Clover Mites

HYG-2095
Date: 
03/09/2010
David Shetlar, Entomology
Barbara Bloetscher, Entomology

Clover mites sometimes invade homes and buildings in enormous numbers, in the early spring and late autumn. They can creep and gather in clusters on walls, drapes, window sills, and furniture; occasionally, they even get into beds and clothing. They may become troublesome in hospitals, nursing homes, apartments, food processing facilities, etc. If crushed, they leave a green stain with red-orange marks, which is quite noticeable on linens, curtains, walls, and woodwork. They are a nuisance by their presence, but they do not bite humans or animals, transmit disease, nor feed on household furnishings or pantry supplies. They feed on various plants and turf outside, but adults seek sheltered places (dark, cool, moist places are preferred) to lay eggs, and the nymphs seek such places to shed their exoskeletons.

Common Name Scientific Name
Clover Mite Bryobia praetiosa Koch

Identification

Clover mites are about 1/30 inch long (smaller than a pinhead), oval-shaped, reddish-brown to dark olive-green with pale orange legs. They are eight-legged, and the front pair of legs is greatly elongated. These front legs are held, antennae-like, in front of the head. There are feather-like bristles on the body and fan-shaped hairs along the back edge of the body; these can be viewed under a high-powered microscope. Young mites are smaller and dark red. Also, eggs are bright red when first laid. Crawling mites are sluggish, slow-moving, and normally invade the home where the sun is warmest (along the south, southwest, and east sides). 

Life Cycle and Habits

Clover mites develop from unfertilized eggs (no males needed). Females lay about 70 eggs each, singly or in masses, in cracks and faults of concrete foundations, in mortar crevices, between building walls, under loose bark of trees, and in other protected places. Eggs laid in late spring will remain dormant during the hot summer, hatching in early autumn when temperatures fall below 85°F. While called clover mites, it appears that these mites prefer to feed on the cell contents of turfgrasses and honeysuckles. There are four stages in the life cycle: the larva (which only has three pairs of legs), nymph I, nymph II, and adult. At molting time, the immatures seek out a secluded resting place where they will remain immobile for one to several days. Each immature stage lasts two to six days, and the life cycle is completed outdoors in two to three weeks.

In late fall, adults lay overwintering eggs when the temperatures begin to regularly drop below freezing at night. Overwintered eggs hatch in April, and the mites will remain active into early June. When daytime temperatures regularly exceed 85°F, the females lay oversummering eggs that won't hatch until cooler temperatures return. Fall and spring populations usually undergo two or more generations per season. Adult mites may live one to several months depending on climatic conditions. The mites can remain active during the summer when cool temperatures prevail, and mites can remain active in the winter as long as moderate temperatures persist.

Adult clover mite. Adults, nymphs, and eggs of clover mites.

Clover mites commonly infest homes and buildings from mid-October through December, and again from mid-April through mid-June. The mites are most active between 50°F and 75°F. They may invade a building at the onset of warm summer weather or if host plants are dried up or cut off. Hosts include grasses (heavy feeding gives a silvered appearance), clover, dandelion, shrub honeysuckle, strawberry, and iris, to name a few. Most outbreaks occur in May where well-fertilized lawns are maintained in close proximity to the sunny side of the house foundation. In the fall, thousands of clover mites may congregate on vegetation around homes and on foundation walls, crawling into protected places as cold weather arrives. In buildings, they may hide under shingles, under siding, behind window and door casings, or indoors, especially in basements. Outside, they may hide under mulch, under landscape hardware stones and logs, and under loose tree bark before becoming active again in the spring.

Control Measures

Prevention

Remove all grass and weeds (lush vegetation) from around the structure's foundation, leaving a bare strip 18 to 24 inches wide, especially on the south, southwest, and east sides of the building. Mites do not like to cross bare, loose soil as readily as grassy surfaces touching the foundation. This bare strip can be planted with flowers such as geranium, zinnia, wallflowers, marigold, salvia, rose, chrysanthemum, and petunia, or with shrubs such as juniper, spruce, arborvitae, yew, or barberry, which are unattractive to clover mites. A layer of pea gravel in a band around a building will also discourage mite invasion. If the mites are ending up in the basement or on walls, there are openings through which they are entering the building. Be sure to seal cracks and gaps or other points of entry with caulking compound and putty, or with weather stripping around foundations, windows, and doors.

Mechanical

Clover mites indoors can be removed using a vacuum cleaner with proper attachments to effectively collect live mites without crushing them. Mites can escape from the sweeper bag when the sweeper is turned off, so dispose of the sweeper bag after collection by placing it in a heavy plastic bag that can be completely sealed.

 

Insecticides

It is best to use a perimeter barrier spray around the outside of the house at the onset of mite invasion. Outdoors, spray the foundation, exterior walls up to the bottom of the first floor windows, and a strip 10 to 20 feet wide from the foundation out into the grass. Spray the foundation and walls thoroughly so as to not leave untreated spots. Completely wet to point of runoff, the vegetation and/or turf with the insecticide mix. Granular insecticides can also be spread in a band around the perimeter of a building to kill and discourage clover mites. Many insecticides (and some miticides) are registered for control of clover mites. Inside a building, select an insecticide that is registered for interior use and claims to kill creeping and crawling pests.

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