methods to exclude (e.g., sealing entrances from nesting area) or kill (e.g., insecticides) the residual bat bugs.   In the absence of the bat hosts, these insects ultimately die-out.  Bat bugs and their control are discussed further in Fact Sheet 5.574.

Bat bugs are very similar in shape and size to bed bugs and with careful search often may be discovered in the vicinity where biting has occurred.  The primary question to ask to determine if bat bugs may be the source of a “bite”:

Are bats nesting within the structure?

Bat bugs only breed and develop on bats, although they will incidentally bite humans if they disperse from bat roosts.

Swallow Bugs and Other Wild Bird Bugs

Swallow bugs (Oeciacus vicarius) are a member of the ”bed bug” family that develops on swallows.  Incidentally they may bite humans when swallows nest on the sides of buildings.  Swallow bug biting occurs most frequently in spring, shortly before the return of nesting swallows.  Biting may also occur shortly after swallows leave nests, as the bugs disperse for shelters where they remain largely dormant until the following spring.  Control of swallow bugs involves elimination of nesting bats on or in a residence.  Along with this are methods to exclude (e.g., sealing entrances from nesting area) or kill (e.g., insecticides) the residual bat bugs.   In the absence of the swallow hosts, these insects ultimately die-out.  Swallow bugs and their control are discussed further in Fact Sheet 5.574.

At least two other species in the “bed bug” family that also feed on wild birds occasionally occur in homes.  In western Colorado Hesperocimex coloradensis is associated with woodpecker nests and may wander into homes and bite human if there are nesting woodpeckers behind walls.  The poultry bug, Haematosiphon inodorus is associated with poultry and owls.

Swallow bugs are very similar in shape and size to bed bugs and with careful search often may be discovered in the vicinity where biting has occurred.  Some questions to ask to determine if swallow bugs may be the source of a “bite”:

Are there swallow nests attached to the building now or the previous season?

Swallow bugs only breed and develop on swallows, although they will incidentally bite humans if they disperse from bat roosts.

Is biting most common in late winter/early spring – coincident with the return of nesting migratory swallows?

Peak period of swallow bug biting is around the time when swallows return to nests, as the bugs resume activity following a period of winter dormancy.

(For other species) Are woodpeckers or owls nesting in the building?

Two species in the “bed bug” family known from Colorado are found associated with nests of these bird species.

Masked Hunter

The masked hunter (Reduvius personatus) is a species of assassin bug that is sometimes found in homes.  Adult stages are dark brown or black and about 3/4-inch long.  However, immature stages are smaller and cover themselves with lint or other debris.  Sometimes they may appear as moving balls of dust.

Never common in homes, masked hunter is a predator of other insects.  It is sometimes called the “bed bug hunter” but will likely feed on almost any insect and probably some spiders that it encounters.  The masked hunter paralyzes its prey with saliva it injects from its piercing-sucking mouthparts.

If handled, this insect may also bite humans.  However, the bite is immediately painful and due to its overall size, the insect bite source is readily recognized.

A sheet that gives some more information on this insect is found at the CSU Insect Information Web Site:


Two species of lice are sometimes associated with humans in Colorado.  By far the most common is the head louse (Pediculus humanus capitatis), which sometimes becomes epidemic in schools, day care and other areas where people are crowded together.  As the name applies the head louse is almost exclusively found among the hairs of the scalp.  Biting can cause itching but the cause is easily identified by close inspection of the head.  In addition to the active adult and nymph stages, white eggs are glued tightly to hairs.  Numerous publications are available that can be used to identify head lice and offer treatment.   One good on-line source is through Centers for Disease Control at

Rarely encountered is the crab louse (Phthirus pubis).  This is sometimes known as the “crab” and is a species almost entirely transmitted via very close, usually sexual, contact.  It lives among the coarse hairs of the pubic region, occasionally occurring on facial hair.  Feeding by this species can cause a local irritation.  However, if present it can usually be easily found upon inspection.  The web page produced by the Centers for Disease Control detailing crab lice is at

Non-Arthropod Causes for Itching/Irritation Symptoms

That Mimic Arthropod Infestation

There are a great many things that can produce persisting itching or irritation that may resemble infestation with insect or other arthropods.  This subject is well cover in the University of Kentucky publication Invisible Itches: Insect and Non-Insect Causes authored by M. F. Potter.   Everything that follows, except for comments in parentheses ( ), are from this article.  Comments involving arthropod causes are deleted since they are covered above.

