|Adult Trombidium holosericeum|
Ewing, 1929 
|The distribution of trombiculid species, which is nearly everywhere in the world.|
Trombiculidae (pronounced /trɒmbɨˈkjuːlɨdiː/) is a family of mites called trombiculid mites (also called berry bugs; harvest mites; red bugs; scrub-itch mites; and, in their larval stage, chiggers). The term chigger is sometimes used to refer to a different animal, the Chigoe flea. Trombiculidae live in the forests and grasslands and are also found in low, damp areas where vegetation is rank such as woodlands, berry bushes, orchards, along lakes and streams, and even in drier places where vegetation is low such as lawns, golf courses, and parks.
They are most numerous in early summer when grass, weeds and other vegetation are heaviest. In their larval stage they attach to various animals, including humans, and feed on skin, often causing itching. These relatives of ticks are nearly microscopic measuring 0.4 mm (1/100 of an inch) and have a chrome-orange hue. A common species of harvest mite in Northern America is Trombicula alfreddugesi; in the UK the most prevalent harvest mite is Trombicula autumnalis.
Trombiculid mites go through a life cycle of eggs, larva, nymph, and adult. The larval mites feed on the skin cells, but not blood, of animals, including humans. The six-legged parasitic larva feeds on a large variety of creatures including humans, rabbits, toads, box turtles, quail, and even some insects. After crawling onto their host, they inject digestive enzymes into the skin that break down skin cells. They do not actually "bite," but instead form a hole in the skin called a stylostome and chew up tiny parts of the inner skin, thus causing severe irritation and swelling. The severe itching is accompanied by red pimple-like bumps (papules) or hives and skin rash or lesions on a sun-exposed area. For humans, itching usually occurs after the larvae detach from the skin.
After feeding on their hosts, the larvae drop to the ground and become nymphs, then mature into adults which have 8 legs and are harmless to humans. In the post larval stage, they are not parasitic and feed on plant materials. The females lay 3–8 eggs in a litter, usually on a leaf or under the roots of a plant, and die by autumn.
Trombiculidae, from Greek τρομειν ("to tremble") and Latin culex, gen. culicis ("gnat" or "midge"), was first described as an independent family by H.E. Ewing in 1944. But references to chiggers go as far back as the six century China, and by 1733, the first recognization of trombiculid mites in North America were made. In 1758, Linnaeus described a single species Acarus batatas (Now Trombicula batatas). However, most information about chiggers came from the problems arose during and after World War II.
Then, when the family was first described, it included two subfamilies, Hemitrombiculinae and Trombiculinae. Womersley added another, Leeuwenhoekiinae, which at the time only contained Leeuwenhoekia (Oudemans, 1911). Later he erected the family Leeuwenhoekiidae for the genus and subfamily, having six genera; they have a pair of submedian setae present on the dorsal plate.
Trombiculid mites are found throughout the world. In Europe and North America, they tend to be more prevalent in the hot and humid parts. In the more temperate regions, they are found only in the summer (in French, harvest mites are called aoûtat, or "August" flies). In the United States, they are found mostly in the southeast, the south, and the Midwest. They are not present, or barely found, in far northern areas, in high mountains and in deserts. In the British Isles, the species Trombicula autumnalis are called harvest mites, in North America the species Trombicula alfreddugesi, and the species Trombicula (eutrombicula) hirsti which are found in Australia and are commonly called the scrub-itch mite.
The length of the mite's cycle depends on species and environment, but normally last 2 to 12 months (but may be longer). The number of cycles in a year depends on the region. For example, in a temperate region, there might only be 3 a year, but in tropical regions, the cycle might be continuous all year long. Adult harvest mites overwinter in protected places such as slightly below the soil. Females become active in the spring, and once the ground temperature is regularly above 60 ℉ (15.6 ℃), she lays eggs, up to 15 eggs per day in vegetation when soil temperatures are 60 ℉ (15.6 ℃). Therefore, from April through early autumn up until the first frost, humans are susceptible to chigger bites. The larvae congregate in groups on small clods of earth, in matted vegetation and even on low bushes and plants, where they have more access to a prospective host. The eggs are dormant for about six days, after which the non-feeding pre-larvae emerge, with only three pairs of legs. After about six days, the pre-larva grows into its larval stage.
The larvae, commonly called chigger, are about 0.17–0.21 mm (0.007–0.008 in) in diameter, normally light red, covered in hairs, and move quickly relative to size. There is a marked constriction in the front part of the body in the nymph and adult stage. The eggs are round in shape.Chigger is also an alternate term for the chigoe flea (Tunga penetrans), a sand flea found in tropical and subtropical climates in the Americas and Africa.[16 ]
The name chigger originated as a corruption of chigoe. Also called scrub mite, red mite and several other names, they are found throughout temperate and tropical zones. Chiggers come in 3 stages: the deutovum, unfed larva, and engorged larva. Once in the egg developing, the larvae enclosed in a membrane in addition to the eggshell, are called deutovum. After hatching, the unfed larvae migrate to the highest area and wait for a host.
The larval stage is the only parasitic stage of the mite's life cycle. They are parasites to many animals. About 30 of the many species in this family, in their larval stage, attach to various animals, including amphibians, reptiles, birds, and mammals, and feed on skin. This often causes an intensely itchy red bump in humans (who are accidental hosts). 
Chiggers attach to the host, pierce the skin, inject enzymes into the bite wound that digest cellular contents, and then suck up the digested tissue through a tube formed by hardened skin cells called a stylostome. They do not burrow into the skin or suck blood, as is commonly assumed. Itching from a chigger bite may not develop until 24–48 hours after the bite, so the victim may not associate the specific exposure with the bite itself. The red welt/bump on the skin is not where a chigger laid eggs, as is sometimes believed. The larva remains attached to a suitable host for 3 to 5 days before dropping off to begin its nymph stage.
