Ohio State University Extension Fact Sheet

Entomology

1991 Kenny Road, Columbus, OH 43210-1000

Mites Infesting Stored Foods

HYG-2152-97

Common Name	Scientific Name
Grain Mite	Acarus siro Linnaeus
Mold Mite	Tyrophagus putrescentiae (Schrank)
Cheese Mite	Tyrolichus casei Oudemans

Several species of mites infest stored foods and other organic debris such as grain, flour, cereals, dried fruits and vegetables, pet foods, cheese, dried milk, ham, sugar, paper, tobacco, molds, bird and animal nests, etc. These mites often prefer a moist, damp location. Sometimes the surface of infested materials appears to move due to the enormous numbers of mites (barely visible to the unaided eye). Heavy infestations of grain mites have a sweet, "minty" odor, best detected when mites are crushed between the thumb and forefinger, and held to the nose. Also, a coating or piles of brownish "mite dust" may appear on open shelving, around the base of flour sacks, on the surface of cheese or in other foods. Such piles consist of dead and living mites, cast skins and feces.

Prolonged contact with mite infested foods may produce a mild dermatitis known as "baker's" or "grocer's itch." Other contact may cause bronchial asthma and dust allergies. Also, if mites are taken internally with infested food, stomach disorders may result. However, Acarid Mites are responsible for the sharp flavor of a famous German Cheese, Altenburger "Milkenkase." Anyone ingesting this cheese (with its thousands of mites) for the first time may have gastrointestinal disturbances. Mites can survive temperatures near freezing and may become more prevalent during colder months.

Grain Mite

Identification

The bodies of grain mites are almost colorless, but the mouthparts and legs are pale yellow to reddish-brown. The adults are about 1/2-mm long, the females being slightly larger than the males. The body of the male is smoothly rounded at the hind end; that of the female is more oval with the posterior edges slightly indented in the middle. The body has a groove across it, dividing it into two areas. Two pairs of fairly long hairs trail at the end of the body.

The first two pairs of legs are widely separated from the hind two pairs. The front pair of the male are enlarged, and the femur bears a stout spinelike process on the underside.

Grain Mite Female (greatly enlarged) Top or dorsal view, Bottom or ventral view

Life Cycle & Habits

Grain mites commonly occur in dense populations. As many as 10,000 mites in 200-gram samples often have been found in binds of Canadian wheat. In one study, grain mites comprised 42 percent of the mites taken from 30 farm granaries in Central Canada. A procession of 50 grain mites has been observed emerging from the germ of a single wheat kernel. During summer and autumn, grain mites are commonly attacked by the predatory mite, Cheyletus eruditus Schrank.

Under highly favorable conditions [a relative humidity (RH) of 87 percent and temperature of 73 degrees F the life cycle of grain mites may be completed in 9 to 11 days. A female may deposit between 100 and 500 eggs on food over 10 to 12 days. The larval and nymphal stage may be 19 to 20 days with a complete life cycle of two to five weeks. An RH of 61 percent or lower is detrimental to grain mites. Grain mite development is extremely sensitive to humidity levels outside the range of 75 to 85 percent.

The second nymphal form may be replaced by a special stage known as the "hypopus." This stage is highly resistant to unfavorable conditions, insecticides and fumigation, and may exist for several months without feeding. The "hypopus" does not move much under its own power but is transported from place to place by clinging to small animal forms such as insects or mice. Most hypopi rely on air currents for distribution. They have been known to survive for at least seven months in dry flour and can withstand lower temperatures than the active form. When the hypopi encounters favorable conditions, it sheds its skin and resumes normal growth and development. The peculiar adaptation through the "hypopus" stage makes it very difficult to eradicate this mite.

Mold Mite

Identification

Males and females are about 1/2-mm long and have a small translucent body with almost colorless mouthparts and legs. The body, somewhat slender, bears a train of hairs which are more numerous and longer than those on the grain mite. Hairs do not project so stiffly. On the underside of the males, on either side of the anus, there are two dome-shaped anal suckers. Only trained specialists with proper magnification can identify mite species and separate males from females. Specialists at the Vector-borne Disease Unit of the Ohio Department of Health and/or Acaralogy Unit of The Ohio State University are qualified to make identification.

Life Cycle & Habits

Under moist conditions and summer temperatures, a generation can be completed in 8 to 21 days. As the temperature falls, the length of the life cycle increases greatly. The mold mite will breed readily above 86°F., a temperature lethal to the grain mite. The mold mite is less tolerant to low temperatures and cannot develop below 50°F., whereas the grain mite breeds readily well below this limit. However, in an inactive state, this mite can survive 32°F. At favorable temperatures and 90 to 100 percent relative humidity, the female will lay an average of 437 eggs. At a given temperature, larval and nymphal stages require about equal time for development. Unlike the grain mite, this mite does not produce a hypopus.

