Mites are tiny arthropods, related to ticks. Several types of mites can be found in homes and of these a few may bite humans. Most mites are harmless predators of insects, or feeders on decaying plant material. Some pest mites feed on stored products like cheese and grain. Others are merely nuisance pests, accidentally entering homes from their normal outdoor habitat. Only a few mite species are parasitic on birds or mammals, but these can occasionally become biting pests in homes. Identifying the type of mite and/or likely host is the first step in solving an indoor mite infestation.
HUMAN BITING MITES
Several types of mites are associated with cases of skin dermatitis in humans. The tropical rat mite, Ornithonyssus bacoti, is one of the most common house invading species. The tropical fowl mite, Ornithonyssus bursa, and northern fowl mite, Ornithonyssus sylviarum, are also frequently encountered in homes. The latter two species are found principally on domestic or wild birds. The house mouse mite, Liponyssoides sanguineus, may also be found in structures with house mouse infestations. The tropical rat mite is a parasite on rats. Although none of these species are truly parasitic on humans, they bite people readily, often producing dermatitis and itching.
Rat and bird mite infestations occur in structures where rat or bird nests are located. Infestations are sometimes first noticed following extermination, or after the natural hosts have died or left the structure. Infestations may also occur where heavy mite infestations have developed around a rodent or bird nest. Occasionally rodent or bird mites may be found on rodents kept as pets.
Rat mites are small, approximately the size of the period at the end of this sentence (see video). They move actively and can be picked up with a wet finger, brush or piece of sticky tape. Distinguishing between different species of Ornithonyssus mites to determine whether birds or rodents are the likely source is difficult and requires special expertise. The first course of action when faced with a suspected biting mite problem is to look for all potential bird or rodent sources and collect some of the mites, if possible.
Collecting mites. Most pest control companies will (rightly) not treat a home without proof of pest presence. It is therefore important to collect mites prior to treatment. Parasitic mites are often first noticed when biting. Mites can be collected from the skin with an artist’s brush or tissue dipped in rubbing alcohol. Mites collected in this way should be placed in a small vial or other waterproof container with a small amount of rubbing alcohol. Mites can also be collected from the skin with a piece of tape (although this makes accurate identification of the mite unlikely). Sticky traps are also useful tools for sampling tiny arthropods around the home. Place several sticky traps in rooms where bites are occurring.
The primary mite host must be eliminated before successful control rodent or bird mites can be achieved. Clues to the type of host that has invaded the house can be deduced by the time of year that the mite infestation occurs. Rodent infestations are possible at any time of year, though they seem to occur most frequently in the fall and winter. Bird problems are most common during the spring and summer.
Roof rats are the most common rat encountered in Texas homes. As their name implies, roof rats are good climbers and often enter the home through openings in the roof or soffit areas. Noises in attic or ceilings, especially at night, can indicate roof rat activity.
To seal homes against rodents all vents and electric service entry points should be tightly closed with rodent-proof metal hardware cloth, metal flashing, or copper wool. Entry points around chimneys and between loose shingles should also be checked. Doors and windows should seal tightly. House mice will enter structures near the ground, especially under poorly-sealed doors. Rodent proofing must include the smallest entry holes. Mice can enter a home through a hole as small as a dime; rats can enter through a hole as small as a quarter.
Bird infestations are often first indicated by the sound of chirping coming from a chimney or soffit area. The same rules and materials used for rodent-proofing are effective in keeping birds out of the home. Special screening may be needed on chimneys to deny birds access to chimney areas. Birds nesting in chimneys may also indicate the need for chimney maintenance and cleaning. Chickens and other fowl kept in sheds or coops attached to a home can also be a source of mites indoors.
Pesticides can help suppress mite populations in the home, but must be used in combination with bird or rodent control. Several pesticides can be used indoors to treat mite problems. Sprays and aerosols containing syngergized pyrethrins should kill mites immediately on contact, though the treatment will only remain effective for up to a few hours. Insecticide sprays containing permethrin or bifenthrin are effective against many mites and should retain their killing properties for several weeks. Read the label carefully before spraying to make sure these products allow application to living areas, attics and crawl spaces. Indoor sprays should be applied only to the bases of walls and other potential entry points, not to furniture or other surfaces where people come into direct contact.
When a nest can be located, it’s best to first treat the area around the nest (e.g., the soffit or vent from which a bird nest is removed) with a pesticide, or else dust the area with a desiccant dust, such as diatomaceous earth. This should reduce the risk of live mites dispersing from the site and entering the indoor areas of the structure after the nest is removed. Long sleeves, gloves, and a tight-fitting dust mask are recommended when removing old bird or rodent nests to reduce the risk of exposure to ecto-parasites, like mites, and other pathogens.
If rodent pets, like gerbils, white mice or hamsters, are present in areas where bites are occurring, they should be taken to a vet and examined for mites.
Other mites that can be found in homes include the clover mite and certain mites associated with stored products. Clover mite infestations are common in homes during the late winter and early spring. Clover mites are feeders on grasses and weeds and can sometimes be found invading structures from the outside through windows and doorways. Adult clover mites are approximately 1 mm-long and can be distinguished, under magnification, by their long front pair of legs. These mites sometimes produce a red stain when crushed. Clover mites do not bite people, and are mainly a nuisance pest. Keeping grass and weeds cut short immediately around structural foundations, and maintaining tight seals around windows and doors can help reduce invasion of the home by this pest. Pesticide sprays can be applied to potential entry areas from the outside. Indoor sprays are generally not needed for this pest.
Stored product mites are uncommon pests in homes. The grain mite, Acarus siro, is one of the most common stored product mite pests. This mite is found most frequently on processed cereal products (e.g., flour); whole wheat flour is a preferred food source, as are some fungi and molds. Grain mites have also been recorded from cheese, poultry litter, and even from abandoned bee nests. Parasitic mites can be distinguished from stored product mites only with the use of a high-powered microscope; however the location of an infestation within a home may provide the best clue as to whether the pest is a feeder on stored grains. Most stored grain insects do not bite. Removal of the infested product, and thorough cleaning of the storage area, is usually sufficient to eliminate infestations. High humidity and moisture also favors mite infestations, so moisture control should also be a goal of a storage mite infestation.
For more information
For more information about about mysterious insect bites around the home, see Internet publication Diagnosing Mysterious Bug Bites. For more information about bird and rodent control, see the Texas AgriLife Extension Service wildlife publication website.
Michael Merchant, Ph.D., Urban Entomologist, Texas AgriLife Extension Service
Pete Teel, Ph.D., Professor, Department of Entomology, Texas A&M University