Chiggers do not like sunlight or humidity. During the wet season, chiggers are usually found in tall grass and other vegetation. During dry seasons, chiggers are mostly found underneath brush and shady areas.
Although the harvest mite chigger usually does not carry diseases in North American temperate climates, the Leptotrombidium deliense are considered a dangerous pest in East Asia and the South Pacific because they often carry Orientia tsutsugamushi, the tiny bacterium that causes scrub typhus, which is known alternatively as the Japanese river disease, scrub disease, or tsutsugamushi. The mites are infected by the Rickettsia passed down from parent to offspring before eggs are laid in a process called transovarial transmission. Symptoms of scrub typhus in humans include fever, headache, muscle pain, cough, and gastrointestinal symptoms.
Once the larva has engorged itself on skin and has fallen off its host, the larva develops to its nymph stage. Like the larva, the nymphs are also sexually immature, but more closely resemble the adult.
This stage consists of three phases; the protonymph, deutonymph, and tritonymph, respectively. The protonymph and tritonymph morphology are unusual in species of Trombiculidae. The protonymph phase combines larval and protonymph characteristics with deutonymph and tritonymph morphology. The protonymph is an inactive transitional stage. The active deutonymph develops an additional pair of legs (for a total of eight). Lastly, it re-enters inactivity during its transitional tritonymph phase before growing to adulthood.
As a deutonymph and adult, trombiculid mites are independent predators that feed on small arthropods and their eggs, also found to eat plant material. They live in soil, often found when digging in yards and gardens. Adults can be beneficial to human beings, since they often eat the eggs of other pests, such as mosquitoes.
Trombiculiasis, also called Trombiculidiasis, is the term coined for the rash caused by trombiculid mites.
Chiggers are commonly found on the tip of blades of grasses to catch a host, so keeping grass short, and removing brush and wood debris where potential mite hosts may live, can limit their impact on an area. Sunlight that penetrates the grass will make the lawn drier and make it less favorable for chigger survival.
Chiggers seem to affect warm covered areas of the body more than drier areas. Thus, the bites are often clustered behind the knees, or beneath tight undergarments such as socks, underwear, or brassieres. Areas higher in the body (chest, back, waist-band, and under-arms) are affected more easily in small children than in adults, since children are shorter and are more likely than adults come in contact with low-lying vegetation and dry grass where chiggers thrive.
Chigger bites can be minimized by the use of tightly woven protective clothing, including long pants, which make it hard for them to reach such spots. Application of repellent to the shoes, lower trousers and skin is also useful. Because they are found in grass, staying on trails, roads, or paths can prevent contact. Dusting sulfur is used commercially for mite control and can be used to control chiggers in yards. The dusting of shoes, socks and trouser legs with sulfur can be highly effective in repelling chiggers.
Another good strategy is to recognize the chigger habitat to avoid exposure in the first place. Chiggers in North America thrive late in summer, in dry tall grasses and other thick, unshaded vegetation. Insect repellents containing one of the following active ingredients are recommended: DEET, catnip oil extract - nepetalactone, citronella oil or eucalyptus oil extract. However, in 1993 issue a study reported on tests of two commercial repellants: DEET and citrus oil: "All chiggers exposed on the filter papers treated with DEET died and did not move off the treated papers. None of the chiggers that were placed on papers treated with citrus oil were killed." It was concluded that DEET was more effective than citrus oil.
To reduce the itching, an application of anti-itch cream containing hydrocortisone, calamine, or benzyl benzoate is often used (though calamine has been shown not to be effective). Hydrogen peroxide and capsaicin cream has also been effective. Another good way to relieve itching is to apply heat — either by using a hand held shower with water hot as one can stand, or by heating the bite with a hair dryer. The heat method will relieve itching for about four hours and will require repeating. Applying fingernail polish to the affected area does not kill the chigger; the chigger is actually no longer present by the time a rash is noticed.
The most effective way of removing chiggers is by washing the affected areas with warm water and soap. This must be done as soon as possible after exposure or possible exposure. Carefully wash the ankles, feet, behind the knees, and under the arms and chest. An Epsom salt bath may help alleviate itching. If one is near the seashore, wading for a few minutes in salt water will both get rid of the mites on one's skin and clothing and also alleviate the itching from their bites. Clothing, especially pants and socks, should be immediately discarded after returning from areas where exposure may have occurred. However, once symptoms appear, it may be too late to prevent further bites. Taking a hot bath when already covered with chigger bites may in fact be very uncomfortable and increase itching symptoms. Do not rub and scratch the skin aggressively, as this can break the skin and leave it vulnerable to a more serious infection.
Some claim that the chigger is still in the bite, perhaps mistaking the tiny red center of the bite for the chigger itself. In some cases, the chigger is still present when the bite appears. A 10X magnifier can be used to see the chigger and it may be removed with fine-tipped tweezers. Once it is gone, covering the bite with nail polish, calamine lotion, vaseline or other petroleum jelly, baby oil, or anything else may help the pain and itching, but will neither suffocate the chigger nor help the bites heal any faster. Medication such as antihistamines or corticosteroid creams may be prescribed by doctors, and might help in some instances.
Subfamiliae: Apoloniinae - Gahrliepiinae - Leeuwenhoekiinae - Trombiculinae
[list of subfamiliae after Shatrov & Kudryashova (2008)]
Aplodontophila - Axiogastia - Dermadelema - Endotrombicula - Eutrombicula - Guntheria - Iguanacarus - Kayella - Longisetacarus - Loomisia - Microtrombicula - Neotrombicula - Oaxacarus - Serratacarus - Tanautarsala - Trombicula - Vercammenia - Whartonacarus ...