Foods

The mold mite is a pest of many foods, especially those having a high fat or protein content. Infestations have been found for each of these foods: mixed feeds; mixed feeds and brewer's yeast; whole wheat flour; soy flour; wheat germ; cheese; rye bread; white bread; and mixtures of oats, barley and wheat. Other known foods include cultivated mushrooms, various seeds, fruits, grain and straw stacks in the field, decaying animal and vegetable matter, herring meal, onion, bacon, figs, dried milk, cheese, ham, dried bananas, and copra. Their relative preferences in oilseeds are: peanuts, 14 percent; sunflower seeds, 12 percent; canola seeds, 10 percent; linseed, 6 percent; palm kernels, 6 percent; poppy seeds, 4 percent; and cotton seed, 4 percent. They may become pests of mushroom beds. Apparently, the mold mite has not caused serious damage to stored products in tropical or subtropical regions. It will attack the bodies of workers engaged in the copra industry, but it spares their faces.

Cheese Mite

Identification, Life Cycle & Habits

The cheese mite, known to cause dermatitis, is larger than both the grain mite and the mold mite. It has stout, well-tanned, faintly-wrinkled legs and tanned mouthparts. Males and females are similar except that females are larger. The life cycle requires 15 to 18 days at the ideal temperature of 73°F and an RH of 87 percent. No hypopi are formed.

Control Measures

Controlling mites is difficult when moisture and temperature conditions favor their development.

1. Carefully inspect foods and grains before purchase. Do not carry mite infested stored foods into the home. Pet foods purchased in bulk and stored for long periods of time should be checked routinely. Rotate food materials to remove the older items first.

2. Store foods only in a clean, dry area. Never store foods under damp, poorly ventilated conditions. If necessary, increase air circulation to reduce relative humidity and prevent molds and mildews. (Reduce the RH to below 55 to 60 percent and the moisture content of the media to below 12 percent.) Ventilate and dry areas with a dehumidifier or fan, or simply open doors of a damp room.

3. If bins or large containers are used for storage, do not dump new replacement foods (flour, grains, etc.) on older unused foods. Allow the original contents to become used up or exhausted, if possible. (Unused materials may become damp and moldy over time.) Avoid prolonged storage.

4. Place stored foods in containers with tight-fitting lids, ideally screw type. Stack any flour or packaged foods on pallets to permit air ventilation, and to prevent possible floor dampness from penetrating sacks or packages.

5. Periodically clean the storage areas, especially cracks, crevices, shelving, etc. Vacuum and wipe up any spilled foods, eliminating the foci of infestations by cleaning with attention to horizontal surfaces such as beams and window ledges.

6. When products become infested with mites, locate the source of infestation and eliminate it. (Usually it is a forgotten, opened package, sack, or container unused over a long period of time in a little-disturbed storage area.)

7. Remove any bird or rodent nests near the storage area. Repair any leaky water pipes or damp areas nearby where molds can develop. Rodents may store seeds, pet food, etc. in wall voids and other locations which later become mite infested.

8. Suspected mite infested foods can be supercooled (0°F for seven days in a deep freeze), superheated (140°F for 30 minutes in an oven in shallow pans), (5 minutes in a microwave), or disposed of in several heavily wrapped paper bags for garbage disposal. Some bury infested foods deep in the ground if land and space is available.

9. Remove all remaining food from the storage area. Place uninfested food in containers with tight-fitting lids. Thoroughly clean and scrub all shelves, floors, walls, etc. with hot water, strong detergents and allow to completely dry before using any registered pesticide sprays or before replacing the stored foods.

10. If a pesticide is needed in the house, apartment, structure or garage, spot-treat cracks and crevices only to kill hidden mites. Use only legal pesticides registered for mites in stored food areas such as pyrethrins containing piperonyl butoxide. Carefully follow all label directions and safety precautions. (Be sure the pesticide can be legally used in the house and avoid spray drift onto food, dishes or cooking utensils.) Always read the label and follow directions and safety precautions.

11. Control of mites in laboratories is often difficult. Laboratory white mice, rats, hamsters, and guinea pigs sometimes are heavily infested with grain and other mites. Clean the cages with steam if possible. Remove all bedding cages and pans, and sterilize cage contents with steam or heat.

12. If the stored food mite infestations cannot be successfully controlled, contact a reputable pest control operator. (The licensed, professional pest control operator has the most effective pesticides, equipment and experience.)

13. For warehouses, storage buildings, food processing plants, mills, granaries, seed houses, grain elevators, etc., only licensed, certified pesticide applicators can apply approved fumigants. Bear in mind that heavier dosages are needed for mite control than are necessary for insect control. Fumigation eliminates mites present at the time, but does nothing to correct conditions nor prevent reinfestation.