“It is important to recognize that there are many potential causes of itching and irritation other than pests. Allergies, cosmetics, medications, and environmental contaminants all can produce reactions similar to insect bites. While this makes the experience no less real or unpleasant for the affected individual, it underscores the importance of keeping an open mind to the possibility of non-insect causes of such reactions. Much like a detective, one should attempt to rule out all potential sources of irritation through the process of elimination…..

Household Products

“There are literally hundreds of non-insect agents capable of causing itching and irritation. Household products are involved far more often than are pests and may cause skin reactions similar to insect bites. Products most often implicated include phosphate detergents, soaps, cosmetics, ammonia-based cleaning agents, hair products, medications, printing inks (especially from multi-form carbonless carbon paper), and certain types of clothing, particularly those which contain fire retardants. If a connection can be made between irritation and exposure to one of these potential irritants, avoiding further exposure may correct the problem. A dermatologist can usually confirm that a product, rather than a pest, is causing the irritation.

Environmental Factors

“When two or more individuals experience irritation in the absence of pests, the cause is likely to be environmental conditions or contaminants dispersed in the air. The irritant(s) may be either physical or chemical in nature.

“Physical Irritants. The most common physical irritants are tiny fragments of paper, fabric, or insulation. When these fibers contact the skin, they can produce symptoms ranging from a “crawling sensation” to intense itching accompanied by a rash, welts, or open sores. If fibers or fragments are involved, the irritation usually occurs over exposed areas of the body such as arms, legs, neck, and head.

“Irritation produced by paper fragments is especially common in offices where large quantities of paper are processed daily. Continuous-feed paper from computers and multi-page forms generate large amounts of fragments, resulting in accumulations on desktops and other surfaces. Newly installed or badly worn synthetic carpet, drapes or upholstery also shed fibers which can irritate skin.

“Other potential sources of irritation are insulation fibers released into the air by heating/cooling systems in need of repair and sound-deadening fibers embedded into drop-ceiling tiles. These latter sources are especially suspected if there have been problems with the air-handling system or recent repair work on the ceiling.

“Irritation is aggravated by static electricity which increases the attraction of the tiny charged fibers to exposed skin. Low humidity, electronic equipment, and nylon (e.g., from carpeting, upholstery, or women’s stockings) all increase levels of static electricity and the potential for problems from fragments or fibers. Static electricity may also cause body hair to move, giving the impression of insects crawling over the skin.

“If fibers or fragments are suspected of causing the reactions, floors, rugs, work surfaces, and furniture should be thoroughly and routinely vacuumed, and desktops and tables wiped down with a damp cloth. Static-reducing measures should also be considered such as raising the humidity level of the air and installing static-resistant mats and pads under chairs and electronic equipment in offices. Anti-static sprays can be used to treat seat cushions and nylon stockings.

“Dry air alone can cause irritation, producing a condition known as “winter itch.” As skin loses moisture, itching results. A similar reaction can occur from changes in temperature; these tend to make skin more sensitive. A skin moisturizer is often helpful in these situations.”

(In Colorado, dry air is very likely a primary contributor to many cases of invisible itches. Humidity is very low at any time of the year and drops sharply indoors with cold weather.  Extension offices should anticipate a surge in inquiries of about itching beginning in late autumn.)

Airborne Chemical Irritants.  Indoor air pollution can be a serious problem in modern office buildings and other energy-efficient structures where air is recirculated over and over. Indoor air pollution can also be a problem in homes. As the concentration of chemical contaminants in the air increases, people may experience dizziness, headaches, and eye, nose, or throat irritation. Certain air-borne contaminants can also produce rashes and skin irritation similar to insect bites. Chemical contaminants most often responsible for these reactions include ammonia-based cleaning agents, formaldehyde emitted from wall and floor coverings, tobacco smoke, and solvents and resins contained in paints, glues, adhesives, and pesticides repeatedly applied for control of suspected pest infestations.

