INTRODUCTION.
The common name of redbug probably comes from the usual bright red color of the
larvae which attack humans; in Europe they are called harvest mites because of their
abundance in the autumn which is field crop harvest time. Only the larval stage is a pest,
being an ectoparasits of vertebrates including humans. They are found worldwide including
the United States. The most common chigger in the Western Hemisphere (New World) is the
common chigger, Trombicula alfraddugesi (Oudemans), and this treatment is based primarily
on this species as being representative.
RECOGNITION.
Larva very small 1/128" (0.15-0.25 mm) long unengorged to 1/64" (0.6
mm) long when engorged. Body oval, dorsoventrally flattened (top to bottom), not hard-
shelled; with 6 legs. Color red to reddish orange. Single dorsal shield or scutum
approximately rectangular with 5 hairs (setae), 1 at each corner and 1 midway along front
margin, and 2 sensillas (long hairlike structures) with 6-8 branches in outer half. Hairs
(setae) of body giving a hairy appearance; 22 on dorsum, 14 on venter. Palpal thumb- claw
process distinct. Seta on palpal coxa posterior to palpal femur. Uristigmate (external
opening to respiratory system) always associated with coxae I (1 st pair of legs). Adults
about 1/32" (0.9-1.1 mm) long; figure-8 shaped. Color bright red, very hairy with
velvet appearance.
SIMILAR GROUPS.
- Other red mites when on humans with 8 legs, not 6 legs, and do not
typically attach at hair follicles in areas of tight-fitting clothing.
BIOLOGY.
The developmental stages are egg, deutovum, larva, nymphochrysalis, nymph,
imagochrysalis, and adult; larva with 6 legs, nymph and adult with 8 legs. Eggs are
usually laid in damp but well drained soil. The deutovum develops within the egg and
contains the developing larva. The egg splits open exposing the deutovum from which the
larva emerges. They crawl on the soil searching for a suitable host or crawl up grass
stems to a height of about 2 3/8-2 3/4" (60-80 mm) and await the passage of a
suitable host. The larva attaches to a vertebrate host via its chelicerae, often at the
base of a hair. It feeds by partially digesting the host's tissue with its saliva and then
ingesting the resulting soup; it does not suck blood or burrow under the skin. After
feeding for about 3 days, it drops off the host and burrows into the soil. Here it
undergoes considerable tissue resorption and rearrangement as the nymphochrysalis
develops. Through a dorsal split in the larval skin and nymphochrysalis, the nymph
emerges. The nymph feeds on the eggs and early instars of other arthropods. After feeding
it becomes quiet and develops the imagochrysalis within its cuticle. The adult emerges via
a dorsal split in the imagochrysalis and nymphal cuticle. The adult food is the same as
that of the nymph. Developmental time (egg to adult) may require 1-12 months depending on
conditions of climate, with 1-3 generations per year in temperate regions to 6 generations
in subtropical areas. In Missouri the adults overwinter in an earthen cell about
1-1.5" (25-38 mm) below the soil surface, eggs are laid during the first warm spring
days, and the adults die shortly thereafter.
HABITS.
Only the larva is parasitic, and on a wide range of vertebrate hosts including
mammals, birds, reptiles, and amphibians. The nymphs and adults are free-living predators.
Adults live in the soil of grasslands and forests, in mammal nests and burrows, crevices
of decaying wood, bat caves, lawns, and marsh areas.
In general the larvae are most abundant in
vegetational transition zones such as the junction of forest and grass, along margins of
swamps, blackberry patches, and brush thickets. In areas such as Ohio, they are found in
lawns and in Kansas they have been collected in most habitats. The larval activity period
varies from 2 months in Minnesota and Massachusetts (July - September) to throughout the
year in southern Florida.
When humans are attacked, the larval mites most
frequently attach themselves at hair follicles in areas where clothing is tight fitting
such as the ankles, waist, and armpits. Itching is usually not felt for 3-6 hours after
attachment and may persist for up to 2 weeks. Scratching often removes the offending mite
but can result in secondary infection. Chiggers are not known to transmit any infectious
diseases to humans in the United States; in the Orient and other areas of the Pacific,
chiggers are vectors of the rickettsiae disease called scrub typhus caused by Rickettsia
tsutsugamushi (Hayashi).
