Common Name: Clover Mite
Scientific Name: Bryobia praetiosa Koch
Class / Order / Family: Arachnida/Acari/Tetranychidae
The common name comes from clover being one of the preferred host plants of this mite. This is a nuisance pest which sometimes invades structures by the thousands, primarily in the autumn. It is distributed worldwide and is found throughout the United States.
Adults about 1/64″ (0.75 mm) long. Body oval, soft, dorsoventrally flattened (top to bottom). Color often dark red, also olive green or rusty brown. Dorsum with 2 shields weakly defined, reticulate (with a network of fine lines), and not protruding over mouthparts; propodosoma (dorsal, front area) with 4 pairs of hairs (setae) and propodosoma with 4 lobes, each with 1 fringed hair (seta). Mouthparts with cheliceral based fused, separated form ventral rostrum, chelicerae long, recurved, and whiplike; palpus 5-segmented, with distinct thumb-claw process. Ocelli present on propodosoma (dorsal, front area).
Stigmata (breathing pore opening plate) 2 in number, located behind cheliceral bases, absent on hysterosoma (dorsolateral area between 2nd and 3rd pair of legs). Legs with 1st pair very long, about twice length of 2nd pair, held forward like antennae; tarsi with padlike empodia (enlarged pretarsal/terminal structure) and claws terminating in a pair of tenent hairs (suckerlike adhesive setae). Genital opening transverse. Plant feeders. Note that there are 15-20 different morphological forms of the clover mite. Immature stages are bright red in color.
(1) Brown wheat mite (Petrobia latens; Tetranychidae) has tarsi with claws padlike and empodium hooklike, with 2 rows of ventrally directed tenent (suckerlike) hairs/setae, and propodosoma (dorsal, front area) with 3 pairs of hairs (setae).
(2) Winter grain mite (Penthaleus major; Penthaleidae) with chelicerae short and terminating with 2 teeth, palps 4-segmented and lacking a thumb-claw process; propodosoma (dorsal, front area) apex with a tubercle bearing 2 hairs (setae); 1st and 4th pair of legs longer than 2nd and 3rd pairs, tarsi with 2 claws and empodium; and genital opening ventral, with 2 pairs of suckers.
Clover mites reproduce parthenogenetically (without fertilization): males are unknown in the United States. A female will lay about 70 spherical red eggs, typically throughout the summer and autumn. Eggs become dormant about 75F/24C and also inactive below 40F/4C. Eggs laid in the autumn hatch the following spring. Developmental stages include egg, larva, protonymph, deutonymph, and adult. Larvae have only 3 pairs of legs whereas, nymphs and adults have 4 pairs. One generation or life cycle (egg to egg) typically requires about 1 month, with optimal developmental temperature being 65-69F/ 18-20C. Several generations can be completed during the autumn, winter, and spring.
Clover mites are plant feeders only. They feed on some 200 different plant species including trees, shrubs, flowers, grass, and agricultural/garden crops. Around structures they show a preference for heavily fertilized grass/lawns. The spherical red eggs can sometimes be seen on foundation walls and/or in foundation cracks. They are also laid on vegetation. Sometimes in the autumn the number of eggs can be large enough to give the surface they are on a fuzzy red appearance. The invasion of structures by enormous numbers of clover mites usually occurs in the autumn as the vegetation dies, with them numbering in the hundreds of thousands.
In Pennsylvania, the greatest number of structural invasions occur in April (40%), with May (23%) being second highest. Springtime structural invasions are usually associated with new mulch being applied to plant beds and shrubbery around the foundation. Invasion is typically through cracks around windows and doors or through masonry cracks. They will come up behind siding until an entryway is found. Clover mites are active during times of cooler weather and become dormant/inactive during hot weather. In Illinois and Pennsylvania, they become dormant in May and remain so until September.
In the eastern United States, these mites can be found in infested homes from November until May or June, with nuisance activity highest during the spring. These mites will leave a reddish spot/smear/stain if crushed. This can cause problems on wallpaper, drapes, window shades, stuffed furniture, carpets, etc. White clover and black medic are the preferred host plants with Kentucky bluegrass, bentgrass, red fescue, redtop, and chickweed a close second. Ornamentals and flowering plants are less preferred.
An 18″ (45 cm) grass-free strip/band around the structure’s perimeter can reduce the number of invaders by about 90%. This strip/band combined with the application of an appropriately labeled pesticide as a perimeter treatment are most effective. The application should go up about 2 ft (0.6 m) on the foundation wall or to the bottom of any siding and come out about 6-10 ft (1.8-3 m) from the foundation. Pay particular attention to the soil/grass junction with the masonry wall and cracks in the masonry wall. Micro encapsulated and wettable powder formulations work best.
Timing of the application is crucial and must occur before the summer inactivity period begins. This would be before mid-May in Pennsylvania and probably before the first week of May further south. Inside, clover mites should only be removed with a vacuum which will reduce or avoid red smears and stains. Indoor applications of pesticides will only give temporary relief if outdoor control measures are not done.