“Reactions to airborne chemicals most often occur in buildings with inadequate ventilation, especially those that are new or have been refurbished with new paint or wall or floor coverings. If indoor air pollutants rather than insects are suspected, you may wish to consult an industrial hygienist who is equipped to monitor ventilation levels and the presence of allergy-producing contaminants. Companies specializing in environmental health monitoring have listings in the telephone directories of most metropolitan areas.

Health-Related Conditions

“Health-related conditions may be responsible for irritation mistakenly attributed to insects. Itching and skin irritation are common during pregnancy (especially during the last trimester) and may also occur in conjunction with diabetes, liver, kidney, and thyroid disease, and shingles. Food allergies are another common cause of itching and irritation.

“A person’s emotional state can also induce skin reactions that can be mistaken for insect bites. Stress and conflict at work or home can produce itching and irritation. The itching response can be induced in other individuals simply by the “power of suggestion;” i.e., when one person in a group feels an itch or bite and begins to talk about it, others also feel the urge to scratch as well (a condition known as Bell’s syndrome).

“Delusory parasitosis is a more serious emotional disorder characterized by an irrational fear that living organisms are infesting a person’s body. Cases of delusory parasitosis often have similar symptoms and patterns of behavior. Patients typically report “bugs” invading their ears, nose, eyes, and other areas of their body. The “creatures” frequently disappear and reappear and change colors while being observed. Specimens brought in for identification usually consist of bits of dead skin, hair, lint, and miscellaneous debris. The skin of the individual is often severely irritated from desperate scratching, excessive bathing, and application of ointments, bleaches, gasoline and other solvents. While these occurrences may seem bizarre to persons who are not affected, they are frighteningly real to the patient. Delusory parasitosis as well as other suspected emotional or medical conditions should be brought to the attention of a dermatologist or other physician.”

(Use of certain drugs, notably methamphetamine, is another source that may produce sensations of insects crawling on the body, sometimes described as “crank bug bites”.  Metabolism of the drug near the skin can produce small, hot bumps.  Scratching of these then may produce sores which in turn become infected.  This reinforces a cycle of bug bite sensation.)

(Table 4.  Household Products that May Produce Irritation that Mimics Infestation with Arthropods)

Detergents (especially phosphate-based)
Cosmetics/hair products
Ammonia-based cleansers
Printing inks (e.g., carbonless)
Clothing (especially fire retardant)

(Table 5.  Environmental Factors that May Produce Irritation that Mimics Infestation with Arthropods)

Physical irritants

Paper, fabric, or insulation fibers
Low humidity
Seasonal changes in temperature
Static electricity

Chemical irritants

Formaldehyde (e.g., from particle board, wall and floor coverings)
Solvents/resins associated with paints and adhesives
Volatiles from asphalt and tar installation

(Table 6.  Health-related Conditions that May Produce Irritation that Mimics Infestation with Arthropods)

Communicable diseases (e.g., chicken pox, measles)
Diabetes, liver, or kidney disorders
Food allergies
Insect phobias
(Methamphetamine use)

References Used Most Heavily in This Publication

Benoit, R. And J.R. Suchard.  2006.  Necrotic Skin Lesions: Spider Bite or Something Else?  Consultant. Volume 46 (12) (October 1, 2006)

Potter, M.F.  2007.  Invisible Itches: Insect and Non-Insect Causes.  University of Kentucky Extension Pub. ENT-58.  4 pp.

Ridge, G.A. 2012.  Delusory parasitosis: The belief of being lived on by insects or other organisms.  Guide for Health Professionals, Medical Communities, and Pest Management Professional

Vetter, R. 1999.  Identifying and Misidentifying the Brown Recluse Spider.  Dermatology Online Journal 5(2): 7

Revised January 24, 2017